Patient circuit disconnect system for a ventilator and method of detecting patient circuit disconnect

[0001] Not Applicable

[0002] Not Applicable

[0003] The present invention relates in general to a method and a system for detecting a patient circuit disconnect of a ventilation system, and more particularly, to a method that not only detects patient circuit disconnect occurring in various locations of the circuit, but also informs critical subsystems of the ventilation system of the disconnect and/or a possible disconnect of the patient circuit.

[0004] When a patient's respiratory system cannot provide adequate oxygenation and/or ventilation, a mechanical ventilation system is typically used to provide breathing gas, particularly, supplemental oxygen to a patient through a patient circuit. The patient circuit generally has an inspiratory line and an expiratory line connected to an inspiratory port and an expiratory port of a wye fitting. The wye fitting further has a patient port connected to a patient. The mechanical ventilation is either supplied by intubating the patient by an endotracheal tube connected to the patient port of the wye fitting or by providing the patient with a face mask or a hood. The inspiratory line is also connected to a pneumatic system of ventilator from which the breathing gas is supplied, and the expiratory line conveys the gas exhaled by the patient to the ventilator and/or ambient air.

[0005] Disconnects of the patient circuit can typically occur in various areas of the patient circuit, including disconnect within the inspiratory line, disconnect within the expiratory line, and disconnect at the patient port of the wye fitting. When the patient is intubated with the endotracheal tube, disconnect may also occur at the patient upon endotracheal tube extabation. The disconnect at any of the above areas normally results in insufficient breathing gas being provided from the ventilator to the patient, which may consequently cause impairment or possible collapse of the lung of the patient.

[0006] Prior art systems and methods for detecting disconnect of a patient circuit have been developed which typically generate an alarm indicating the disconnect occurring in the patient circuit so as to prevent lung collapse due to circuit disconnect. However, the reliability of such conventional systems and methods is often affected by the location of the disconnect, the ventilation mode being used, the tubing size and length selection, and the physical size of the patient.

[0007] Therefore a substantial need exists in the art to provide a method and/or a device which detects circuit disconnects in a reliable and timely manner for a ventilation system regardless of disconnect locations, ventilation mode, patient size, and/or patient breathing pattern.

[0008] The present invention addresses and alleviates the above referenced deficiencies associated in the art by providing an apparatus and a method of detecting disconnect of a patient circuit of a ventilation system in a reliable and timely manner, wherein the circuit disconnect is preferably detected by a first detection algorithms and a second detection algorithm. The first detection algorithm provides a primary output for the main alarm of the ventilation system. The second detection algorithm provides a secondary output for a more reactive indication of a disconnect or a potential disconnect to inform other onboard subsystems (typically closed-loop subsystems) of the ventilation system whose feedback signals can be severely affected by the disconnect of a circuit disconnect. For example, the exhalation valve servo controlling positive end expiratory pressure (PEEP) must be informed by the start of the expiratory phase of a circuit disconnect which may have caused a PEEP error during previous breath. This will prevent the adaptive system from inappropriately compensating for the pressure loss due to disconnect or potential disconnect.

[0009] As the first detection algorithm provides the primary output for the main alarm, to avoid activating the main alarm by disconnect condition caused by noise or short-term fluctuation, the first detection algorithm is less sensitive than the second detection algorithm. That is, the first detection algorithm requires the disconnect condition to be detected for at least one predetermined period of time or two consecutive breaths, while the second detection algorithm basically requires detection of only one single breath.

[0010] The first detection algorithm includes three sub-algorithms for detecting long-term volume loss, consecutive-breath pressure loss, and continuous pressure loss. In the sub-algorithm for detecting long-term volume loss, a volume loss is determined based on the percentage of volume leakage, which reflects the volume difference between the inspiratory gas and expiratory gas. When the volume leakage exceeds a predetermined percentage for the first predetermined period of time, a long-term volume loss is detected. In the sub-algorithm for detecting consecutive-breath pressure loss, a threshold pressure is calculated based on the baseline pressure setting. When a selected pressure reading is less than the threshold pressure during a breath longer than a second predetermined period of time, or when the selected pressure reading is less than the threshold pressure for two consecutive breaths, a pressure loss is detected. In the continuous pressure loss detection sub-algorithm, a threshold pressure is determined in the similar manner used in the consecutive-breath pressure loss sub-algorithm. When a selected pressure reading is lower than the threshold pressure for a third predetermined period of time, the continuous pressure loss is detected. However, instead of selecting the peak expiratory gas pressure, the expiratory gas pressure that may change anytime when a disconnect occurs is used to compare with the threshold pressure, so as to determine whether a continuous pressure loss is detected.

[0011] The second detection algorithm also includes three sub-algorithms for detecting a short-term volume loss, a single-breath pressure loss and a compliance change. In the sub-algorithm for detecting a short-term volume loss, the percentage of volume leakage for a single breath is compared to the predetermined percentage as defined in the long-term volume loss detection. When the percentage of volume leakage exceeds the predetermined percentage, the short-term volume loss is detected. The short-term volume loss can thus be referred as a single-breath volume loss. The sub-algorithm for detecting single-breath pressure loss is basically the same as the sub-algorithm for detecting consecutive-breath pressure loss apart from that the pressure loss is detected without the requirement of a breath longer than the second predetermined period of time or two consecutive breaths.

[0012] The sub-algorithm for detecting the compliance change basically declares a disconnect when the compliance increase reaches a certain range. Preferably, the compliance increase is calculated based on the volume of inspiratory gas and the pressure change between the expiratory pressure in an inspiratory phase and the positive end expiratory pressure.

[0013] The present invention also provides a ventilation system comprising an inspiratory flow sensor, an expiratory flow sensor, an expiratory pressure sensor, a first set of processing units and a second set of processing units. The inspiratory flow sensor is operative to measure an inspiratory flow of a patient circuit of the ventilation system, the expiratory flow sensor is operative to measure an expiratory flow of the patient circuit, and the pressure sensor is operative to measure an expiratory pressure of the patient circuit. The first set of processing units is operative to perform the first detection algorithm, and the second set of processing units is operative to perform the second detection algorithm. Thereby, the main alarm can be triggered in a more reliably and time manner, while the on-board sub-systems can be properly adjusted when a short-term disconnect occurs.

[0031] Referring now to the drawings wherein the showings are for purpose of illustrating preferred embodiments of the present invention only, and not for purposes of limiting the same, FIG. 1 shows a ventilation system which comprises a ventilator 10 and a patient circuit 20. The ventilator 10 has an inspiratory port 12 from which breathing gas is supplied to the patient circuit 20 and an expiratory port 14 for receiving gas exhaled from the patient through the patient circuit 20. The patient circuit 20 has an inspiratory line 22 connected to the inspiratory port 12 and an expiratory line 24 connected to the expiratory port 14. Both of the inspiratory line 22 and the expiratory line 24 are connected to a wye-fitting 26, which establishes the connection between the inspiratory and expiratory lines 22 and 24 to the patient via an endotracheal tube, mask or hood 30. Typically, the ventilator 10 has a pneumatic system for providing the breathing gas and an expiratory gas processing unit processing the gas exhaled by the patient.

[0032] As shown in FIG. 2, a flow sensor 42 is installed in the inspiratory line 22 or the inspiratory port 12 for monitoring the flow rate of the breathing gas Q_insp; a flow sensor 44 is installed in the expiratory line 24 or the expiratory port 14 for monitoring the flow rate Q_exp of the gas exhaled by the patient; and a pressure sensor 46 is installed at the expiratory port 14 or the expiratory line 24 for measuring the gas pressure P_exp in the expiratory line 24. Other flow and pressure sensors may also be installed in various locations of the patient circuit 20. For example, pressure sensors may also be installed in the inspiratory port 12 or line 22 and/or the Wye fitting 26. In the preferred embodiment, the output readings of the flow sensors 42 and 44 and the pressure sensor 46 are input to a signal processing unit 62. The signal processing unit 62 integrates the flow rates Q_insp and Q_exp into inspiratory and expiratory volumes V_insp and V_exp and captures both the real time expiratory pressure P_exp and the peak expiratory pressure P_peak for every breathing cycle as parameters for detecting disconnects occurring in various locations of the patient circuit 20.

[0033] The ventilation system of the present invention preferably comprises a processor 64, in which a set of high-sensitivity detection units for performing a high-sensitivity detection algorithm and a set of low-sensitivity detection units for performing a low-sensitivity detection algorithm are installed. The high-sensitivity detection units include a compliance change detection unit 641, a short-term volume loss detection unit 642, and a single-breath pressure loss detection unit 643. The low-sensitivity detection units include a long-term volume loss detection unit 644, a consecutive-breath volume loss detection unit 645, and a continuous pressure loss detection unit 646. The detection outputs of the compliance change detection unit 641, the short-term volume loss detection unit 642 and the single-breath pressure loss detection unit 643 are input into a high-sensitivity disconnect determination unit 650. The high-sensitivity disconnect determination unit 650 provides an output signal to inform other on-board system such as a servo control system 52 of subsystems of the ventilator 10 of the disconnect condition of the patient circuit 20. The detection outputs of the long-term volume loss detection unit 644, the consecutive-breath pressure loss detection unit 645 and the continuous pressure loss detection unit 646 are input into a low-sensitivity disconnect determination unit 652, which then provides an output signal to a breath control system 54. When the signal indicates a disconnect in the patient circuit 20, the breath control system 54 is operative to generate an alarm such as an audio or visual warning signal.

[0034] The ventilation system preferably includes a monitor 50 for displaying the settings of the ventilator 10, the breathing condition of the patient, and/or the condition of the gas supplied by the pneumatic system of the ventilator. FIG. 3 shows the pressure at the Wye fitting 26, the inspiratory flow rate Q_insp and the expiratory flow rate Q_exp for normal breaths of a patient under a pressure-control mode. As shown, in normal breaths, the pressure at the Wye fitting 26 increases up to a peak pressure value P_peak from the beginning of each inspiratory phase I, and starts to fall to a low pressure value from the beginning of each expiratory phase E. The pressure at the Wye fitting 26 stays at the low pressure value until the beginning of the next inspiratory phase I. The inspiratory gas flow Q_insp rises sharply to a peak value at the beginning of each inspiratory phase, and falls quickly to zero within the same inspiratory phase I. At the beginning of the following expiratory phase, the inspiratory gas flow Q_insp starts to rise from zero to a peak bias flow until the beginning of the next inspiratory phase I. The expiratory gas flows Q_exp is a negative bias flow with respect to the inspiratory gas flow Q_insp. At the beginning of each inspiratory phase I, the expiratory gas flow Q_exp rises from a negative flow rate to zero. Such zero flow rate continues for the remaining inspiratory phase I. At the beginning of the following expiratory phase E, the flow rate drops sharply to a trough value and immediately reflects back to the previous negative flow rate as the patient exhales.

[0035] The low-sensitivity detection algorithm is performed to generate an output for activating the main circuit-disconnect alarm, such that a timely and reliable detection of the circuit disconnected can be obtained, while a false alarm caused by noise or fluctuation within a short term is avoided. The high-sensitivity detection program is performed to generate an output to inform devices of various subsystems of the ventilation system which are typically equipped with self-learning functions or software to adjust the operation parameters in accordance with variation of breathing conditions. For example, when an expiratory pressure loss is detected, an on-board adaptive system may add pressure for maintaining the positive end expiratory pressure (PEEP). This consequently increases the PEEP value when the patient circuit 20 is reconnected. Therefore, it is desirable that the adaptive system be informed of the circuit disconnect with a high sensitivity to avoid the false or incorrect learning and/or inappropriate self adjustment.

[0036] The high-sensitivity detection algorithm is also considered critical to control of patient flow delivery or gas pressure within the patient circuit which is limited or even stopped when feedback signals such as those representing gas pressure and flow in the patient circuit are disturbed or even unavailable due to circuit disconnect. Therefore, the circuit disconnect algorithm generates a disconnect alarm with a first, low sensitivity when a circuit disconnect is detected, but also preferably generates a signal to inform subsystems of the ventilation system of disconnect or potential disconnect of the patient circuit with a second, high detection sensitivity. The low-sensitivity detection of patient circuit disconnect is normally obtained when a pressure loss or volume loss occurs for a relatively long period of time compared to the high-sensitivity detection. The relatively long period of time depends on many factors including the settings of the ventilation system and physical conditions, including the size, weight, and health condition of the patient. In contrast, the high-sensitivity detection of patient circuit disconnect is normally obtained when the pressure or volume loss occurs for a single breath regardless how much time such disconnect has occurred. Thereby, the user or the therapist can reconnect the circuit to prevent lung collapse while preventing false learning or improper operations of on-board subsystems of the ventilation system.

[0037] FIGS. 4 and 5 show the high- and low-sensitivity detection algorithms, respectively, FIG. 6 shows the complete algorithm including both the high- and low-sensitivity detection algorithms as shown in FIGS. 4 and 5. FIG. 7 illustrates the response of inspiratory gas flow Q_insp, expiratory gas flow Q_exp, expiratory gas pressure P_exp, inspiratory gas volume V_insp—brth for a single breath, expiratory gas volume V_exp—brth for the single breath, accumulated inspiratory and expiratory gas volumes V_insp and V_exp when a disconnect occurs in the ventilation system 10.

[0038] As shown in FIG. 4, when any of single-breath pressure loss, compliance increase and short-term volume loss is detected in steps 402, 404 or 406, whether a disconnect output has been set is determined in step 408. If the disconnect output has not been set, the high-sensitivity disconnect detection output is set as logic “1”, and a counter for counting breaths upon reconnection is set to 3 breaths in step 418. If no disconnect has been detected in steps 402, 404 and 406, or when disconnect has been detected in any of the steps 402, 404, 406 and the detection output has been set at “1” in step 408 already, whether there is a short-term volume loss is determined in step 410. If no volume loss has been detected and the high-sensitivity detection output has been set at “1”, the reconnect breath counter is decremented in step 412. If a volume loss has been detected in step 410, or after the reconnect breath counter has been decremented in step 412, the number of breaths taken by the patient after a circuit reconnect is counted in step 414. If there has been at least 3 breaths after the circuit reconnect is performed, the high-sensitivity disconnect detection output is cleared as “0” in step 416. After the breath counter is set to 3 breaths in step 418, or the high-sensitivity disconnect detection output is set at “0” in step 416, or when there is less than 3 breaths upon circuit reconnection after the high-sensitivity disconnect detection is set as “1” in step 414, a timer BRTH_TMR for counting the breath interval is updated, and the process will be iterated (referring to FIG. 6).

[0039] FIG. 5 is a flow chart showing the low-sensitivity disconnect detection algorithm. As shown, when any of consecutive-breath pressure loss, long-term volume loss and continuous pressure loss is detected in steps 502, 504 and 506, the output of the low-sensitivity circuit disconnect detection is set as logic “1” in step 508, and the main alarm is activated in step 510. When none of the consecutive-breath pressure loss, long-term volume loss and continuous pressure loss has been detected in steps 502, 504 and 506, the output of the low-sensitivity circuit disconnect detection is set as logic “0” in step 512. In one embodiment, the output of the long-term volume loss detection will not be set at logic “1” unless the volume loss has been lasting for minimum periods of time, while the consecutive-pressure loss detection is set at logic “1” only when the breath during which the loss was detected is longer than a minimum period of time or the loss has lasted for two consecutive breaths. Therefore, a volume counter VOL_TMR for counting the volume loss time and a breath interval counter BRTH_TMR for counting the total breath time are required. In addition, continuous pressure loss is detected whenever such loss is continuous for at least a period of time, thus a continuous pressure loss timer CONTPRS_TMR is required for the continuous pressure loss detection.

[0040] FIG. 6 shows the complete algorithm of circuit-disconnect detection at both low-sensitivity and high-sensitivity. According to the nature of each detection unit, the consecutive-breath pressure loss detection (step 604), the short-term volume loss detection (steps 606) and the long-term volume loss detection (step 608) normally initiate at the beginning of the inspiratory phase of a breath (step 602). In contrast, the single-breath pressure loss detection (step 616) and compliance change detection (steps 618) typically initiate at the end of the inspiratory phase of the breath (step 614). Therefore, whether the breath taken by the patient is at the beginning of the inspiratory phase is determined in step 602, followed by the consecutive-breath pressure loss detection 604 and the detections of short- and long-term volume loss detections in steps 606 and 608. As the output of consecutive-breath pressure loss is determined based on the peak expiratory pressure P_peak, the peak expiratory pressure P_peak is reset after step 604 at step 610. In addition, the consecutive-breath pressure loss detection requires such loss to be detected when the loss occurs in a breath longer than a minimum period of time or when the loss lasts for two consecutive breaths. Therefore, the breath interval timer BRTH_TMR is reset at step 612 after the output of the consecutive-breath pressure loss is obtained in step 604.

[0041] When the breath taken by the patient is not at the beginning of the inspiratory phase, whether the breath is at the end of the inspiratory phase is determined in step 614. If, in step 614, it is determined that the patient is at the end of the inspiratory phase, the single-breath pressure loss and the compliance change detections are performed in steps 616 and 618, and the high-sensitivity disconnection output is computed in step 620. The breath interval timer BRTH_TMR is then updated in step 622. At any stage of the breath, including the beginning or the end of the inspiratory phase or a stage in between, the continuous pressure loss detection is performed in step 624. This is because the continuous pressure loss is determined based on the expiratory pressure P_exp which may vary any time throughout the breath and can be measured any time during the breath, such that the continuous pressure loss is basically independent of breath phase. The peak expiratory pressure P_peak is updated in step 626, and the short-term and accumulated inspiratory and expiratory volumes V_exp—brth, V_insp—brth, V_exp and V_insp are updated in step 628. The volume timer VOL_TMR is updated in step 630, and the low-sensitivity disconnect detection output is computed in step 632. The whole process is re-iterated after step 632.

[0042] FIG. 7 shows illustrate the inspiratory gas flow Q_insp, the expiratory gas flow Q_exp, the expiratory gas pressure P_exp, the peak expiratory gas pressure P_peak, the inspiratory and expiratory gas volume V_insp—brth and V_exp—brth for a single breath, and the accumulated inspiratory and expiratory gas volumes V_insp and V_exp in response to a circuit disconnect. As shown, in the first two expiratory phases and the first inspiratory phases, the inspiratory flow Q_insp, the expiratory flow Q_exp and the expiratory pressure P_exp are similar to those as shown in FIG. 3. When a volume loss or a pressure loss occurs in the second inspiratory phase, the ventilation system automatically elevates inspiratory flow to compensate the loss caused by the circuit disconnect. As shown, the inspiratory flow Q_insp is elevated steeply for compensating the loss caused by the circuit disconnect. The elevation of inspiratory flow Q_insp is not reset until the patient circuit 20 is reconnected. As disconnect occurs, no expiratory flow can be detected at the expiratory line 24. Therefore, the expiratory flow Q_exp remains at zero. The expiratory pressure P_exp also falls to a lower value as shown in FIG. 7. As the high-sensitivity disconnect detection is performed for each breath, the output of the high-sensitivity disconnect is typically produced at the end of each inspiratory phase. Depending on the sub-algorithm that detects the low-sensitivity circuit disconnect, the circuit disconnect may be declared at various stages of the breath, so as to activate the main alarm.

[0043] FIGS. 8, 9 and 11 illustrates the sub-algorithms for detecting short-term volume loss (step 606), the single-breath pressure loss detection (step 616), and the compliance change (step 618), and FIGS. 13, 15 and 16 illustrate the sub-algorithms for detecting the long-term volume loss detection (step 608), the consecutive-breath pressure loss detection (604) and the continuous pressure loss detection (step 624), respectively. Each of the sub-algorithms will be explained in detail as follows with references to the respective drawing.

[0044] As shown in FIG. 8, the short-term volume loss detection initiates at the beginning of the inspiratory phase of each breath in step 800. In step 802, a percentage of volume leakage of the last breath PCT_LEAK_brth can be obtained from equation (1) as:
PCT_LEAK_brth=100×(V_insp—brth−V_exp—brth)/V_insp—brth  (1).
In one embodiment, the inspiratory gas volume V_insp—brth of the last breath can be obtained by integrating the inspiratory gas flow Q_insp, and the the expiratory gas volume V_exp—brth of the last breath can be obtained by integrating the expiratory gas flow Q_exp. In step 804, whether the baseline pressure setting has been changed must be confirmed before detection of volume loss since the change of the baseline pressure setting will cause unbalanced volumes that appear as a volume leakage to ultimately affect the volume loss detection. When a change of the baseline pressure is detected, the short-term volume loss output is set at logic zero in step 808A, and the short-term volume calculation for the inspiratory volume V_insp—brth and the expiratory volume V_exp—brth are reset in step 810. The short-term volume loss detection unit 642 is ready to detect short-term volume loss for the next breath. When the baseline pressure remains unchanged, the percentage of volume leakage is compared to a reference percentage in step 806. The percentage of volume leakage is variable according to various parameters of the ventilation system as well as the physical condition of the patient. In this embodiment, the reference percentage is set at 75%. Therefore, when the percentage of volume leakage is less than or equal to 75%, the process goes to step 808A. The short-term volume loss output is set as 0, and the calculations are reset in step 810. When the percentage of volume leakage is higher than 75%, the short-term volume loss is detected, and the output of the short-term volume loss detection is set at logic 1 in step 808B. After outputting the short-term volume loss detection result, the calculations of the inspiratory and expiratory volumes V_insp—brth and V_exp—brth are reset in step 810, and the short-term volume loss detection unit 642 is ready to detect short-term volume loss for the next breath. Therefore, short-term volume loss detection can also be referred as the single-breath volume loss detection.

[0045] FIG. 9 illustrates the process for detecting single-breath pressure loss performed by the single-breath pressure loss detection unit 643. As discussed above, the single-breath pressure loss detection initiates at the end of the inspiratory phase of a breath in step 900. In steps 902 to 908D, a threshold pressure P_thrs—disc is calculated based on a baseline pressure setting BS_lineset. The threshold pressure P_thrs—disc is used as a reference for determining whether a circuit disconnect occurs due to pressure loss of a single breath. In step 902, the baseline pressure setting BS_setline is compared to a first predetermined reference pressure, which is 20 cmH₂O in this embodiment. When the baseline pressure is larger than or equal to 20 cmH₂O, a threshold pressure P_thrs—disc is calculated as the baseline pressure minus 10 cmH₂O in step 908A. If the baseline pressure setting BS_lineset is smaller than 20 cmH₂O, it is further compared to a second reference pressure in step 904, which is 11 cmH₂O in this embodiment. When the baseline pressure setting BS_lineset is larger than 11 cmH₂O, in step 908B, the threshold pressure P_thrs—disc is set at 10 cmH₂O. If the baseline pressure setting BS_lineset is smaller than 11 cmH₂O, it is further compared to a third reference pressure in step 906. In this embodiment, this third reference pressure is 3 cmH₂O. Therefore, when the baseline pressure setting BS_lineset is equal to or larger than 3 cmH₂O, the threshold pressure P_thrs—disc equals to the baseline pressure setting BS_lineset minus 2 in step 908C. If not, the threshold pressure P_thrs—disc is set at zero in step 908D. When the threshold pressure P_thrs—disc is determined, in step 910, the peak expiratory pressure P_peak is compared to the threshold pressure P_thrs—disc. If the peak expiratory pressure P_peak is not lower than the threshold pressure P_thrs—disc, no pressure loss is detected, that is, the output of single-breath pressure loss detection is set as 0 in step 912A. If the peak expiratory pressure P_peak is lower than the threshold pressure P_thrs—disc, single-breath pressure loss is detected, and the output for such detection is set as 1 in step 912B.

[0046] Referring to FIGS. 3, 7 and 9, the single-breath pressure loss detection typically initiates at the beginning of the inspiratory phase of a breath, while the detection output of the single-breath pressure is normally generated at the beginning of the expiratory phase or the end of the inspiratory phase. For example, as shown in FIG. 7, when a circuit disconnect occurs at the middle of an inspiratory phase, although the expiratory pressure P_exp drops lower than the threshold pressure P_thrd—disc immediately, the highest expiratory peak pressure, that is, the peak peak pressure P_peak remains higher than the threshold pressure throughout the following expiratory phase. Therefore, no pressure loss can be detected during at the beginning of the inspiratory phase of the following breath. However, as the circuit disconnect has occurred in the previous breath, the expiratory pressure during the following inspiratory phase will remain lower than the threshold pressure P_thrd—disc. Therefore, a disconnect can be detected at the beginning of the expiratory phase of the following breath as shown in FIG. 7.

[0047] Table I shows the relationship between the threshold pressure P_thrs—disc and the baseline pressure setting BS_lineset in this embodiment.

[0048]

	TABLE I
		Pressure Loss
	Baseline Pressure Setting (BSlineset)	Threshold (Pthrs—disc)
	BSlineset 20 cm H2O	BSlineset − 10
	20 cm H2O > (less than only) BSlineset >	10
	11 cm H2O
	11 cm H2O > BSlineset 3 cm H2O	BSlineset − 2
	3 cm H2O > BSlineset	0

It will be appreciated that apart from the relationship as shown in Table I, the relationship can be adjusted according to various parameters of the ventilation system and/or the physical conditions of the patient. For example, Tables II and III show other relationships for determining the threshold pressure P_thrs—disc in accordance with the baseline pressure setting BS_lineset.

[0049]

	TABLE II
		Pressure Loss
	Baseline Pressure Setting (BSlineset)	Threshold (Pthrs—disc)
	BSlineset 20 cm H2O	BSlineset − 10
	20 cm H2O > BSlineset 3 cm H2O	BSlineset − 2
	3 cm H2O > BSlineset	0

[0050]

	TABLE III
		Pressure Loss
	Baseline Pressure Setting (BSlineset)	Threshold (Pthrs—disc)
	BSlineset 20 cm H2O	BSlineset − 15
	20 cm H2O > BSlineset 9 cm H2O	BSlineset − 5
	9 cm H2O > BSlineset 4 cm H2O	BSlineset − 3.5
	4 cm H2O > BSlineset	0

[0051] As mentioned above, the inspiratory and expiratory gas flows Q_insp and Q_exp are typically measured at the inspiratory and expiratory lines 22 and 24, while the expiratory pressure P_exp is typically measured at the Wye fitting 26. The detection based on pressure loss and volume loss thus does not provide direct indication of a circuit disconnect occurring at the endotracheal tube. For example, as shown in FIG. 10, when a circuit disconnect occurs, similar to FIG. 7, the inspiratory gas flow is elevated by the ventilation system to compensate the circuit disconnected. However, the reading of the expiratory pressure does not indicate any loss because the disconnect occurs at the endotracheal tube instead of the Wye fitting 26 and the expiratory line 24. Therefore, in the detection algorithm as provided, compliance change caused by circuit disconnect is detected, such that a more direct indication or information regarding disconnect at the endotracheal tube can be obtained.

[0052] FIG. 11 is a flow chart showing the compliance change detection algorithm performed by the compliance change detection unit 641. The compliance C_k is defined as the ratio of the change in volume of inspiratory gas flow ΔV and the pressure change ΔP during an inspiratory phase. In this embodiment, the changing volume of inspiratory gas flow ΔV is equal to the inspiratory gas volume V_insp minus zero, which is equal to the inspiratory gas volume V_insp. It will be appreciated that the changing volume of inspiratory gas flow may vary from the inspiratory gas flow V_insp under specific conditions. The pressure change ΔP is the difference between the expiratory pressure P_exp measured at the end of the inspiratory phase and the positive end expiratory pressure (PEEP). That is, ΔP=P_exp−PEEP and C_k=V_insp/ΔP. In this embodiment, the compliance of the previous breath and the current breath is compared to determine whether there is a compliance change due to patient circuit disconnect. As mentioned above, the compliance change detection initiates at the end of the inspiratory phase of a breath in step 1100. In step 1102, the compliance of the previous breath C_k-1 is captured, and the pressure difference between the expiratory pressure and last PEEP pressure for the inspiratory phase of a current breath is calculated. In step 1104, the compliance of the current breath C_k is calculated. In step 1106, an error P_E of baseline pressure setting is calculated as: P_E=|BS_lineset−BS_line|. In step 1108, the baseline pressure error P_E and the pressure change during inspiratory phase ΔP are compared with predetermined values, such as 1 cmH₂O and 2 cmH₂O, respectively. When the pressure error P_E is smaller than 1 cmH₂O or the pressure change ΔP is less than 2 cmH₂O, the compliance increase detection output is set as 0 in step 1110. When the pressure error P_E is larger than 1 cmH₂O and the pressure change ΔP is no smaller than 2 cmH₂O, the compliances C_k-1 and C_k for two consecutive breaths are compared in step 1112. When C_k-1 is larger than 1 and C_k is larger than 2×C_k-1, a compliance increase due to disconnect is declared in step 1116, and the compliance increase detection output is set as 1 in step 1118. Also, when C_k-1 is less than or equal to 1 and larger than 0, and C_k is larger than 3×C_k-1, the circuit disconnected is declared in step 1116, and the compliance increase detection output is set as 1 in step 1118. If C_k-1 is no larger than 0 or C_k is no larger than 3×C_k-1, the compliance increase detection is cleared, and the output of compliance increase detection is set at “0” in step 1110.

[0053] Referring to FIG. 12, when a circuit disconnect occurs at the endotracheal tube at the end of the expiratory phase, an error of PEEP occurs. The compliance increase is checked at the end of the inspiratory phase for each breath, such the circuit disconnect at the endotracheal tube can be detected.

[0054] The detailed procedure of the long-term volume loss detection is illustrated in FIG. 13. Similar to the short-term volume loss detection, the long-term volume loss detection initiates at the beginning of an inspiratory phase in step 1300. In step 1302, the accumulated inspiratory gas volume V_insp and the expiratory gas volume V_exp obtained by integrating the inspiratory gas flow Q_insp and the expiratory gas flow Q_exp, respectively, are provided to calculate the percentage of volume leakage PCT_LEAK according to equation (1) as previously described. In step 1304, whether the baseline pressure setting has been changed must be confirmed before detection of volume loss since the change of the baseline pressure setting will cause unbalanced volumes that appear as a volume leakage to ultimately affect the volume loss detection. When there is no change of baseline pressure setting, in step 1306, the percentage of volume leakage is compared to a reference percentage, such as 75% in this embodiment. It will be appreciated that such reference percentage may be altered as desired and/or according to the physical conditions of the patient and/or parameters of the ventilation system. When the percentage of volume leakage of the current breath is higher than 75%, whether the computation of the percentage of volume leakage has lasted over a minimum period of time is determined in step 1307. More specifically, the time from the beginning of the inspiratory phase when the long-term volume loss detection initiates is counted by a volume timer or counter VOL_TMR. The volume timer VOL_TMR will not be reset unless the percentage of volume leakage PCT_LEAK drops lower than 75%, since the breathing circuit has been reconnected or due to previous detection of a low-sensitivity circuit disconnect. When the percentage of volume leakage PCT_LEAK over 75% lasts no less than 5 seconds, the long-term volume loss is detected, and the detection output of long-term volume loss is set as “1” in step 1308B. When the leakage percentage V_leak—brth is less than 75% in step 1306, no volume loss is detected, and the volume loss detection output is set at 0 at step 1308A. The calculation of the long-term volume loss, including the volume timer VOL_TMR, the accumulated expiratory and inspiratory volumes V_exp and V_insp are reset in step 1310. The long-term volume detection unit 644 is thus ready for detecting the next long-term volume loss, should it occur.

[0055] Referring to FIG. 7, when a circuit disconnect occurs to a patient having a breath rate more than 12 BPM, in which each breath is shorter than 5 seconds. As shown, when the percentage of volume leakage is over 75%, the accumulated inspiratory and expiratory gas volumes V_insp and V_exp are not reset until a long-term volume detection output has been generated although the single-breath inspiratory and expiratory gas volumes V_insp—brth and V_exp—brth are reset for each breath. The volume timer VOL_TMR is also continuously updated before the long-term volume loss detection is generated, or the circuit has been reconnected. When the percentage of volume leakage PCT_LEAK over 75% lasts for at least 5 seconds, the long-term volume loss is detected, and the output is set at “1” at the end of the immediately following expiratory phase. Therefore, when each breath taken by the patient is shorter than the minimum period of time defined in the long-term volume loss detection algorithm, the long-term volume loss is normally detected over multiple breaths.

[0056] FIG. 14 shows a patient having a breath rate less than 12 BPM, in which each breath is longer than 5 seconds. Similarly, the volume timer VOL_TMR will not be reset unless a long-term volume loss detection output has been generated or a circuit reconnection has been performed. When the percentage of volume leakage PCT_LEAK over 75% lasts for 5 seconds, with the slow breath rate, the long-term volume loss is detected within a single breath, and the detection output is set at “1” at the end of the current breath. That is, at the end of the expiratory phase of the same breath, or the beginning of the inspiratory phase of the immediately following breath.

[0057] FIG. 15 is a flow chart showing the process of the consecutive breath pressure detection algorithm. Similar to the single-breath pressure loss detection algorithm, a pressure loss threshold is set or determined according to a baseline pressure, which, in many ventilation systems, is referred as the positive end expiratory pressure (PEEP). When a peak pressure is lower than the threshold is detected for a breath which lasts over a minimum duration or for two consecutive breaths, the consecutive-breath pressure loss is detected. In step 1500, the consecutive-breath pressure loss algorithm initiates at the beginning of each inspiratory phase. In steps 1502 to 1510, a threshold value for determining the pressure loss caused by circuit disconnect is determined based on the relationship as shown in Table II. Similarly, the threshold pressure can also set up by other relationships such as those shown in Table I and III. In step 1502, if the baseline pressure BS_lineset is initially determined to be larger than or equal to a first predetermined value. In this embodiment, the first predetermined value is preferably selected at 20 cmH₂O. When the baseline pressure BS_lineset is larger than or equal to 20 cmH₂O, the pressure loss threshold is computed as P_thrs—disc=BS_lineset−10 in step 1504. When the baseline pressure BS_lineset is less than 20 cmH₂O, whether the baseline pressure BS_lineset is larger than or equal to a second predetermined value, such as 3 cmH₂O is determined in step 1506. If the baseline pressure BS_lineset is larger than or equal to 3 cmH₂O, the threshold value is calculated by P_thrs—disc=BS_lineset−2 in step 1508. When the baseline pressure is set less than 3 cmH₂O, the threshold value P_thrs—disc is determined equal to zero in step 1510.

[0058] When the threshold value P_thrs—disc for determining circuit disconnect is calculated, the peak expiratory pressure P_peak is compared to the threshold value P_thrs—disc in step 1512. When the peak expiratory pressure P_peak is no smaller than the threshold value P_thrs—disc, whether a pressure loss has been detected has to be checked in step 1514. When the disconnect has already been declared and the patient circuit is reconnected, the long-term volume loss calculations, are reset in step 1516 to avoid volume loss disconnect detection upon reconnection. Otherwise, the consecutive-breath pressure loss detection output is set at 0 in step 1518. When the peak pressure P_peak falls under the threshold value P_thrs—disc in step 1512 in step 1520, a counter is used to increment the consecutive breath cycles during which the pressure loss detected in step 1512 continues. When the pressure loss detected in step 1512 lasts for more two consecutive breaths, that is, when the breath counter reading is 2, or when the breath interval timer BRTH_TMR is over a minimum period of time, which is 5 seconds in this embodiment, the detection output is set at “1” in step 1524 at the end of the expiratory phase when either one of such conditions is satisfied in step 1522. As shown in FIG. 6, the peak expiratory pressure P_peak and the breath interval timer BRTH_TMR are reset in steps 610 and 612 occurring at the beginning of every inspiratory phase.

[0059] FIG. 16 shows the process flow of the continuous pressure loss detection algorithm. The continuous pressure loss detection is particularly applicable when the patient has a breath rate so low, that the circuit disconnect based on two consecutive breaths may endanger the patient. For example, the response of the continuous pressure loss detection algorithm applied to a patient having a breath rate as low as 5 Breaths Per Minute (BPM) is illustrated in FIG. 14. The continuous pressure loss detection algorithm is based on the expiratory pressure P_exp and the threshold value of pressure loss P_thrs—disc as defined previously. In step 1602, the expiratory pressure P_exp is compared to the threshold pressure P_thrs—disc. When the expiratory pressure P_exp is higher than the threshold pressure P_thrs—disc, a timer for counting the time duration of the continuous pressure loss is zeroed in step 1604, and the continuous pressure loss detection output is set as 0 in step 1606. When the expiratory pressure P_exp is again lower than the threshold pressure P_thrs—disc, any change in the baseline pressure setting BS_lineset is determined. If the baseline pressure setting BS_lineset has been changed, the timer is reset to zero in step 1604, and the output of continuous pressure loss detection is set at zero in step 1606. When the baseline pressure setting BS_lineset has not been changed, the duration of the pressure loss observed in step 1602 is counted by the timer in step 1610. When the duration exceeds or reaches 8 seconds in step 1612, that is, when the pressure loss continues for at least 8 seconds, the continuous pressure loss detection output is set at 1 in step 1614. Otherwise, the continuous pressure loss detection output is set at 0 in step 1606.

[0060] The threshold value of the continuous pressure loss can also be varied from that defined in the consecutive-breath pressure loss detection algorithm. For example, as shown in Table III, the threshold pressure P_thrs—disc can also be set as 15, BS_lineset−5, BS_lineset−3.5 and 0 when the baseline pressure is no less than 20 cmH₂O, less than 20 cmH₂O and no less than 9 cmH₂O, less than 9 cmH₂O and no less than 4 cmH₂O, and less than 4 cmH₂O, respectively. In addition, as the expiratory pressure varies at any time or any stage when a disconnect occurs, the continuous pressure loss detection does not depend on the phase of the breath. In other words, the detection output may be generated any time regardless the phase of the breath. For example, as shown in FIG. 14, the continuous pressure loss detected for 8 seconds generates an alarm output during an expiratory phase.

[0061] The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention. Further, the various features of this invention can be used along, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the invention is not to be limited by the illustrated embodiments but is to be defined by the following claims when read in the broadest reasonable manner to preserve the validity of the claims.