Liquid Separator

This invention relates to liquid separators and more particularly to separators for removing liquid from breath samples and breath analyzer systems incorporating the liquid separators.

Gas analyzers are used to measure gases exhaled by a patient. In particular, carbon dioxide is monitored to determine the physiology of a patient. This is of prime importance during and immediately after surgery, and during respiration therapy. A major problem area in the analysis is the removal of entrained liquid either from the patient or the condensation from the breadth or humidified oxygen supplied by a ventilator. This problem is further complicated by the need for rapid pneumatic response time, so that the visual waveform of the patient's exhaled CO.sub.2 displayed electronically on a breath-by-breath basis will be faithfully reproduced. Any large and/or dead volumes anywhere in the sample train can cause mixing of the gas and tend to integrate rapid peaks and ultimately degrade the waveform.

Filters have heretofore been used in breath sampling lines, such as hydrophobic membranes (U.S. Pat. No. 4,558,708), sintered metal (U.S. Pat. No. 4,558,709) and water absorbents (U.S. Pat. No. 4,446,869). Such filters, however, become saturated and must be changed over extended sampling periods. Mechanical liquid traps in which liquid is collected in a reservoir have greater liquid capacity, see, for example, U.S. Pat. No. 4,579,568, but the increased volume of the system increases response time and convection or diffusion of the gas from the liquid reservoir dilutes the breath sample.

It is an object of this invention to provide a liquid separator that effectively separates liquid from a breath sample to be analyzed without significantly degrading pneumatic response time. Another object of the invention is to provide a breath sampling system that permits accurate waveform analysis over a breathing cycle. A further object is to provide a breath sampling system that has a minimum volume when sampling and that provides an isolated reservoir for collection of entrained liquid.

The liquid separator comprises a liquid trap having a substantially horizontal conduit with an inlet and an outlet, a reservoir chamber, a bottom outlet from the conduit opening to the reservoir chamber, a one-way valve means closing said bottom outlet and permitting flow only from the conduit to the reservoir chamber; means to flow the breath sample through the conduit; and means to divert the sample flow to the reservoir chamber and flow air back through the outlet and the bottom outlet to the reservoir chamber whereby liquid is backflushed from the conduit to the reservoir chamber.

The means to divert the sample flow and back-flow air back through the outlet may be responsive to the presence of liquid in the sample line, or periodically activated by a timer, or both. When using an infrared CO.sub.2 analyzer, the reverse air flow, freed of CO.sub.2, is used as a zero calibration gas.

FIG. 1 is a Flow Diagram of a breath analyzer system in accordance with this invention.

FIG. 2 is a plan view of the liquid trap of FIG. 1.

FIG. 3 is a sectional view on line III--III of FIG. 2.

FIG. 4 is a sectional view of the liquid trap of FIG. 1.

Referring to FIG. 1, small diameter tubing 2 is connected to cannula or airway 4 and subjected to a vacuum drawn by diaphragm pump 6 to withdraw the breath sample from the airway. Tubing 2 is connected to inlet 8 of liquid trap 10, shown in detail in FIG. 2 and FIG. 3.

The trap has a cover member 12 with a horizontal bore 14 therethrough forming inlet 8 to accept a luer-loc fitting and outlet 16. Vertical bore 18 opens the bottom of bore 14 to chamber 20, formed by the cover and cup-shaped housing 21 which are sealed together by friction fit or threads. Bore 18 is closed by a duckbill check valve 22 supported by integral sleeve 24. The check valve may be a Model VA 3143 of silicone rubber available from Vernay Laboratories. Outlet 26 is connected to a normally closed port of solenoid valve 28.

Outlet 16 from the liquid trap is connected through a length 29 of per fluorinated ion exchange polymer tubing, such as Nafion.RTM. tubing, to a liquid sensor 32. As is well known, water, but not gas, will be absorbed and pass through the wall of the Nafion tubing to the drier ambient air removing some moisture from the breath sample. A preferred liquid sensor of minimal volume, shown in FIG. 4, consists of two metal tubes 34, 34a, separated by a gap 36 and held in place by insulating tubing 38. Tubes 34 and 34a are connected by leads 40 and 42 to a control circuit 44 that is responsive to a change in resistance resulting from water droplets bridging gap 36 and operates solenoid valves 28 and 46.

The liquid sensor outlet is connected by tubing to the inlet of an infrared analyzer cell or other analytical device for measuring carbon dioxide or other gas component of the breath sample. The cell outlet is connected to solenoid 3-way valve 46 through a common port to a normally open port and a normally closed port openable to the ambient air through CO.sub.2 scrubber 48. The normally open port of valve 46 is connected through an optional second analytical device 50, a pulse damper 52, a flow sensor 54, and solenoid valve 28, to pump 6.

The second analytical device may be a thermal conductivity cell for measuring N.sub.2 O when the patient is being given N.sub.2 O anesthesia or other analytical device which is not subjected to back flushing. The pulse damper may be a small chamber with a flexible wall. The flow sensor may be a conventional hot wire miniaturized mass flowmeter connected to a null balance bridge flow control circuit 56, that regulates the speed of pump 6 to provide constant flow. Water droplets in the patient sample line will increase the resistance to flow, but this resistance is overcome to provide a constant flow through the analytical device by increasing the pump speed.

In the normal sample mode, the breath sample is drawn through the trap, Nafion tubing, liquid sensor, analytical devices and the normally open solenoid valves. The system provides a minimum volume between the sampling point and the infrared analyzer cell, so response time is fast to produce an accurate wave-form analysis of CO.sub.2 over the entire breathing cycle. The effective volume of the liquid trap is only the volume of the bores 14 and 18 and check valve 12, inasmuch as the chamber 20 is isolated from the breath sample by valve 12. This effective volume can be as little as about 150 microliters, for example, in a typical trap one inch in diameter with 1/16-inch bores. The dead volume not directly in the flow path, i.e. the volume of bore 18 and the duckbill valve 22, is only about 20 microliters.

Liquid that passes the trap and perfluorinated tube 30 will form droplets in the liquid sensor, reducing the resistance across tubes 34 and 34a and activating the control circuit to open the normally closed ports and close the normally open ports of the solenoid valves. In this liquid removal mode, ambient air is drawn by pump 6 in a reverse direction through solenoid valve 46, the infrared analyzer cell, the liquid sensor, and the trap outlet, and through valve 22 to the reservoir chamber 20. Thus liquid is prevented from entering and damaging the analyzer cell or causing inaccurate readings. The vacuum created in the reservoir by this action also serves to maintain flow from the patient sampling line through the duckbill valve 12, thus ensuring a fast, updated breath sample on return to the normal mode. This operation also does not allow back flushed air to reach the patient airway. In the liquid removal mode the flow sensor sees no flow, resulting in the pump operating at full capacity.

Timing circuit 58 periodically activates the solenoid valves to operate the system in the liquid removal mode in synchronization with a zero calibration cycle of the infrared analyzer. Any CO.sub.2 in the ambient air is removed by scrubber 48, providing a CO.sub.2 -free calibration gas for the infrared analyzer. A calibration procedure by which both zero and span are calibrated with CO.sub.2 -free air is described in the copending application, Ser. No. 07/047,650, filed May 8, 1987 of common ownership with the application.

A vacuum regulator 60 downstream of the pump allows the exhausted sample to be vented to a hospital scavenger system. The regulator renders the CO.sub.2 sampling system immune to variations of vacuum in the scavenger system.