Abstract:
Apparatus for measuring expired gas concentration in infants whose breathing rate is too high for normal analyzers by sampling the breath, for instance from the tube between the respirator and the infant, and drawing that sample of gas into an elongated, small diameter tube which serves to store the sample in a linear array substantially without intermixing. Several breaths are thus stored after which time the sampling may be interrupted and the stored gas drawn slowly through a gas analyzer.

Description:
BACKGROUND OF THE INVENTION 
     In the treatment of infants with respiratory disease it can be of great importance to measure the concentration of carbon dioxide and oxygen in the expelled air of the infant. It is even more useful if these gases, particularly the carbon dioxide, can be measured at the end of expiration so as to obtain the highest carbon dioxide concentration of the expiration, usually called end-tidal peak carbon dioxide (ECO 2 ). Likewise, in the study of infants liable to &#34;Sudden Infant Death Syndrome&#34;, the measurement of end-tidal peak carbon dioxide can be of critical importance because such infants may hypoventilate with a rising ECO 2  before going into apnea and dying. 
     In adults, who breathe at a rate of about 20 breaths per minute, it is relatively easy to take measurements of ECO 2 . But most current rapid analyzers are limited in their response time to indicating only about 90% of the actual concentration in two tenths of a second. This means that in infants, who breath at a high rate, often over sixty or eighty, the response of a rapid gas analyzer is too slow to pick the peak of the end-tidal carbon dioxide because the next inspiration intervenes before the instrument has a chance to come to equilibrium for a valid measurement. Reliable measurements of end-tidal carbon dioxide in infants have heretofore only been possible using specially adapted mass-spectrometers which are very expensive and even then the end-tidal peak is often blurred by the rapid respiration rate. 
     Another problem in such measurement with respect to infants is that in order to measure, it is necessary to draw a continuous sample from the airway, typically at the rate of about 0.5 liters per minute and pass that volume through the gas analyzer. But in an infant who is only breathing one or two liters per minute and who is on the partially closed circuit of a ventilator, the half liter is a dangerously large amount to suck out of the airway. 
     SUMMARY OF THE INVENTION AND OBJECTS 
     The invention is incorporated in apparatus including means for sampling the gas from an infant&#39;s airway and simultaneously returning gas to that airway in the same volume so that there is no net loss of gas from the airway. Means are further provided to discontinue sampling of gas from the airway and thereafter direct the previously drawn sample to a gas analyzer at a relatively slow rate such that the analyzer may come to equilibrium and give valid measurements of end-tidal peak carbon dioxide. 
     It is, therefore, a general object of the present invention to provide an improved apparatus for measurement of expired gas concentration in infants. 
     It is a further object of the present invention to provide such an improved apparatus for measuring expired gas concentration in infants wherein accurate measurements can be taken with existing equipment and without subjecting the infant to the dangers of drawing large volumes of gases from the airway. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE is a schematic diagram of apparatus for measurement of expired gas concentration in infants in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the FIGURE there is shown an infant 11 utilizing an airway 13 coupled to a respirator 15. The airway 13 may employ a face mask 17, as shown, or may include an intratracheal tube or the like. The airway 13 includes a baffle 19 extending longitudinally in the area of taps 21 and 23. The baffle 19 serves to isolate the tap 21 from the tap 23 whereby gases going to or from either will not interfere with the gas from the opposite tap. 
     The tap 21 is connected to a loop of fine tubing 25 through lines 27 and 28 and valve 29. Valve 29 includes an exhaust port 31 and when rotated counterclockwise from the position as shown will serve to connect loop 25 to the atmosphere through the port 31. 
     The opposite end of the loop 25 is connected by lines 33, 35 and 37 and valves 39 and 41 to a reversible pump 43. The opposite side of the pump 43 is connected to the tap 23 in the airway 13. As shown this route is defined by the lines 45, 47 and 49 together with the valves 51 and 53. 
     Alternatively, with the valves 51 and 53 rotated 90° clockwise and counterclockwise respectively, from the position shown, the pump 43 is connected to the outer chamber 55 of a captive bag assembly 57. The captive bag 59 of that assembly communicates with the tap 23 through the lines 61, 63 and 49 and the valves 65 and 53. The valve 65 includes an exhaust port 67 which communicates with the interior of the captive bag 59 when the valve 65 is rotated 90° clockwise from the position shown. 
     The valve 39 includes a port 69 which communicates with the loop 25 when the valve 39 is rotated 90° clockwise from the position shown. Tap 69 further communicates with a rapid gas analyzer 71 through the line 73. Gas from the loop 25 may be drawn through the rapid gas analyzer by means of a pump 75 connected to the analyzer 71 by the line 77. The discharge of the pump 75 may be vented to atmosphere through the line 79. 
     In the operation of the apparatus with the valves 51 and 53 set as shown in the drawing, the unit operates without using the captive air bag assembly 57. With the valves 29 and 39 set as shown in the drawing the apparatus is set for the first of two phases of operation. In this first phase, which lasts for several seconds or longer, the pump 43 operates to circulate gas from the airway 13 through the valve 31, the loop 25, the valve 39, the pump 43 itself, the valves 51 and 53 to the tap 23 of the airway 13. Phase one operates for a sufficiently long period of time to permit the infant to breath several breaths. The tube loop 25 is chosen to be long enough so that the transit time of a gas particle through it is longer than the time required for several breaths at the normal infant respiratory rate. Conveniently, this tube may be about 10 meters long with an internal diameter of 1 millimeter or less with the pump 43 operating at a rate of approximately one-half liter per minute. 
     Even though the pump draws a half liter of gas per minute from the airway 13 at the tap 21, the same amount of gas is being simultaneously returned to the airway through the tap 23. Consequently, there is no net loss of gas volume in the airway itself. 
     During the second phase of operation the valve 29 is rotated 90° counterclockwise from the position shown and valve 39 rotated 90° clockwise from the position shown. The pump 75 is then operated to draw the gas from the loop 25 slowly through the rapid gas analyzer 71. During this phase of operation measurements of gas concentrations are actually made and no gas is drawn from the airway 13. After sufficient time for the gas measurements to be made the system returns to phase one and the operation is repeated. 
     The apparatus then does not measure every breath of the infant but it does measure several breaths in sequence. If it were felt necessary to measure every breath, multiple apparatus of the type shown could be employed and phased in such a manner that when one is in the first phase of operation the other is in the second phase. 
     It may be considered advantageous to prevent mixing of the gases at inlet and outlet lines of the loop 25 and if this mode of operation is required the captive gas bag assembly 57 may be employed by rotating the valves 51 and 53 clockwise and counterclockwise respectively for 90°. The line 47 is then incapacitated and during phase one of the operation gas is drawn through the loop 25 by the pump 43 and, rather than being returned to the airway tap 23, is directed to the outer chamber 55 of the assembly 57. During this time gases within the bag 59 are thus forced through the valves 65 and 53 to the tap 23. 
     During the second phase of operation, gases from the loop 25 are drawn through the gas analyzer in the same fashion as during phase two of the first described mode of operation. However, when the captive bag assembly 57 is employed pump 43 is also utilized during the second phase to refill the bag 59. Thus during phase two of the operation utilizing the assembly 57, valves 41 and 65 are rotated 90° counterclockwise and clockwise respectively and atmospheric air, or if desired some other gas, is drawn into the captive bag 59. When phase one recommences, the valves 41 and 65 are returned to their original position and the pump 43 is again operated in its forward direction. 
     Thus it is seen that the invention comprises apparatus whereby gas can be sampled from the airway and simultaneously returned to that airway in the same volume so that there is no net loss of gas volume from the airway. A conventional rapid gas analyzer can then be given adequate time to equilibriate and make a valid measurement of ECO 2  in spite of the rapid respiration of the infant. 
     An important aspect of the invention is the utilization of a very small internal diameter tube for the loop 25 such that when gas is drawn through the loop at a reasonable rate the flow is laminar with very little mixing of the gas. With this construction, if a gas of an undulating concentration is drawn into the loop 25 it remains in that undulating concentration throughout the length of the loop 25. The concentration of gas flowing from the opposite end of the loop will undulate in approximately the same wave form as it is admitted to the loop with very little slurring or mixing. It may be said then that the gases from the airway 13 are stored in analog form throughout the length of the loop 25.