Gas mixing in the lungs of dogs and pigs

Design and validation of an analyser to measure sulphur hexafluoride gas during respiration

The study presents the results of the development of an analyser to measure sulphur hexafluoride (SF6) gas in breathing circuits, for application is studies of lung function. The analyser consists of an in-line breathing circuit measurement transducer and a compact unit for signal treatment. The detector unit of the analyser consists of a near-infrared light source, a bandpass filter and a pyro-electrical detector. When incremental steps of SF6 gas between 0 and 2% were presented to the analyser, the maximum deviation from the theoretical calibration curve was calculated to be 0.01% SF6. The step response of the analyser (10-90%) was 250 ms. The sensitivity of the analyser to ambient temperature was 0.01% SF6 degrees C(-1) in the range between 0 and 2% SF6. It is concluded that the analyser presented is accurate, and has a sufficient response speed to be used in clinical measurement settings. Furthermore, the analyser is resistant to changes in temperature, gas flow, orientation and movement, which are likely to occur in clinical measurement settings.

Unevenness of ventilation assessed by the expired CO(2) gas volume versus V(T) curve in asthmatic patients

Recently, we have shown that the expired CO2 gas volume versus tidal volume (VCO2-VT) curve is a useful tool for assessing unevenness of ventilation because it allows the separation of tidal volume into three functional compartments: (a) the CO2-free expired air (V0), (b) the transitional volume (Vtr), (c) the alveolar volume (VA) and the measurement of alveolar FCO2 during resting breathing in normal subjects and patients with COPD. In this paper, we have investigated whether changes pertaining to unevenness of ventilation taking place immediately after the administration of methacholine can be assessed using the VCO2-VT curve in asthmatic patients. The VCO2-VT curve was obtained during tidal breathing from 16 stable asthmatic patients who underwent a methacholine challenge test. It has been found that the Vtr, and hence Bohr's dead space (VD,Bohr = V0 + Vtr), over tidal volume ratios were significantly increased immediately after the methacholine administration, whilst the V0 over tidal volume ratio was not affected. The change of the above ratios was not related to the percentage decrease of FEV1.0 following methacholine administration.

A fast, digitally controlled flow proportional gas injection system for studies in lung function

The aim of this paper is to describe a device for flow proportional injection of tracer gas in the lungs of mechanically ventilated patients. This device may then be used for the study of the multiple breath indicator gas washout technique to determine the end-expiratory lung volume. Such a tracer gas injection device may also be used in the study of other techniques that rely on uptake and elimination of tracer gas by the lungs. In this paper, an injector is described which enables injection of indicator gas at a predetermined concentration in a breathing circuit independent of the type of breathing. The presented setup uses a control computer to produce steering signals to a multivalve array in proportion to the input breathing signals. The multivalve array consists of ten circular valves, each with a different diameter, which can be opened or closed individually according to the input signal of the array. By opening of a certain combination of valves an amount of sulphur hexafluoride gas proportional to the inspiratory breathing signal is released. The rate of transmission between the components of the injection system was 80 Hz. The injector has a full flow range between 0-10 L/min. The delay time between the breathing signal and the flow response was 70 ms. The aimed washin gas concentration of 1% SF6 was achieved after 0.5 s. The study describes the results of tests to determine valve-flow ratios, step response and dynamic response of the injector. The flow output response of the injector system was shown to increase in input frequencies above 3 Hz. The valve flow ratios showed the largest relative deviation in the two smallest valves of the 10 valve array, respectively 0.005 L/min (25%) and 0.002 L/min (20%). We conclude that the injector can achieve a stable concentration of indicator gas in a breathing system with an accuracy of 0.005 L/min to execute the multiple breath indicator washout test in human subjects. The results of the study indicate that the injector may be of use in other application fields in respiratory physiology in which breathing circuit injection of indicator gas is required.

Ventilation heterogeneity is increased in hypocapnic dogs but not pigs

Hypocapnia increases ventilation/perfusion (VA/Q) heterogeneity in dogs, possibly by adversely affecting distribution of ventilation through its effects on collateral ventilation. Because pigs lack collateral ventilation, we compared the effects of hypocapnia on ventilation heterogeneity in pentobarbital-anesthetized, mechanically-ventilated dogs and pigs. Simultaneous multiple breath washouts of helium and nitrogen were used to assess the uniformity of the ventilation distribution by the phase III (SnIII) method. Ventilation heterogeneity was partitioned into two components, e.g. convective-dependent inhomogeneity (cdi) and diffusive-convective-dependent inhomogeneity (dcdi). Pulmonary gas exchange was also measured in pigs by the multiple inert gas elimination technique. Ventilation heterogeneity was increased (P < 0.01) in hypocapnic dogs. Inspiration of CO2 decreased ventilation heterogeneity by decreasing dcdi (P < 0.01). In contrast, ventilation heterogeneity was not increased in hypocapnic pigs. However, hypocapnia increased VA/Q heterogeneity by 18% (P < 0.05) in pigs. We conclude that hypocapnia increases ventilation heterogeneity in dogs but not in pigs, most likely related to an interspecies difference in collateral ventilation.

Transfer of carbon monoxide during an inspiratory pause procedure in mechanically ventilated pigs

We studied the effect of forced inflation at different alveolar volumes (VA) on carbon monoxide diffusing capacity (DLCO) in anaesthetized, paralysed and mechanically ventilated healthy pigs. An inspiratory pause procedure (equivalent of the single-breath technique) consisting of a pause between an inflation and expiration, both at a constant flow rate, was used. The procedure was computer-controlled and could easily be standardized. In five pigs, VA was varied at constant inflation volume by increasing positive end-expiratory pressure (PEEP) from 2 to 10 cmH2O. Inspiratory pause time was varied from 1 to 8 s to verify whether the decay of CO was exponential. In nine pigs, DLCO was estimated at four different VA values by inflating with 15-30 ml kg-1 at 2 cmH2O PEEP. An exponential decay of CO was always obtained. With increasing VA by either an increase in PEEP or inflation volume, DLCO remained constant. Since the diffusing capacity of the pulmonary membrane is expected to increase with increasing VA, the constant DLCO may be attributed to a decrease in capillary blood volume.

Sloping alveolar plateaus of He and SF6 measured in excised cat lungs ventilated at constant volume by pressure changes

Single-breath washout experiments with He and SF6 were performed in excised cat lungs placed in a closed, liquid-filled reservoir, where lung volume was clamped by the surrounding liquid and breathing was accomplished by hyperbaric pressure changes (pressure breathing) produced by a piston pump. Under these conditions the flow into each lung unit was assumed to be proportional to its volume, and sequential filling and emptying of lung units by convection probably did not occur. Thus, implicitly, gravity-dependent patterns of sequential filling and emptying of lung regions were also excluded. Different lung volumes (VL = 50%, 75%, 100% TLC, where TLC is total lung capacity), tidal volumes (VT = 21%, 34%, 47% TLC) and durations of post-inspiratory apnea (tA = 0,1,2,4,8 sec) were applied. The expirograms showed that the slopes of the alveolar plateau (S) were significantly positive for both He and SF6. For tA = 0 sec SHe ranged from 8.7 to 62.8% and SSF6 ranged from 24.4 to 87.8% (S is expressed in % per unit VE/TLC, where VE is expired volume). The ratio SSF6/SHe was larger than unity for each combination of VL and VT. Further, for tA = 0 sec both SHe and SSF6 showed a tendency to decrease with increasing VL and with increasing VT. For tA = 8 sec both SHe and SSF6 were close to zero. Additional single-breath washout experiments were performed with the same cat lungs by applying normal breathing where lung volume was not clamped and asynchronous unequal ventilation might have occurred. For comparable values of VL and VT, there were no clear differences between the slopes obtained at normal breathing and those obtained at pressure breathing. We conclude that asynchronous unequal ventilation plays only a minor role in the sloping alveolar plateau during normal breathing, and that the mechanism underlying the sloping alveolar plateau is diffusion dependent.