BTPS calibration of heated Fleisch pneumotachometer.

The Fleisch pneumotachometer (PTM) gives the instantaneous respiratory gas flow, and when integrated provides the volume of gas displaced. Its output signal is proportional to the product of the flow and the viscosity of the gas. The calibration factor is thus different for inspiration and expiration. Since the exact value of the viscosity is unknown, accurate figures for flow cannot be obtained. This study examines the use of a precisely sinusoidal pump with adjustable speed and capacity, displacing air in a thermostatically controlled chamber containing water at 37 degrees C stirred by a propellor. Thus air at ambient temperature flows through the PTM on inspiration, and on expiration the gas exactly simulates in temperature and humidity that normally expired by a human subject. This makes the output signal asymmetrical, with the expired volume VTE being greater than the inspired value, the ratio VTE/VTI = 1.035. Other sources of error, notably temperature and pressure changes in the chamber and differences in the proportions of O2 and CO2 in the expired gas, have been considered from both a theoretical and experimental standpoint. Their combined effects produce a less than 0.5% error. Using this pump, the Fleisch PTM can be calibrated empirically without making any assumptions about the temperature and viscosity of the expired gas mixture.

Alveostat, an alveolar PACO2 and PAO2 control system.

In the study of the physiological regulation of respiration through a control system model it is necessary to test the ventilatory response to various forcing functions of either the parrtial pressure of alveolar carbon dioxide (PACO2) or oxygen (PAO2). Since PACO2 and PAO2 are both functions of alveolar ventilation and metabolic rate, such a result cannot be obtained by merely changing the composition of the inspired gases without a feedback control. Thus a servomechanism is necessary. The input to the servomechanism is an instantaneous determination of PACO2 and PAO2. This is accomplished by using the criterion of equality of the exchange ratio in mean alveolar gas and mean expired gas. The servomechanism described has three essential characteristics: rapidity, accuracy, and stability. In experiments of step, ramp, and sinusoidal forcing functions, variations of PACO2 have been obtained without change in PAO2, and step variations of PAO2 have been obtained without change in PACO2.

Kinematics of spontaneous breathing: the ventilatory system as a non-linear oscillator.

The kinematics of spontaneous breathing at rest and during moderate exercise is described exactly by a non-linear differential equation, the parameters of which are determined by observation with a pneumotachograph. Analogue circuits are used for the determination of the coefficients and for the comparison by superimposition of the actual spirogram with its simulation. The ventilatory system, taken as a whole and without any assumption concerning its structure, works as a non-linear oscillator. If the classical distinction between a passive and an active ventilatory system is accepted, the concept of a linear equivalent of a non-linear oscillator is valid for the description of the properties of the passive system. It affords some of the advantages of linear mechanics and indicates the restrictions put upon the use of a linear hypothesis. The role of the different terms in determining the pattern of breathing is displayed and the correlation of scale factors with body size is shown. The physiological meaning of the components of muscle action is discussed.