Effects of Changes of Frequency and Tidal Volume of Controlled Ventilation: Measurement at Constant Arterial Pco 2 in Dogs

Abstract

Seven Alsatian dogs were anaesthetized with thiopentone, paralysed with pancuronium, and ventilated with 1% halothane in a mixture of air and oxygen in such a way as to maintain Paco2 at 40 mm Hg and Pao2 at 150 mm Hg. From various respiratory and circulatory measurementts the following variables were determined: physiologicaldeadspace, cardiac output, venous admixture, respiratory compliance and resistance, and oxygen and carbondioxide exchcnage. After a controlseries of measurements at a ventilator frequency of was doubled to 50/min and adjustments to tidal volume and inspired oxygen concentration made to maintain constancy of blood gas tensions. The same procedure was followed again at 25/min, l/min and finally at 25/min. At each frequency the I:E ratio was kept at 1/2, the shape of the inspiratory flow waveform was kept constant (a rapid increase followed by a steady decline to zero) and expiration was passive to atmosphere. Mean results showed that physiological deadspace changed relatively little with frequency so that the deadspace:tidal volume ratio increased significantly on changing to 50/min (by 24%) and decreased significantly on changing to 6/min (by 46%). Changes of cardiac output and venous admixture were either not significant or on the borderline of significance and the 95% confidence limits of these changes were within +14%, --12% for cardiac outpur and within plus or minus1.4% of cardiac output for venous admixture. Tt is concluded that, provided Paco2 and Pa02 and mean air way pressure are kept constant, the frequency of ventilation is not important, even over a wide range of values.

Control of Intermittent Positive Pressure Breathing (Ippb) by Extracorporeal Removal of Carbon Dioxide

“…Despite a decrease in FT we did not observe significant changes in the alveolar-arterial oxygen tension gradient. These findings are in agreement with Lunn, Mapleson and Chilcoat (1975) who found no change in venous admixture in dogs undergoing mechanical ventilation of the lungs at low FT and high/with constant Pa C02 . Moreover, it is known also that intermittent sigh can prevent collapse of lung tissue (Bendixen, Hedley-White and Laver, 1963).…”

Cardiorespiratory Effects of High Frequency Intermittent Positive Pressure Ventilation in the Dog

“…In other studies in humans and in dogs (Williams and Rayford, 1956;Severinghaus and Stupfel, 1957;Cooper, 1969;Hedenstierna and McCarthy, 1975;Lunn, Mapleson and Chilcoat, 1975), the apparatus deadspace was either included as a part of the ventilatory pathway or a nominal value (usually determined by water displacement) was deducted from the calculated physiological deadspace. Since the effective volume of the apparatus deadspace does not necessarily equal the volume determined by water displacement, the assumption of a fixed value may have introduced a variable error.…”