Effects of changes of frequency and tidal volume of controlled ventilation: measurements at constant arterial Pco2 in dogs.

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.