The Response of Patients With Respiratory Failure and Cardiopulmonary Disease to Different Levels of Constant Volume Ventilation

Abstract

The purpose of this study was to determine the effect of increasing the tidal volume on pulmonary ventilation-perfusion relations in patients with respiratory failure caused by cardiopulmonary disease.The frequent occurrence of an increased physiologic dead space (2, 3) and of increased physiologic shunting (3) in patients undergoing prolonged artificial ventilation has previously been described. These changes in ventilation-perfusion ratios mean that very large tidal volumes and high percentages of inspired oxygen are often required to maintain life during prolonged intermittent positive pressure ventilation. The aim of our study was to measure the consequences of changing the tidal volume, but not the respiratory frequency, of patients with respiratory failure secondary to either acute intrapulmonary infection or chronic pulmonary emphysema. MethodsSelection of patients. At the time of the study none of the twelve patients had a total vital capacity above 400 ml, and hence all were completely dependent on intermittent positive pressure ventilation. Eight of the patients studied were selected because they had been free of symptomatic pulmonary disease on admission to the hospital, but had later developed nonemphysematous cardiopulmonary disease and acute intrapulmonary infection that caused respiratory failure. Polyethylene catheters were placed percutaneously in either a radial or femoral artery and in the superior vena cava. These catheters were used for blood sampling and continuous pressure recording via a Sanborn 350 system. Continuous intratracheal pressure and the electrocardio- 1 The apparatus dead space, average volume 28 ml, extended from the tip of the tracheostomy tube lying in the trachea to the junction of the inspiratory and expiratory tubes of the ventilator, a distance of 25 cm; the internal diameter of the tubing that comprised the apparatus dead space was 1.2 cm. 1543

Dead Space, Compliance and Venous Admixture During Heart Surgery

“…The ratios for ventilation to perfusion, VD/VT, and the amount of venous admixture in five patients with mitral disease are plotted in figures 3 and 4 (Nos. 7,8,13,19,27).…”

“…The predicted VDpat values are given in table 19. The average predicted volume calculated as body surface x0.05 corresponded to 94 ml BTPS.…”

Oxygen therapy

“…The proportion of blood passing nonfunctional alveoli (physiologic shunt) may be affected more by the state of the circulation 5 than the degree to which the lung is expanded. 116 The percentage physiologic shunt is increased by a higher total flow of blood through the lungs ( Fig. 10).…”

“…With volume-constant machines the oxygen inflow can generally be regulated by a flow meter and the inspired oxygen concentration quantitatively diluted by air entrainment. 116 To calculate the inspired oxygen tension in such instances we use the formula shown in Fig. 6.…”

“…116 It is thus possible that passive deep breaths do not reduce the physiologic shunt by re-expansion of alveoli so much as by lowering cardiac output and thereby redistributing pulmonary blood flow away from shunt channels. 116 In gross pulmonary infection both physiologic shunt and deadspace are increased ( Fig. 5 86 and the measured physiologic shunt is less.…”