Influence of decreased vascular pressures on mechanics of ventilation in dogs

¹ Intensive Treatment Center and Department of Medical Research, Division of Laboratories, St. Joseph Hospital, Burbank, California

In closed-chest and open-chest dogs the peak insufflation tracheal pressure during constant-volume, positive pressure ventilation increased whenever the systemic arterial pressure was lowered by bleeding. The tracheal pressure was also recorded in open-chest dog experiments, where the pulmonary and the systemic circulations were perfused by separate perfusion systems, the output of which was known. When the blood flow in one circuit was kept constant while the flow in the other was decreased, increases of the tracheal pressure were observed. The effect of systemic hypotension upon tracheal pressure disappeared after cooling or sectioning of the vagi. On the other hand, the effect of decreased pulmonary flow upon tracheal pressure persisted after denervation of the lungs and was observable even in dead dogs. We conclude that a) decrease of the systemic arterial pressure (bleeding) causes reflex increase of the peak insufflation tracheal pressure which is mediated by the vagus nerve. b) decrease of the pulmonary blood pressures and of the pulmonary blood volume below normal cause increased peak insufflation pressures, which are believed to represent decreased pulmonary compliance.