The coupling patterns between the rhythm of a mechanical ventilator and the rhythm of spontaneous breathing were studied in enflurane-anesthetized adult human subjects. The spontaneous breathing pattern was altered in response to different frequencies and amplitudes of forced lung inflations. A 1:1 phase locking (the frequency of the mechanical ventilator is matched by the frequency of spontaneous breathing with a fixed phase between the 2 rhythms) was observed in a range of up to +/- 40% of some of the subject's spontaneous breathing frequencies. During 1:1 phase locking, there were marked changes in the expiratory duration as measured from the electromyogram of the diaphragm. The phase relationship between onset of inflation and onset of inspiration depended on the frequency and amplitude of mechanical inflation. At ventilator settings that did not give 1:1 phase locking, other simple phase-locked patterns, such as 1:2 and 2:1, or irregular non-phase-locked patterns were observed. Reflexes arising from lung inflation, which may underlie the entrainment, are discussed in the context of these results.
The cardiovascular responses, ventilation and pulmonary gas exchanges of 8 treated and 6 untreated hypertensive patients were studied during artificial ventilation under nitrous oxide/oxygen/relaxant anaesthesia, with and without halothane (1%). Mean arterial pressure (m.a.p.) fell to 69% and 62% of the awake values in the treated and untreated patients respectively during hypocapnia (mean Pa CO2 23 mm Hg) induced by IPPV under nitrous oxide anaesthesia. When halothane (1%) was added under the same conditions of IPPV, m.a.p. fell to 60% and 53% of the awake values respectively, as a result of reduced cardiac output (50% and 53% respectively of awake values), whereas systemic vascular resistance was raised above the awake values in all patients. Electrocardiographic evidence of myocardial ischaemia was observed during these periods of arterial hypotension in 50% of the treated patients and in all the untreated patients. Increased alveolar-arterial Po, differences during IPPV were due to desaturation of mixed venous blood associated with raised arteriovenous oxygen content differences. Pulmonary venous admixture did not change significantly during the course of anaesthesia with IPPV, and recovery. VD/VT increased during spontaneous ventilation following induction of anaesthesia, but decreased significantly below awake values during IPPV, returning to control values in the postoperative period.
SERIOUS PROBLEMS arise frequently during anaesthesia for major vascular surgery associated with clamping of the aorta. Haemodynamic changes at the moment of recirculation are readily observed and have been the subject of numerous publications. 1-4 The hypotension, at times severe, that follows the unclamping of the aorta is not without danger for these arteriosclerotic and hypertensive patients. It is obvious that hypotension, even of short duration, can reduce cerebral, coronary or renal perfusion and jeopardize an otherwise successful operation.While the period of unclamping has been thoroughly examined, the events associated with clamping have not received much attention except for Perry's animal experiments, which showed decrease in cardiac output# The proximal occlusion of the aorta during operation for thoracic aneurysm can induce alarming increases in blood pressure and experienced anaesthetists will look for signs of left ventricular failure. The use of nitroprusside is often mandatory to relieve the acute afterload while the surgeon prepares a temporary bypass. It is not uncommon to see pulsus alternans, indicating ventricular failure probably due to discrepancy in regional perfusion of an already diseased myocardium.The clamping of the distal aorta also induces haemodynamic changes, the most obvious response again being the increase in systolic pressure due to an afterload which could be of clinical significance for these patients. With that in mind we decided to investigate the haemodynamic changes due to cross-clamping of the abdominal aorta. We also attempted to evaluate renal function during and following operation and to correlate changes, if any, to haemodynamic alterations. The latter will be the subject of a future publication.
MATEBIAL AND METHODOLOGYEighteen patients requiring surgery of the abdominal aorta were included in this study. None presented clinical evidence of cardiac insufficiency or dysrythmia before operation. They were distributed at random in two groups involving nine Leriches and nine aneurysms of the infrarenal aorta. Anthropometrie data and type of surgery are summarized in Table I.After premedication with diazepam 5 mg intramuscularly, induction of anaesthesia was carried out with thiopentone 5 mg/kg, and pancuronium 0.15 mg/kg for tracheal intubation and muscular relaxation.
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