This study examines the effects of nitrous oxide on haemodynamics, anterior left ventricular (LV) function and incidence of myocardial ischaemia in abdominal vascular surgical patients with coronary artery disease. Forty-seven patients were randomly assigned to isoflurane-fentanyl anaesthesia with nitrous oxide-oxygen vs air-oxygen (control). Systemic and coronary haemodynamics, 12-lead ECG, LV anterior wall motion by cardiokymography (CKG) and myocardial lactate balance were recorded at four intervals: before and during anaesthesia and 10 and 30 minutes into surgery. Systemic haemodynamics were controlled by anaesthetic dose, and, when insufficient, by i.v. nitroglycerine (NG) in case of LV failure (PCWP > 18 mmHg) and by phenylephrine during hypotension. We found that nitrous oxide was associated with greater need for i.v. nitroglycerin (patients: P = 0.031, episodes P = 0.005) and more myocardial ischaemia (patients P = 0.012, episodes P = 0.001) despite systemic and coronary haemodynamics comparable to the control group. We conclude that nitrous oxide, known to have both sympathomimetic and cardiodepressive actions, produced cardiodepression in the face of sympathetic stimulation. Our study design did not allow to conclude if myocardial ischaemia was the consequence of increased wall stress or a reason for the observed LV dysfunction. The higher incidence of introperative myocardial ischaemia and need for NG did not cause increased cardiac morbidity.
The effects of anaesthesia for major abdominal vascular surgery on coronary flow regulation and mechanisms of myocardial ischaemia were studied in 56 patients with CAD, using a randomized, partly double-blinded protocol. After induction with fentanyl (3 micrograms.kg-1) and thiopentone (2-4 mg.kg-1) and tracheal intubation, principal anaesthetics were nitrous oxide/oxygen (60/40) with isoflurane (n = 20), halothane (n = 19) or fentanyl (15-20 micrograms.kg-1) (n = 17). Conventional invasive techniques and coronary venous retrograde thermodilution were used to assess systemic and coronary haemodynamics. Coronary vascular resistance was estimated from myocardial oxygen extraction. Myocardial ischaemia was diagnosed by 12-lead ECG and/or anterior wall motion abnormalities by cardiokymography and/or myocardial lactate production. When adjustment of anaesthetic dose was insufficient for haemodynamic control, i.v. phenylephrine and nitroglycerine were administered to treat hypotension and hypertension or cardiac failure respectively. Measurements were performed at four specific intervals; awake, before surgery and 10 and 30 min after abdominal incision. Comparable changes of systemic haemodynamics and myocardial oxygen consumption were observed in the three groups. Coronary vasodilation was evidenced in isoflurane patients only and was linearly dose-dependent (P < 0.001). Partial Least Squares Projections to Latent Structures modelling with cross validation confirmed this dose-dependency and ruled out a clinically measurable influence by intervention drugs or simultaneous systemic haemodynamic abnormalities. The incidence of myocardial ischaemia during anaesthesia and surgery was comparable in the three groups (35, 37 and 24%, respectively) and there was an association with systemic haemodynamic aberrations in 19 of the 27 ischaemic episodes. In contrast to ischaemic halothane and fentanyl patients, isoflurane patients with ischaemia had significantly lower myocardial oxygen extraction (P = 0.008 and P = 0.001, respectively), indicating that the oxygen extraction reserve was not utilized in a normal way during ischaemia.
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