EFFECTS OF RESPIRATORY ALKALOSIS AND ACIDOSIS ON MYOCARDIAL BLOOD-FLOW AND METABOLISM IN PATIENTS WITH CORONARY-ARTERY DISEASE

Background: Variation of the arterial carbon dioxide partial pressure (Pa-CO2) is not uncommon in anesthetic practice. However, little is kn own about the myocardial consequences of respiratory alkalosis and aci dosis, particularly in patients with coronary artery disease. The aim of the current study was to investigate the effects of variation in Pa -CO2 on myocardial blood flow (MBF), metabolism, and systemic hemodyna mics in patients before elective coronary artery bypass graft surgery. Methods: In 10 male anesthetized patients, measurements of MBF, myoca rdial contractility, metabolism, and systemic hemodynamics were made i n a randomized sequence at Pa-CO2 levels of 30, 40, and 50 mmHg, respe ctively. The MBF was measured using the Kety-Schmidt technique with ar gon as a tracer. End-diastolic left ventricular pressure and the maxim al increase of left ventricular pressure were assessed using a manomet er-tipped catheter. Results: The cardiac index significantly changed w ith varying Pa-CO2 levels (hypocapnia, - 9%; hypercapnia, 13%). This r eaction was associated With inverse changes in systemic vascular resis tance index levels. The MBF significantly increased by 15% during hype rcapnia, whereas no change was found during hypocapnia. Myocardial oxy gen and glucose uptake and the maximal increase of left ventricular pr essure were not affected by varying Pa-CO2 levels. Conclusions: In ane sthetized patients with coronary artery disease, short-term variations in Pa-CO2 have significant effects on MBF but do not influence global myocardial oxygen and glucose uptake. Changes in systemic hemodynamic s associated with respiratory alkalosis and acidosis are caused by cha nges in systemic vascular resistance rather than by alterations in myo cardial contractility.