Myocardial and systemic effects of nitroglycerin, given awake and during anaesthesia in coronary patients

The haemodynamic and myocardial energy-metabolic effects of intravenous nitroglycerin (NTG) were investigated in seven male patients about to undergo coronary artery bypass grafting. Effects of one mg of NTG given at a rate of 100 micrograms . min-1 to sedated, awake hypertensive patients, were compared to effects of the same dose of NTG in the same patients, after sternotomy. Coronary sinus flow was measured by a thermodilution catheter and myocardial oxygen consumption (MVO2) was calculated using the arterio-coronary sinus difference of oxygen content. With the patients awake, cardiac index and stroke volume and work indices fell during NTG administration. There was only a small fall in arterial pressure as heart rate and systemic resistance increased. Pulmonary capillary wedge pressure decreased but not MVO2. With NTG administration during anaesthesia, arterial pressure fell significantly to normal, as systemic resistance decreased, without a change in heart rate or cardiac index. Calculated MVO2 decreased as the arterial-coronary venous difference of oxygen content increased. Both oxygen and lactate balance of the myocardium were maintained. NTG given during anaesthesia was more effective in reducing blood pressure and MVO2 than when given to awake patients.