EFFECT OF PREMEDICATION AND GENERAL ANAESTHESIA ON ARTERIAL BLOOD GASES

Effect of premedication on arterial blood gases prior to cardiac surgery

The effect of premedication on arterial blood gas tensions was studied in thirty adult surgical patients with valvular disease. They were divided into three groups, each group having a different premedication regimen. Blood gas tensions were compared in these patients when awake on the night before surgery, asleep, after premedication and just prior to induction of anaesthesia. Samples were taken while the patient breathed air and each patient acted as his/her own control. The patients were randomised into one of three premedication regimens: 1. intramuscular lorazepam, 2. intramuscular morphine and hyoscine (scopolamine) and 3. oral lorazepam plus intramuscular morphine and hyoscine. There was a statistically significant though not clinically significant rise in PaCO2 and fall in pH following premedication with lorazepam, morphine and hyoscine. There was also a significant fall in PaO2 associated with morphine and hyoscine premedication which was greater than that which occurred with unsedated sleep. Patients who are to undergo cardiac valvular surgery should receive supplementary oxygen following premedication and during transfer to the operating room.

PEEP and the Bain circuit

The feasibility of applying positive end expiratory pressure within the Bain circuit using an Emerson PEEP valve was studied. It was determined that varying degrees of PEEP may be applied within the Bain circuit using an Emerson PEEP valve during either mechanically or manually controlled ventilation but not during spontaneous breathing. There was some loss of measured tidal volume with increasing PEEP, due primarily to compression gas losses but also from leakage throughout the system. The maximum gas loss was 100 ml per minute at PEEP 15 cm H2O under test conditions. PEEP may be applied between the Bain tubing and the manifold. However, one should then never allow the patient to breathe spontaneously as the PEEP valve presents an obstruction to inspiratory flow. With the PEEP valve relocated between the Bain manifold and the ventilator hose it was found that application of PEEP was possible without this disadvantage, but only during mechanical ventilation.

Arterial hypoxaemia following premedication in patients with coronary artery disease

The effect of premedication with morphine and scopolamine, morphine alone, and scopolamine alone on arterial oxygen tension has been studied in patients with coronary artery disease undergoing bypass operations. Control arterial blood samples were obtained with the patients in the supine position breathing room air the day before operation. A second arterial blood sample was taken in the induction room with the patient breathing room air in the supine position. In the first group (scopolamine only) and second group (morphine only) cH+ (pH), PaCO2 and PaO2 did not change. In the third group (morphine and scopolamine), however, PaO2 decreased, while PaCO2 and cH+ increased (pH decreased). The changes in cH+ and PaCO2 were statistically significant, but of no clinical importance. The present study suggests that, when morphine and scopolamine premedication is used for patients with coronary artery disease, simultaneous administration of oxygen is important in helping to eliminate a possible cause of preoperative myocardial ischaemia.