Mechanical ventilation
Ventilation necessitated a thoracotomy at the fifth intercostal space as, in closed-chest rats, excessive contractions of the diaphragm and intercostal muscles was found to prevent effective respiration. A positive end-expiratory pressure was exerted and air ventilation provided at 54 strokes min-1 (3-4.5 ml per stroke) using a Harvard Bioscience small animal respirator. Rabbits were ventilated with air at 38 strokes min-1 (13-18 ml per stroke). A thoracotomy was not necessary in anaesthetized rabbits. Blood gases were measured using a Corning 158 or 850 pH / blood gas analyser. Stroke volumes were adjusted for pre-drug PO2>80 mmHg and PCO2 >30 mmHg.