The effects of spontaneous and mechanical ventilation on central cardiovascular function and peripheral perfusion during isoflurane anaesthesia in horses

Objective To compare the effects of spontaneous breathing and mechanical ventilation on haemodynamic variables, including muscle and skin perfusion measured with laser Doppler flowmetery, in horses anaesthetized with isoflurane. Study design Prospective controlled study. Animals Ten warm-blood trotter horses (five males, five females). Mean mass was 492 kg (range 420584 kg) and mean age was 5 years (range 4-8 years). Materials and methods After pre-anaesthetic medication with detomidine (10 mu g kg(-1)) anaesthesia was induced with intravenous (IV) guaifenesin and thiopental (4-5 mg kg-1 IV) and maintained using isoflurane in oxygen. The horses were positioned in dorsal recumbency. In five animals breathing was initially spontaneous (SB) while the lungs of the other five were ventilated mechanically using intermittent positive pressure ventilation (IPPV). Total anaesthesia time was 4 hours with the ventilatory mode changed after 2 hours. During anaesthesia, heart rate (HR) cardiac output ((Q) under circlet) stroke volume (SV) systemic arterial blood pressures (sAP), and pulmonary arterial pressure (pAP) were recorded. Peripheral perfusion was measured in the semimembranosus and gluteal muscles and on the tail skin using laser Doppler flowmetry. Arterial (a) and mixed venous ((v) over bar) blood gases, pH, haemoglobin concentration [Hb], haematocrit (Hct), plasma lactate concentration and muscle temperature were measured. Oxygen content, venous admixture ((Q) under circles/(Q) under circlet) oxygen delivery (DO2) and oxygen consumption (VO2) were calculated. Results During mechanical ventilation, FIR, sAP, pAP, (Q) under circlet, SV, (Q) under circles/(Q) under circlet and PaCO2 were lower and PaO2 was higher compared with spontaneous breathing. There were no differences between the modes of ventilation in the level of perfusion, DO2, (V) under circleO(2), [Hb], (Hct), or plasma lactate concentration. After the change from IPPV to SB, left semimembranosus muscle and skin perfusion improved, while muscle perfusion tended to decrease when SB was changed to IPPV. Low-frequency flow motion was seen twice as frequently during IPPV compared with SB. Conclusions Mechanical ventilation impaired cardiovascular function compared with SB in horses during isoflurane anaesthesia. Muscle and skin perfusion changes occurred with ventilation, although further studies are needed to elucidate the underlying mechanisms.