Epidural clonidine does not attenuate hypoxic pulmonary vasoconstriction in anaesthetized swine

Abstract

Clonidine has been shown in vitro to vasodilate precontracted porcine pulmonary artery ringlets [1]. Hypoxic pulmonary vasoconstriction (HPV) in vivo is a predominantly local response to hypoxia reducing ventilation-perfusion mismatch and shunt. During one lung ventilation in dogs, inhalational agents have been shown to increase the shunt fraction by attenuating HPV, whereas induction agents have no effect on HPV [2]. This study was designed to investigate the effects of epidural clonidine on HPV in vivo in the intact anaesthetized pig. It was also performed under a licence obtained under the Cruelty to Animals Act and EC Directive 86/609. General anaeshesia was induced in five pigs (20–25 kg) with intramuscular ketamine and halothane and maintained solely with continuous intravenous propofol infusion of 10 mg kg−1 h−1. The lungs were ventilated with oxygen and air to normocapnia. Halothane washout after induction resulted in an endtidal halothane concentration which did not exceed 0.2%, a concentration previously shown to have an insigificant effect on HPV [3]. Arterial pressure (SBP), central venous (CVP), pulmonary artery (PAS) and pulmonary artery occlusion pressures (PAOP) and cardiac output (CO) were monitored via the femoral vessels using an intraarterial 20-G cannula and a 7-F pulmonary artery catheter. An 18-G lumbar epidural catheter was sited. Measurements were made at baseline and 30 min after an epidural bolus of 2 μg mL−1 clonidine. After 5 min of ventilation with a hypoxic mixture, F1O2 0.1–0.15, arterial and mixed venous partial pressures of oxygen (PaO2, Pv¯O2) were obtained and HPV was measured indirectly by calculating pulmonary vascular resistance (PVR). Statistical analysis was by paired Student's t-test.Table 1Table 1: abstract 2).Epidural clonidine does not result in a decrease in PVR. It is concluded from this study that epidural clonidine does not attenuate HPV in the intact pig. It is therefore suggested that the adjunctive use of epidural clonidine in surgery requiring one lung ventilation will not result in deleterious effects on oxygen transfer because of an increased shunt.