Respiratory mechanics during and after anaesthesia for major vascular surgery

Abstract

SummaryTo evaluate the effects of major vascular surgery on respiratory mechanics, 11 patients undergoing general anaesthesia for abdominal aortic surgery were studied. Before aortic cross-clamping, chest wall elastance and resistance both increased (by 126% and 58%, respectively) when surgical retractors were placed. After aortic cross-clamping, lung elastance increased by 29%, accompanied by a decrease in cardiac index (22%) and an increase in pulmonary (17%) and systemic (15%) vascular resistance. After aortic unclamping, lung elastance decreased, although it remained higher than baseline values (by 12% Changes in respiratory system elastance have been described previously during anaesthesia. This has been attributed to various mechanisms, for example an increased thoracic blood volume [1][2][3][4]. During major vascular surgery aortic cross-clamping (ACC) is likely to cause a redistribution of blood to the upper part of the body [5]. If this phenomenon also occurs at the lung level, lung elastance might increase. This hypothesis was proposed almost 60 years ago, when Barcroft & Samaan [6] suggested that an increase in blood volume in the lungs can occur during and after ACC. In contrast, Paterson et al. [7] observed interstitial pulmonary oedema on chest radiography 4-8 h after surgery, even though pulmonary arterial occlusion pressure did not exceed 12 mmHg. They also described increased peak airway pressure, although the latter could arise because of an increase not only of respiratory elastance, but also of respiratory resistance, and therefore should not be used to quantify pulmonary disease. Moreover, a high percentage of pulmonary failure [5] has been described following repair of thoracoabdominal aneurysm, which is likely to represent a more severe clinical condition. We sought to study repair of abdominal aneurysm as a model of cardiac-lung interaction during major vascular surgery. Methods Patients and monitoring proceduresEleven patients (10 males) undergoing general anaesthesia for infrarenal aortic surgery were studied. The research was approved by our institutional ethics committee and informed consent was obtained from all subjects. No premedication was administrated. Anaesthesia was induced with fentanyl and thiopentone and maintained with 60% nitrous oxide/0.2-0.3% isoflurane in oxygen according to our standard practice, with further fentanyl as required. Orotracheal intubation was facilitated by vecuronium 0.1 mg.kg ¹1 with further doses to provide a level of twitch depression at which no palpable response to train-of-four stimulation was obtained. The patients' lungs were ventilated mechanically (Ohmeda Excel 210, BOC Health Care) with tidal volume (V T ) and ventilatory frequency ( f ) chosen to maintain normocapnia.Because the quantity and quality of the administered fluids could have an effect on respiratory and cardiovascular variables, peri-operative fluid management consisted of an almost fixed amount of both quality (crystalloid : colloid 3 : 1) and quantity (12 Ϯ 3 ml.kg ¹1 .h ¹1 )...