Physiology of gas exchange during anaesthesia

Abstract

Mechanical ventilation is a prerequisite for many surgical interventions. Furthermore, during states of severe gas exchange disturbance or impaired neurological conditions with the threat of aspiration or cardiovascular instability, it is a life-saving intervention on every ICU. Even the induction of anaesthesia disturbs the physiological lung function, due to changes in chest wall mechanics and diaphragm relaxation, generating atelectases, gas exchange disturbance and ventilation-perfusion mismatch. Additionally, the application of positive pressure to lung structures elicits ventilator-induced lung injury, with the severity of injury dependent on the applied volume, peak pressures and levels of positive end-expiratory pressure. Although these pathophysiological changes may be of minor importance for the majority of ventilated patients in the operating room, these mechanisms may harm patients during surgical interventions with the need for one-lung ventilation or with underlying co-morbidities such as chronic obstructive pulmonary disease (COPD) or acute respiratory distress syndrome (ARDS). This review provides an outline of the major components of the pathophysiological changes associated with general anaesthesia and describes the additional risks in patients with COPD and ARDS as common co-morbidities in every hospital.

Incidence and risk factors of hypoxaemia after preoxygenation at induction of anaesthesia

“…In patients with COPD, induction of anaesthesia and preoxygenation may exacerbate gas exchange disturbance by decreasing the ventilationeperfusion ratio (atelectasis formation). 10 Furthermore, in patients with COPD, preoxygenation may dampen hypoxic pulmonary vasoconstriction, increasing the pulmonary shunt. 11 The link between hypertension and hypoxaemia after anaesthetic induction is not obvious; we can hypothesise that anaesthetic drugs may impact on the vascular tone and promote vasodilation.…”

Perioperative Evaluation of Patients with Pulmonary Conditions Undergoing Non-Cardiothoracic Surgery

“…The development of atelectasis can lead to hypoxemia and the translocation of bacteria to the bloodstream, which can increase the risk for ventilator-induced lung injury and sepsis. 4 Preoperative evaluation. The most important tools for assessing the risk for PPCs during a preoperative evaluation are a careful and thorough history and physical examination followed by targeted laboratory testing.…”

Successful 1:1 proportion ventilation with a unique device for independent lung ventilation using a double-lumen tube without complications in the supine and lateral decubitus positions. A pilot study

“…Differences in gas distribution between lungs in the supine position probably do not have a great clinical relevance, but are informative and could supplement literature findings, while uneven gas distribution in the lateral decubitus position during general anaesthesia with artificial ventilation lowers the functional residual capacity (FRC) [ 17 , 18 ] causing alveolar collapse [ 19 , 20 ] and compression atelectasis [ 4 , 20 ]. The second issue is the perfusion inequality caused by gravitational force [ 11 , 21 ] and what is more dynamic hyperinflation diverts perfusion towards the dependent hypo-inflated lung [ 22 , 23 ].…”

“…Should these parameters not be met, oxygen delivery to the cells will be inadequate, leading to anaerobic metabolism and potential multi-organ failure. One of the reasons for clinical ventilation/perfusion mismatch can be pulmonary pathology, leading to so called shunt [ 1 – 3 ], another is—patient positioning on the operating table [ 4 , 5 ]. While young and healthy subjects can cope this scenario due to the efficiency of their physiological reflexes, in medicine, many surgical procedures involve elderly and critically ill patients with significant health impairments [ 6 , 7 ].…”