Respiratory Function During Anesthesia: Effects on Gas Exchange

Abstract

Anaesthesia causes a respiratory impairment, whether the patient is breathing spontaneously or is ventilated mechanically. This impairment impedes the matching of alveolar ventilation and perfusion and thus the oxygenation of arterial blood. A triggering factor is loss of muscle tone that causes a fall in the resting lung volume, functional residual capacity. This fall promotes airway closure and gas adsorption, leading eventually to alveolar collapse, that is, atelectasis. The higher the oxygen concentration, the faster will the gas be adsorbed and the aleveoli collapse. Preoxygenation is a major cause of atelectasis and continuing use of high oxygen concentration maintains or increases the lung collapse, that typically is 10% or more of the lung tissue. It can exceed 25% to 40%. Perfusion of the atelectasis causes shunt and cyclic airway closure causes regions with low ventilation/perfusion ratios, that add to impaired oxygenation. Ventilation with positive end-expiratory pressure reduces the atelectasis but oxygenation need not improve, because of shift of blood flow down the lung to any remaining atelectatic tissue. Inflation of the lung to an airway pressure of 40 cmH2O recruits almost all collapsed lung and the lung remains open if ventilation is with moderate oxygen concentration (< 40%) but recollapses within a few minutes if ventilation is with 100% oxygen. Severe obesity increases the lung collapse and obstructive lung disease and one-lung anesthesia increase the mismatch of ventilation and perfusion. CO2 pneumoperitoneum increases atelectasis formation but not shunt, likely explained by enhanced hypoxic pulmonary vasoconstriction by CO2. Atelectasis may persist in the postoperative period and contribute to pneumonia.

Respiratory Management of Perioperative Obese Patients

“…20,21 The functional residual capacity of anesthetized patients decreases in part due to a decrease in respiratory muscle tone, resulting in a new lung-chest wall equilibrium at a lower overall volume of the respiratory system. 22 There is also a cranial displacement of the diaphragm that occurs during general anesthesia and in the supine position that contributes to atelectasis formation especially in obese patients, because the abdominal content that is displaced is of tremendous weight (Fig. 1).…”

“…24 Lung mechanics are also impaired in the atelectatic lung, because the lung is less compliant at lower volumes. 22 In normal-weight anesthetized subjects, Rothen et al 25 showed by chest CT imaging that atelectasis is enhanced by the use of 1.0 F IO 2 during preoperative induction compared with the use of 0.3 F IO 2 as a result of absorption atelectasis. Lower oxygen concentrations might reduce the formation of atelectasis but also shorten the safe apnea period during intubation.…”

WHO Needs High FIO2?

“…Macroscopically, atelectasis will be more easily produced during anesthesia and postoperatively when breathing hyperoxic gas (77,78). Thus, atelectasis is caused by absorption of alveolar gas, and oxygen is the gas that is absorbed the fastest since it is being consumed in the body.…”

Effects of anaesthesia on ventilation/perfusion matching

“…Again, there is not space enough here to deal with these issues, but the interested reader is referred to the review article of Hedenstierna and Rothen. 8 Different steps can be taken to improveV=Q matching and will be discussed as follows:…”