Oxygenation Impairment during Anesthesia: Influence of Age and Body Weight

Background: Anesthesia is increasingly common in elderly and overweight patients and prompted the current study to explore mechanisms of age- and weight-dependent worsening of arterial oxygen tension (Pao(2)). Methods: This is a primary analysis of pooled data in patients with (1) American Society of Anesthesiologists (ASA) classification of 1; (2) normal forced vital capacity; (3) preoxygenation with an inspired oxygen fraction (FIO2) more than 0.8 and ventilated with FIO2 0.3 to 0.4; (4) measurements done during anesthesia before surgery. Eighty patients (21 women and 59 men, aged 19 to 69 yr, body mass index up to 30 kg/m(2)) were studied with multiple inert gas elimination technique to assess shunt and perfusion of poorly ventilated regions (low ventilation/perfusion ratio [(V) over dot(A)/Q]) and computed tomography to assess atelectasis. Results: Pao(2) /FIO2 was lower during anesthesia than awake (368; 291 to 470 [median; quartiles] vs. 441; 397 to 462 mm Hg; P = 0.003) and fell with increasing age and body mass index. Log shunt was best related to a quadratic function of age with largest shunt at 45 yr (r(2) = 0.17, P = 0.001). Log shunt was linearly related to body mass index (r(2) = 0.15, P < 0.001). A multiple regression analysis including age, age(2), and body mass index strengthened the association further (r(2) = 0.27). Shunt was highly associated to atelectasis (r(2) = 0.58, P < 0.001). Log low (V) over dot(A)/Q showed a linear relation to age (r(2) = 0.14, P = 0.001). Conclusions: Pao(2)/FIO2 ratio was impaired during anesthesia, and the impairment increased with age and body mass index. Shunt was related to atelectasis and was a more important cause of oxygenation impairment in middle-aged patients, whereas low (V) over dot(A)/Q, likely caused by airway closure, was more important in elderly patients. Shunt but not low (V) over dot(A)/Q increased with increasing body mass index. Thus, increasing age and body mass index impaired gas exchange by different mechanisms during anesthesia.