Causes of Oxygenation Impairment During Anesthesia
Mild to moderate hypoxemia (arterial oxygen saturation of between 85–90%) occurs in about half of all patients undergoing anesthesia and elective surgery, despite an inspired oxygen fraction (FiO2) of 0.3–0.4 [1]. In 20% of the patients the saturation is
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Introduction Mild to moderate hypoxemia (arterial oxygen saturation of between 85-90%) occurs in about half of all patients undergoing anesthesia and elective surgery, despite an inspired oxygen fraction (Fi0 2 ) of 0.3-0.4 [1]. In 20o/o of the patients the saturation is below 81 o/o for up to 5 min [ 1]. It may be argued that such hypoxemia does not cause any harm in most patients, but it can hardly be considered a desirable condition and the causes of such hypoxemia should be identified and prevented, if possible. Moreover, post-operative pulmonary complications occur in 1-3o/o of patients undergoing elective thoracic or abdominal surgery, and the complication rate may increase to 20o/o in emergency surgery [2, 3]. To what extent post-operative complications are caused by a respiratory dysfunction during anesthesia is not clear. However, atelectasis that has developed during anesthesia remains in the post-operative period, and impairment in arterial oxygenation and decrease in forced spirometry correlate to the size of the atelectasis [4]. Moreover, in view of the large number of anesthestics that are given in the western world, some 30-50 000 per million inhabitants, a moderate complication rate will have considerable social and economic consequences.
Gas Exchange Venous Admixture and Ventilation-Perfusion Mismatch The impairment in gas exchange corresponds to a venous admixture of approximately 10o/o [5]. It is generally held that the impairment of arterial oxygenation during anesthesia is more severe at older ages [6]. Obesity worsens the oxygenation of blood [7], and smokers show more gas exchange impairment than non-smokers [8]. The impairment in arterial oxygenation has made it a routine procedure to increase the Fi0 2 during anesthesia to 0.3-0.4 in an uncomplicated anesthetic procedure, and even higher during anesthesia in obese, bronchitic or elderly patients. It should be remembered that although venous admixture often is used synonymously with shunt, it includes both lung regions that are non-ventilated but still perfused (true shunt) and regions that are poorly ventilated in relation to their perfusion, so called 'low VA/Q regions' (see below). The separation of these two components of venous admixture is not only of academic interest but gives information as J.-L. Vincent (ed.), Yearbook of Intensive Care and Emergency Medicine 2000 © Springer-Verlag Berlin Heidelberg 2000
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G. Hedenstierna
to the morphological causes of the oxygenation impairment. Such separation can be achieved by the multiple inert gas elimination technique [9] which enables the assessment of the ventilation/perfusion relationships (VA/Q). In young, healthy volunteers only a small VA/Q mismatch was seen, with a widened VA/Q mode and the appearance of a small shunt [10]. However, in a similar group, Prutow et al. [11] described a much larger shunt of mean 8% in anesthetized patients who were to undergo surgery. In a study by Bindslev and co-workers [12] of a middle-aged group of patients during anesthesia (spontaneous breath
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