Kinetics of absorption atelectasis during anesthesia: a mathematical model

Abstract

Recent computed tomography studies show that inspired gas composition affects the development of anesthesia-related atelectasis. This suggests that gas absorption plays an important role in the genesis of the atelectasis. A mathematical model was developed that combined models of gas exchange from an ideal lung compartment, peripheral gas exchange, and gas uptake from a closed collapsible cavity. It was assumed that, initially, the lung functioned as an ideal lung compartment but that, with induction of anesthesia, the airways to dependent areas of lung closed and these areas of lung behaved as a closed collapsible cavity. The main parameter of interest was the time the unventilated area of lung took to collapse; the effects of preoxygenation and of different inspired gas mixtures during anesthesia were examined. Preoxygenation increased the rate of gas uptake from the unventilated area of lung and was the most important determinant of the time to collapse. Increasing the inspired O2 fraction during anesthesia reduced the time to collapse. Which inert gas (N2 or N2O) was breathed during anesthesia had minimal effect on the time to collapse.

Modeling Alveolar Volume Changes During Periodic Breathing in Heterogeneously Ventilated Lungs

“…Poorly ventilated lung units-such as those that are the cause of periodic breathing-are likely to collapse due to atelectasis, especially for high oxygen concentrations. 11,12 By allowing compartment volume to vary, and therefore to be dependent on the ventilation of the lung, the effect that lung volume has on periodic breathing can be investigated. Subsequently, whether there is a link to the prevention, or delay, of total lung collapse may be determined.…”

“…The percentage of inspired oxygen is increased to F I = 0.8, in agreement with experimental observation that the incidence of pulmonary collapse is greater with higher levels of inspired oxygen, even in the absence of restrictions in ventilation. 11,12 Indeed, it has been observed that atelectasis can result from high levels of inspired oxygen. 13 It should be noted that the parameters used to produce these solutions do not lead toward total lung collapse.…”

“…3,11,12 Given that lung collapse is known to occur when a subject is administered high levels of oxygen, this parameter is assigned a value of 0.8 for the extended model. Figure 6 compares the results for F I = 0.8 (solid line) with the effects of a normal inspired oxygen level, F I = 0.21 (dotted line), and the equivalent of a patient being administered 100% oxygen, F I = 1 (dashed line).…”

“…3 This occurs when the net flux of gas across the alveolar membrane into the blood is greater than that supplied by inspired ventilation, causing the lung volume to decrease. Models exist that examine the onset and aggravation of atelectasis 11,12 and such work has lead to the conclusion that increasing the fraction of inspired oxygen reduces the time to collapse.…”

Atelectasias durante anestesia: fisiopatologia e tratamento

“…Joyce e Williams postularam que após a indução anestésica as vias aéreas das regiões pulmonares dependentes se fecham e essas áreas se comportam como uma cavidade fechada passível de colapso. Utilizando um modelo matemático, esses autores propuseram que a oxigenação prévia e o uso de altas frações inspiradas de oxigênio aumentam a captação de gás das áreas pouco ventiladas do pulmão e eram os principais responsáveis pela formação de atelectasias 34 . Rothen e col. observaram em 12 pacientes submetidos à intervenção cirúrgica eletiva sob anestesia geral, que quando FiO 2 de 1 era utilizada após manobra de expansão pulmonar até a capacidade vital (pressão de 40 cmH 2 O nas vias aéreas), atelectasias eram observadas em tomografias de tórax obtidas em até cinco minutos.…”