A Model Analysis of Arterial Oxygen Desaturation during Apnea in Preterm Infants

Abstract

Rapid arterial O2 desaturation during apnea in the preterm infant has obvious clinical implications but to date no adequate explanation for why it exists. Understanding the factors influencing the rate of arterial O2 desaturation during apnea () is complicated by the non-linear O2 dissociation curve, falling pulmonary O2 uptake, and by the fact that O2 desaturation is biphasic, exhibiting a rapid phase (stage 1) followed by a slower phase when severe desaturation develops (stage 2). Using a mathematical model incorporating pulmonary uptake dynamics, we found that elevated metabolic O2 consumption accelerates throughout the entire desaturation process. By contrast, the remaining factors have a restricted temporal influence: low pre-apneic alveolar causes an early onset of desaturation, but thereafter has little impact; reduced lung volume, hemoglobin content or cardiac output, accelerates during stage 1, and finally, total blood O2 capacity (blood volume and hemoglobin content) alone determines during stage 2. Preterm infants with elevated metabolic rate, respiratory depression, low lung volume, impaired cardiac reserve, anemia, or hypovolemia, are at risk for rapid and profound apneic hypoxemia. Our insights provide a basic physiological framework that may guide clinical interpretation and design of interventions for preventing sudden apneic hypoxemia.

Intermittent Hypoxic Episodes in Preterm Infants: Do They Matter?

“…It should be noted that several physiologic parameters contribute to the desaturation that accompanies cessation of ventilation. As indicated in figure 1 , these include pulmonary oxygen stores that are a reflection of lung volume, total blood oxygen capacity comprising blood volume and hemoglobin content, as well as tissue oxygen consumption [5] . Nonetheless, pulse oximetry offers a technologically feasible, noninvasive technique for quantifying the relationship between intermittent hypoxic episodes and various pathophysiologic outcomes in preterm infants.…”

Simulating hypoxia and modelling the airway

“…3a) and blood volumes (Fig. 3b) in models of the preterm infant [4]. This model shows extreme desaturation, as the patients have high metabolic rates compared with their lung volumes and total haemoglobin content.…”

“…According to Sands et al [4], the first theoretical analysis of the factors influencing apnoeic desaturation was that of Farmery and Roe [3]. This analysis was constructed from first principles using discrete equations, solved symbolically, and so this now seems rather quaint compared with more contemporary analyses that invariably use computational numerical techniques or simulation tools, such as Simulink The basis of all these analyses lies in a physical model with stated assumptions.…”

“…Figure 3b shows the effect of progressively decreasing total blood volume: 120, 80, 40 ml.kg )1 (iv, v and vi, respectively). Figure adapted from Sands et al [4]. .…”

“…If we wanted more precision, we could certainly take account of air resistance, but if we try to include an exhaustive array of influencing factors, we will sacrifice simplicity of the model for enhanced precision. So often, in modelling, the balance between the beauty of parsimony and the necessity of complexity has to be carefully considered [1][2][3][4].…”