This study demonstrates that smoothened averages for hemodynamic and clinical values in the first 48 hours after a Blalock-Taussig shunt in children with parallel circulation are associated with risk of cardiopulmonary arrest, need for extracorporeal membrane oxygenation, or inpatient mortality. More specifically, combination of a central venous pressure greater than 7.8 mmHg, a serum lactate greater than 1.8, a renal oxygen extraction ratio of greater than 32, and a vasoinotrope score greater than 8.7 were associated with increased risk.
Such a score can be helpful in identifying those at high risk and increasing situational awareness. Identification of patients at high risk for cardiopulmonary arrest can be helpful in intensive care units to assign cardiopulmonary resuscitation roles, prophylactically. Early identification of patients at high risk for cardiopulmonary arrest can also facilitate having certain medications at the bedside in case of clinical deterioration, specifically lower dose epinephrine. Such situation awareness and medication availability has been associated with decreases in cardiopulmonary arrests.
Previous studies have identified factors associated with cardiopulmonary arrest in critically ill children. Previous studies have demonstrated ST-segment variability, vasoinotrope score, higher inadequate oxygen delivery index, venous saturation, serum lactate, cerebral dysfunction by electroencephalography, a history of a previous cardiac arrest, specific cardiac lesions, and specific cardiac surgeries among others [3–16]. Risk factors for mortality have often mirrored the risk factors for cardiac arrest.
Compared to previous studies, the current study aimed to identify postoperative hemodynamics and routine clinical variables in a more static, binary approach so that the findings can be more easily applied at the bedside.
The cardiovascular system’s sole purpose is to ensure that adequate oxygen is delivered to other organs. Oxygen content is a function of arterial oxygen saturation, hemoglobin, and partial pressure of oxygen while. Cardiac output the quotient of oxygen consumption and the arteriovenous oxygen content difference. Systemic oxygen delivery is then the product of both oxygen content and cardiac output. Looking at systemic oxygen delivery broadly, arterial oxygen saturation, hemoglobin, partial pressure of oxygen, and venous oxygen saturation become the major components. From a more pragmatic standpoint, systemic oxygen delivery is proportional to the arteriovenous oxygen saturation difference [17]. From this it is not surprising that renal oxygen extraction ratio and serum lactate were found to be significantly associated with adverse outcomes in the current study, as both are markers of the adequacy of systemic oxygen delivery. Vasoinotrope score likely represents human action in response to the perceived inadequacy in systemic oxygen delivery. As for central venous pressure, central venous pressure does increase as a response to lower cardiac output, thus increased central venous pressure preceding cardiorespiratory arrest may be an indicator of lower cardiac output and subsequently lower systemic oxygen delivery [18].
Also of note is that blood pressure was not found to be a predictor of adverse events. Blood pressure is the product of flow and systemic vascular resistance and thus increase in systemic vascular resistance alone can lead to increased blood pressure. When this happens, and flow is not increased, then cardiac output is not increased. Thus, if systemic vascular resistance is not being monitored, then blood pressure is not a good surrogate for cardiac output. Often, blood pressure is augmented by increasing systemic vascular resistance and not cardiac output, thus not increasing systemic oxygen delivery [19, 20]. In fact, increasing systemic vascular resistance may lead to no change or a decrease in the adequacy of systemic oxygen delivery [21, 22]. Low systemic vascular resistance with high cardiac output has been demonstrated as the physiologic state associated with the lowest likelihood to be associated with poor outcomes in parallel circulation [23]. Thus, interventions to decrease systemic vascular resistance and increase cardiac output lead to optimization of systemic oxygen delivery and clinical outcome [24–37]. In the current study, systemic oxygen delivery was quantified using the renal oxygen extraction ratio calculated using renal near infrared spectroscopy. Inferior caval vein saturations are also helpful in this regard as inferior caval vein saturation and renal near infrared spectroscopy correlate well [38–40].
While these data are clinically applicable and may be helpful in early identification of those at with parallel circulation with a high risk for cardiopulmonary arrest after Blalock-Taussig-Thomas shunt placement, they are not without their limitations. The exact cutoffs for variables defined here may be variable at different institutions due to institution specific nuances in management. However, the overall physiologic states that are more favorable shouldn’t change and it is unlikely that the absolute cutoffs are greatly different. The absolute frequency of the composite outcome was also relatively low which does limit the effect size that could be detected. Nonetheless, statistically significant differences were identified.