Apnea testing in suspected brain dead children - physiological and mathematical modelling

Objective: To study the validity and safety of the traditional apnea test in children, and to evaluate a mathematical equation estimating the hemodynamic response to the apnea test. Design: A prospective clinical study. Setting: Pediatric ICU Patients and participants: 38 pediatric patients suffering severe brain injury aged 2 months to 17 years, undergoing apnea testing for brain death. Measurements and results: Apnea tests were performed 61 times (once in 19 patients, twice in 15, and 3 times in 4 patients). Mean PaCO₂ was 41.1±10.6 mmHg before apnea and increased to 68.0±17.6 at 5 min. PaCO₂ increased to 81.8±20.1 and 86.0±25.6 at 10 and 15 min, respectively. There was a mean PaCO₂ increase by 5.38±1.4 mmHg/min in the first 5 min, and 2.75±0.5 mmHg/min during the next 5 min. We found a statistically significant (p<0.05) linear relationship between the natural logarithm of PaCO₂, time, and the logarithm of the initial level of PaCO₂. An inverse linear relationship (p<0.05) was found between systemic mean arterial pressure (MAP) and initial level of PaCO₂ presented as mathematical correlations and nomograms. Conclusions: By using our model for predicting MAP and PCO₂ prior to apnea testing, hemodynamic embarrassment can be anticipated and prevented, thus allowing a safer procedure in the detection of brain death. Despite the fact that continuous cardiorespiratory monitoring is important, hemodynamic disturbances can be estimated before the apnea test, thus allowing a safer approach to brain death detection.