A Thesis submitted to The University of Arizona College of Medicine - Phoenix in partial fulfillment of the requirements for the Degree of Doctor of Medicine. / Children with prolonged mechanical ventilation after cardiac surgery have a higher risk for poor outcome due to a variety of ventilator‐associated morbidities. It therefore becomes essential to identify these children at higher risk of prolonged mechanical ventilation as well as find methods to identify children ready to be extubated as early as possible to avoid these complications. One physiological variable, the pulmonary dead space fraction (VD/VT), has been suggested as a possible indicator of prolonged mechanical ventilation. VD/VT essentially measures the amount of ventilated air that is unable to participate in gas exchange. Can VD/VT be used successfully in children undergoing cardiac surgery to identify those at risk for prolonged mechanical ventilation and identify those ready for extubation? Retrospective chart review of 461 patients at Phoenix Children’s Hospital in the Pediatric Cardiac Intensive Care Unit since the initiation of standard application of the Philips NM3 monitors in October 2013 through December 2014. From the 461 patients screened, only 99 patients met all the inclusion criteria. These 99 patients consisted of 29 patients with balanced single ventricle physiology and 61 patients with two ventricle physiology. Initial postoperative and pre‐extubation VD/VT values correlated with length of mechanical ventilation for patients with two ventricle physiology but not for patients with single ventricle physiology. Additionally, pre‐extubation VD/VT values of greater than 0.5 indicated higher rates of extubation failure in two ventricle patients. Conclusion: For children with two ventricle physiology undergoing cardiac surgery, VD/VT should be used clinically to estimate the length of mechanical ventilation for these children. VD/VT should also be checked in these patients before attempting to extubate. If VD/VT is found to be higher than 0.5, extubation should not be attempted since the patient is at a much higher risk for extubation failure.