A59-10 Hypoxemia Despite Successful Volume-target Mechanical Ventilation in Congenital Myasthenia Syndrome

The patient is a 30-month-old female with a new diagnosis of presumed congenital myasthenia syndrome (CMS) of unknown variant. History The child experienced unpredictable episodes of complete unresponsiveness and low muscle tone. During these episodes, the child was unresponsive to pain and hypoxemic despite receiving non-invasive ventilation with oxygen. The child had no history of cardiac shunting but was born premature at 26 weeks. A tracheostomy was placed to protect against respiratory failure. Treatment plan The patient responded positively to a trial of pyridostigmine; approximately 30 minutes after administration, the child would recover from her low-tone states and be active. Despite regular medication dosing however, the child would still experience low-tone states and hypoxemia while receiving invasive mechanical ventilation. Mechanical ventilation The child was mechanically ventilated through a cuffed tracheostomy tube. A volume-targeted algorithm delivered a volume approximately 8.5 mL/kg of the child’s weight. Other settings: respiratory rate (RR) 20, a variable inspiratory time between 0.4 s and 1.0 s; a positive end expiratory pressure (PEEP) of 10 cm H2O; and fraction of inhaled oxygen (FiO2) of 21% during normal-tone states. Findings During low-tone states in either seated or supine position, FiO2 needs increased from 21% to 60% to achieve an oxygen saturation (SpO2) of 90%. The child received all intended ventilatory support: peak inspiratory pressure to achieve the set volume ranged between 20 and 25 cm H2O; total RR was 4 bpm over the set rate; spontaneous trigger was 100% and flow-cycling was 100%; Both end-tidal carbon dioxide and capillary blood gas CO2 values were 31 mm Hg. Positioning corrected hypoxemia Suspecting that ventilation/perfusion (V/Q) mismatch was causing the hypoxemia, the patient was positioned prone. Almost instantly, SpO2 increased to 100%, allowing FiO2 to be weaned from 60% to 25%. In the absence of meaningful inspiratory effort, positive pressure ventilation was hypothesized to preferentially inflate non-dependent lobes. Placing the child in a prone position improved ventilation to dorsal regions of perfusion until she recovered from her low-tone state. Prone positioning is known to improve V/Q matching in dorsal lung regions in acute respiratory distress syndrome, but its utility for unusual presentations of CMS is not well documented. Hypopnea or episodic apnea are known causes of hypoxemia in CMS, but the effect of these etiologies was alleviated by the successful delivery of mechanical ventilation. Positioning was an essential component with mechanical ventilation and supplemental oxygen to correct the child’s hypoxemia.