Effect of synchronized intermittent mandatory ventilation on respiratory workload in infants after cardiac surgery

Background: Synchronized intermittent mandatory ventilation (SIMV) is commo nly used in infants and adults. However, few investigations have examined h ow SIMV reduces respiratory workload in infants. The authors evaluated how infants' changing respiratory patterns when reducing SIMV rate increased re spiratory load. The authors also investigated whether SIMV reduces infant r espiratory workload in proportion to the rate of mandatory breaths and whic h rate of SIMV provides respiratory workloads similar to those after trache al extubation. Methods: When 11 post-cardiac surgery infants aged 2-11 months were to be w eaned with SIMV, the authors randomly applied five level, of mandatory brea thing: 0, 5, 10, 15, and 20 breaths/min. Ali patients underwent ventilation with SIMV mode: pressure control ventilation, 16 cm H2O; inspiratory time, 0.8 s; triggering sensitivity, 0.6 l/min; and positive end-expiratory pres sure, 3 cm H2O. After establishing steady-state conditions at each SIMV rat e, arterial blood gases were analyzed, and esophageal pressure, airway pres sure, and airflow were measured. Inspiratory work of breathing, pressure-ti me products, and the negative deflection of esophageal pressure were calcul ated separately for assisted breaths, for spontaneous breaths, and for tota l breaths per minute. Measurements were repeated after extubation. Results: As the SIMV rate decreased, although minute ventilation and arteri al carbon dioxide tension were maintained at constant values, spontaneous b reathing rate and tidal volume increased. Work of breathing, pressure-time products, and negative deflection of esophageal pressure Increased as the S IMV rate decreased. Work of breathing and pressure-time products after extu bation were intermediate between those at a SIMV rate of 5 breaths/min and those at 0 breaths/min. Conclusion: When the load to breathing was increased progressively by decre asing the SIMV rate In post-cardiac surgery infants, tidal volume and spont aneous respiratory rate both Increased. in addition, work of breathing and pressure-time products were increased depending on the SIMV rate.