Bedside Assessment and Monitoring of Pulmonary Function and Power of Breathing in the Critically Ill

Work of breathing per minute, or power of breathing (POB), reflects the balance between patient spontaneous breathing demand (driven by metabolic and neural factors) and the support provided by the ventilator. Increases in respiratory muscle loading and, thus, POB result primarily from increased physiologic elastance and resistance. Because compliance is the reciprocal of elastance, as total compliance (lungs and chest wall) decreases, elastic loading of the respiratory muscles increases. The total resistive load is affected by physiologic airways and breathing apparatus resistances. Elastance, resistance, or both can significantly increase the POB or load on the respiratory muscles, predisposing to muscle fatigue (loss of the forcegenerating capacity of the muscles), carbon dioxide retention, and hypoxemia. Ventilatory support may be applied to partially or totally unload respiratory muscles. High levels of ventilatory support totally unload the muscles and, if applied for too long a period, may lead to atrophy. Conversely, too little support risks muscle fatigue. Unfortunately, in either case, the duration of mechanical ventilation may be needlessly prolonged for reconditioning/training if respiratory muscle atrophy is present or to provide needed rest if the muscles are fatigued. Optimization of ventilatory support to each patient’s unique needs requires information of the load on the respiratory muscles as well as gas exchange. This manuscript will focus on POB measurements as my approach to assess the load on the muscles and to provide a quantitative and goal-oriented method for appropriately setting pressure support ventilation (PSV).