What derived variables should be monitored during mechanical ventilation?

Abstract

Without a careful definition, it is very difficult to propose a list of essential derived variables that should be monitored during mechanical ventilation. The list of essentials will vary not only with disease type and severity but also with the expertise of the operator in interpreting the data, and willingness to incorporate it into his/her surveillance and treatment plan. It can be cogently argued that the only variables of crucial significance to the vast majority of patients are the primary ones--airway pressure, flow, tidal volume, and minute ventilation. My own view is that end-inspiratory (PD, Ps, and Pz), end-expiratory (total PEEP), and mean airway pressures must be checked at frequent intervals, especially in ARDS. Partitioning of the total pressure into its flow-driving and elastance-counterbalancing components is always wise, whether or not resistance and compliance or elastance are formally calculated. Incremental changes in the pressure-volume relationship should be monitored whenever adjustments in PEEP or VT are made. Ventilatory demand, strength, and power-reserve assessment are often instrumental in the care of the ventilator-dependent patient who presents as a weaning problem. The most valuable indicators of these include the VE, the maximum voluntary inspiratory pressure, and the frequency-to-tidal-volume ratio. Measurements of the work of breathing, P0.1, and Pes should be reserved for unusually difficult clinical questions. Finally, the variability of the Paw tracing yields valuable data regarding the synchrony of patient-ventilator interactions.