Assessment of "Active" Components
When the patient is spontaneously breathing and his/her respiratory muscles are working, irrespective of the work done by the ventilator, P MUS becomes a significant component of Eq. 14.1. Under this condition, the lung-distending pressure is due to both the action of the ventilator and the patient, and the measurement of PES, an estimation of PPL, is needed for an accurate estimation of PTP. In presence of spontaneous breathing efforts while receiving mechanical ventilation, direct measurement of effort may help the clinician to better adjust the ventilator settings and/or the sedation level [47].
In the clinical setting, respiratory muscle effort can be assessed by either assessing the work of breathing (WOB) or by the pressure-time product (PTP), which are both useful approaches to estimate the energy dissipated or consumed by the respiratory muscles [48].
The work performed by the respiratory muscles during each inspiration (i.e., from the beginning of inspiratory flow to the end of inspiration) is expressed as the area enclosed between the dynamic esophageal pressure (PES)-volume (V) curve and the static relaxation curve of the chest wall. This is classically referred as the Campbell diagram [49], in which at any time during inspiration, the pressure developed by the inspiratory muscles in expanding the chest wall (PMUS) is determined as the horizontal distance between the current PES-V point on the diagram and the corresponding point on the chest relaxation curve at the same volume [50]. WOB can be expressed per breathing cycle, per minute (WOB per cycle multiplied by respiratory frequency), or per liter (work per minute divided by minute ventilation).
Respiratory muscle activity can also be assessed using the pressure-time product (PTP) index, which refers to the integral of pressure over time (the cumulative sum of sample-by-sample product of the pressure multiplied by the time difference between two consecutive samples) and not over volume, and, therefore, it can be used whether or not volume is generated. When PTP is calculated using esophageal pressure (PTPES), it provides an index of the effort done by all of the respiratory muscles. When PTP is calculated using transdiaphragmatic pressure (PDI = PGA - PES, where PGA is gastric pressure, that provides an estimation of abdominal pressure) (PTPDI), it reflects the effort performed by the diaphragm.
WOB is usually highly correlated to oxygen consumption [51]. In conditions like isometric contraction, however, WOB is insensitive to energy expenditure, and PTPES or PTPDI has to be considered to estimate respiratory muscle oxygen consumption [52].
Although much less accurate than measurements of PMUS or PDI, in the clinical setting, inspiratory effort may be monitored simply by assessing PES changes during inspiration, in other words not taking into account the static recoil pressure of the relaxed chest wall [53].
