Monitoring Patient Respiratory Effort During Mechanical Ventilation: Lung and Diaphragm-Protective Ventilation

P_es can be used as a surrogate measure of P_pl, bearing in mind regional variations [20]. It can therefore be used to measure P_L (P_aw − P_pl), by substituting P_pl with P_es. As shown in Fig. 1, P_L can easily reach an injuriously high value during assisted mechanical ventilation (where both patient and ventilator distend the lung). An acceptable upper limit for P_L has not yet been defined; a “precautionary” peak inspiratory value of 20 cmH₂O in a lung-injured patient is a reasonable target to limit the risk of injury [2, 21].

Of note, in the presence of regional ventilation heterogeneity and pendelluft, the measured value of P_L will underestimate lung stress in the dependent lung areas. While the quasi-static plateau P_L obtained during an end-inspiratory occlusion reflects lung stress during passive ventilation, the dynamic swing in P_L (ΔP_L) may perhaps be more reflective of injury risk during spontaneous breathing because of the pendelluft phenomenon [22]. ΔP_L likely reflects the upper limit of mechanical stress experienced in dorsal regions of the lung under dynamic conditions [23]. Moreover, various lines of evidence suggest that the dynamic (tidal increase) in lung stress is a more important driver of lung injury than the global (peak) lung stress [24‐26].

P_es permits measurement of inspiratory effort. The inspiratory muscle pressure (P_mus) corresponds to the global force generated by the inspiratory muscles. Although the diaphragm is the most important respiratory muscle, accessory inspiratory muscles (rib cage, sternomastoid, and scalene muscles) contribute significantly during vigorous effort, especially when diaphragm function is impaired. As shown in Fig. 3, P_mus is computed from the difference between P_es and the additional pressure required to overcome the chest wall elastic recoil (P_cw) (P_mus = P_cw − P_es).

Optimal levels of P_mus during assisted mechanical ventilation are uncertain; recent data suggest that P_mus values similar to those of healthy subjects breathing at rest may be safe and may prevent diaphragm atrophy (5–10 cmH₂O) [4, 27]. In routine clinical practice, one can generally disregard the correction for P_cw because chest wall elastance is usually relatively low (even when pleural pressures are elevated). Hence, a target ΔP_es of around 3–8 cmH₂O can be considered reasonably comparable to a normal P_mus of 5–10 cmH₂O.

The gold standard measurement of respiratory effort is the integral of P_mus over the duration of inspiration (pressure-time product [PTP]) (Fig. 3). PTP is closely correlated to inspiratory muscle energy expenditure. PTP values between 50 and 100 cmH₂O/s/min probably reflect appropriate oxygen consumption and acceptable respiratory effort [28].

In routine clinical practice, the magnitude and frequency of the swing in ΔP_es are probably sufficient to monitor respiratory effort.

A double balloon catheter can be used to monitor inspiratory swings in P_es and gastric pressure (P_ga) to specifically quantify the pressure generated by the diaphragm (transdiaphragmatic pressure [P_di]). During an inspiratory effort (depending on the pattern of thoracoabdominal motion), the diaphragm’s contractile effort moves the abdominal organs downwards, increasing abdominal pressure (positive swing in P_ga) and expanding thoracic cavity (negative swing in P_es). Even when thoracoabdominal motion is such that the diaphragm moves upward during inspiration (i.e., P_ga decreases), the contractile effort of the diaphragm is reflected by the fact that P_ga declines less than P_es (and thus P_di increases). This technique is used mainly in research rather than clinical practice.

Brief end-inspiratory occlusion maneuvers are widely used to measure plateau pressure (P_plat) in passive mechanical ventilation. Driving pressure (ΔP), calculated as the difference between PEEP and P_pl, reflects dynamic lung stress and lung injury risk and closely correlates to mortality in patients with ARDS [25]. Bellani et al. [29] suggested that a brief inspiratory occlusion maneuver can enable reliable measurements of P_plat even in assisted mechanical ventilation. During an inspiratory occlusion in assisted mechanical ventilation, patients relax the contracting inspiratory muscles at end-inspiration, resulting in an increase in ΔP_aw, easily detectable on the ventilator waveform. When the patient is over-assisted and respiratory effort is low, P_aw drops during the occlusion (Fig. 4). A high P_plat and ΔP measured in this way raises concern for hyperdistention and lung injury. Bellani and colleagues [29] recently reported that ΔP and compliance measured by end-inspiratory occlusion maneuvers during assisted mechanical ventilation predict mortality, supporting the validity and relevance of these measures.

The measurement technique has some limitations. First, because the pressure is obtained under quasi-static conditions this measurement may underestimate the risk of regional lung injury due to the pendelluft mechanism of P-SILI [23]. Second, clinicians need to carefully evaluate the stability and pattern of the P_aw tracing during the occlusion to determine whether the measurement is confounded by the action of the abdominal muscles which may rapidly increase P_aw at the onset of neural expiration during the occlusion.

Expiratory occlusions are ordinarily employed to measure intrinsic PEEP in passively ventilated patients or to measure maximal inspiratory pressure in spontaneously breathing patients during maximal volitional inspiratory efforts. However, the airway pressure swing during a brief, randomly applied end-expiratory occlusion maneuver (duration equal to one respiratory cycle) may actually be used to assess inspiratory effort. Under occluded conditions, the swing in airway pressure is exactly correlated to the swing in pleural pressure. Consequently, the airway pressure swing during the occlusion (ΔP_occ) can be used to assess the presence and magnitude of pleural pressure swings due to patient respiratory effort (taking into account differences in pleural pressure swing between occluded and dynamic conditions). On this basis, ΔP_occ can be used to predict ΔP_es, P_mus, and ΔP_L during the respiratory cycle so long as the patient’s respiratory drive during the tidal breath is unchanged by a single, brief, and unexpected end-expiratory occlusion [30, 31]. A transient end-expiratory occlusion maneuver is a practical and noninvasive method to routinely detect insufficient or excessive respiratory effort and P_L during assisted mechanical ventilation [32, 33].

The P_0.1 (airway pressure generated in the first 100 ms of inspiration against an expiratory occlusion) provides a measure of the patient’s respiratory drive (Fig. 5) [34]. Whitelaw et al. [35] demonstrated that an occlusion does not modify cortical respiratory output until it is prolonged beyond 200 ms. Additionally, during the first 100 ms, respiratory pressure generation is independent of pulmonary mechanics or diaphragm function [35, 36]. Although the reliability of P_0.1 has been confirmed only in small studies, a value between 1.5 and 3.5 cmH₂O [37, 38] seems to be an easy method to guide clinicians to adjust ventilation during assisted mechanical ventilation [34, 39‐41]. P_0.1 values less than 1.5 cmH₂O might suggest that respiratory effort is inadequate [42], and values greater than 3.5 cmH₂O suggest high respiratory drive [37].

P_0.1 has several advantages: it is easy and practical to obtain, and most modern ventilators have a function for measuring it. A method for setting the pressure support level based on the P_0.1 value has been described [43]. P_0.1 may have substantial intra-patient variability and several repeated measurements are required to estimate a stable mean value. Moreover, in hyperinflated patients, the intrinsic PEEP causes a delay in the fall in P_aw, which might give rise to underestimation of P_0.1. Conti et al. demonstrated that in this condition, commencing the 100 ms for the P_0.1 measure when expiratory flow is equal to zero overcomes this problem [44].

Table 1 summarizes the different methods available to monitor inspiratory effort and respiratory drive in assisted mechanical ventilation, along with possible targets for safe spontaneous breathing as discussed throughout this chapter. The interpretation and application of measurements must always be guided by the clinical context. Different forms and phases of acute respiratory failure require somewhat different priorities: in early ARDS, close attention must be taken to avoid high inspiratory effort to limit VILI and P-SILI. Adjustments to ventilation and sedation to obtain a low level of inspiratory effort should be implemented as early as possible to avoid myotrauma.

It remains uncertain whether it is possible to achieve an acceptable level of respiratory effort during the acute phase of illness and this remains a key area for clinical investigation. For the present, clinicians should strive to be aware of patient respiratory effort and appreciate the potential benefits and harms of manipulating respiratory effort during acute respiratory failure.