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04-03-2024 | Editorial

Measuring patient’s effort on the ventilator

Authors: Rodrigo Cornejo, Irene Telias, Laurent Brochard

Published in: Intensive Care Medicine | Issue 4/2024

Excerpt

Allowing spontaneous breathing with optimal inspiratory effort should be a priority in patients with acute respiratory failure under mechanical ventilation. Low and absent effort promote diaphragmatic dysfunction, creating difficulties in weaning, but also atelectasis and hypoxemia, while intense inspiratory efforts generate negative alveolar pressures and can induce lung (i.e., patient self-inflicted lung injury—P-SILI) and diaphragmatic injury (myotrauma) [1]. Low effort even occurs despite apparently spontaneous “triggered” breaths, because assisted ventilation deliver minimal minute ventilation once triggered. The transition from controlled to assisted ventilation can be associated with vigorous efforts in patients with hypoxemic failure, difficult to control [2]. In addition, lung injury and systemic inflammation are primers for P-SILI and myotrauma with excessive concentric or eccentric loading [1, 3]. Mechanisms of P-SILI include excessive global and regional lung stress (due to pendelluft) and increased transvascular pressure, and are directly associated with the magnitude and timing of inspiratory effort [3, 4]. There is also evidence of interaction between lung and other organs such as brain and kidneys. Therefore, monitoring respiratory drive and effort to adjust ventilatory settings and sedation seems necessary to achieve lung- and diaphragm-protective targets that should result in better clinical outcomes. This review describes the tools and the different parameters for monitoring respiratory effort, and suggests targets for respiratory drive and effort (Fig. 1).

Fig. 1

Invasive and non-invasive monitoring of inspiratory effort and respiratory drive. Figure 1 shows representative traces from a mechanically ventilated ARDS patient resuming spontaneous breathing under pressure support ventilation (pressure support, 10 cmH₂O; PEEP, 12 cmH₂O; Flow-trigger sensitivity, 2 LPM). From top to bottom: flow, airway pressure [Paw], esophageal pressure [Pes] and transdiaphragmatic pressure [Pdi]). Two types of effort-related measurements are depicted: esophageal-pressure-derived measurements and non-invasive measurements. For invasive monitoring, esophageal pressure signal is needed. In panel A, the esophageal pressure swing (ΔPes), which represents the magnitude of inspiratory effort, is -13.8 cmH₂O in the representative cycle. Muscular pressure (Pmus) generated by all the respiratory muscles is 19.7 cmH₂O, calculated as the difference between chest wall elastic recoil pressure (Pcw = 5.9 cmH₂O) and ΔPes during inspiration. Pcw is the product of tidal volume (V_T = 578 ml in this cycle) and chest wall elastance (Ecw = 10.2 cmH₂O/L). The integral of Pmus during inspiration corresponds to the esophageal pressure–time product or PTP which is 10 cmH₂O*s in the same cycle. When gastric pressure (Pga) signal is also available (not shown in the Figure), the difference between Pga and Pes represents the transdiaphragmatic pressure (Pdi is 15.1 cmH₂O in the same cycle). For non-invasive effort monitoring, inspiratory hold (Panel A) and end-expiratory occlusion (B) maneuvers are needed. During an inspiratory hold, (i.e., zero flow), a plateau Paw greater than the peak inspiratory pressure is observed, which accounts for the inspiratory effort. The difference between the plateau pressure and the peak inspiratory pressure is called pressure muscle index (PMI) and correlates with effort magnitude (in this example, PMI = 5.2 cmH₂O). One should be aware that the activation of expiratory muscles during the inspiratory hold does not allow a reliable measurement of plateau Paw. On the other hand, during an end-expiratory occlusion, two variables may be obtained from the airway pressure signal: P_0.1, which is the negative pressure measured during the first 100 ms of occlusion (in this cycle, – 3 cmH₂O); and the occlusion pressure (Pocc), defined as the swing in airway pressure generated by respiratory muscle effort when the airway is briefly occluded (in the representative cycle, – 24.8 cmH₂O). Pocc allows to estimate muscular pressure (with the formula Pmus = – 0.75 × Pocc). Thus, estimated Pmus is 18.6 cmH₂O. This Pmus value is concordant with the average (18.6 cmH₂O) between the Pmus values directly measured using ΔPes and Pcw in panel A and B (19.7 and 17.5 cmH₂O, respectively). Please note that the values of ΔPes and ΔPaw are similar during the end-expiratory occlusion test

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Title: Measuring patient’s effort on the ventilator
Authors: Rodrigo Cornejo
Irene Telias
Laurent Brochard
Publication date: 04-03-2024
Publisher: Springer Berlin Heidelberg
Published in: Intensive Care Medicine / Issue 4/2024
Print ISSN: 0342-4642
Electronic ISSN: 1432-1238
DOI: https://doi.org/10.1007/s00134-024-07352-4

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