A clinical review examined the invasive and non-invasive monitoring of inspiratory effort and respiratory drive in patients undergoing assisted mechanical ventilation (AMV). Following the acute phase of respiratory failure, AMV is commonly used to support spontaneous breathing, improve gas exchange, and reduce sedation requirements. However, excessive or insufficient patient effort during AMV can contribute to patient self-inflicted lung injury (P-SILI). Accurate assessment of inspiratory effort and respiratory drive is essential to optimise ventilatory support and minimise harm.
The review highlights that inspiratory effort and respiratory drive influence lung stress, diaphragm workload, and patient outcomes. Excessive effort may increase transpulmonary pressure and lung distension, whereas insufficient effort can promote diaphragm disuse and ventilator-induced diaphragm dysfunction. Monitoring these variables allows clinicians to tailor ventilatory assistance and avoid both over- and under-assistance.
Oesophageal pressure measurement is presented as the reference standard for assessing inspiratory effort. When correctly positioned, an oesophageal catheter provides an accurate estimate of pleural pressure. During assisted ventilation, the negative swing in oesophageal pressure during inspiration reflects the intensity of patient effort. From this measurement, global inspiratory muscle pressure (Pmus) can be calculated for precise quantification of inspiratory effort.
Work of breathing (WOB) represents the total energy expended by respiratory muscles and is calculated as the integral of muscle pressure over volume. An alternative but closely related measure is the oesophageal pressure–time product (PTPes), which integrates pressure over time and is particularly useful when inspiratory contractions are partially isometric. Because WOB and PTPes are complex to calculate at the bedside, the oesophageal pressure swing (ΔPes) is commonly used as a practical surrogate. In patients with low chest wall elastance, ΔPes values between 3 and 8 cmH₂O generally correspond to physiological levels of inspiratory effort.
To specifically assess diaphragm function, transdiaphragmatic pressure can be measured using combined oesophageal and gastric pressure monitoring. This technique quantifies the diaphragmatic contribution to inspiratory effort and has informed the concept of lung- and diaphragm-protective ventilation. Although accurate, both oesophageal and transdiaphragmatic pressure monitoring are invasive, require expertise, and may cause patient discomfort.
As a result, several non-invasive indices derived from ventilator pressure and flow signals have been developed. The pressure muscle index (PMI) is obtained during an end-inspiratory pause and reflects the relaxation of inspiratory muscles. Low PMI values are associated with over-assistance, while higher values indicate under-assistance, although its positive predictive value is limited, and measurement artefacts may occur.
Airway occlusion pressure during an end-expiratory pause (ΔPocc) represents the maximal negative pressure generated by the patient. Compared with PMI, ΔPocc shows stronger correlations with oesophageal and transdiaphragmatic pressures and performs better in identifying both low and high inspiratory effort. Suggested thresholds allow clinicians to identify safe ranges of effort, although universally accepted cut-offs are still lacking. In contrast, airway occlusion pressure measured 100 ms after inspiratory onset (P0.1) is more closely related to respiratory drive than total inspiratory effort. P0.1 is relatively independent of respiratory mechanics, but its interpretation may be affected by airway resistance, neuromuscular weakness, and ventilator-specific measurement biases.
Another non-invasive method discussed is the inspiratory flow index, which analyses the shape of the inspiratory flow curve during pressure support ventilation. Increased downward concavity of the flow curve reflects greater patient effort. This index correlates linearly with invasive measures of inspiratory effort and is unaffected by intrinsic PEEP. However, it requires algorithmic analysis and is not yet widely available on standard ventilators.
The review also explores the role of ultrasound in assessing diaphragm function. Diaphragm excursion, diaphragm thickness, and diaphragm thickening fraction (DTf) can be measured non-invasively and provide insight into diaphragm activity and recovery. Among these, DTf has shown the strongest association with weaning success and stable diaphragm function. Values between 15% and 30% are associated with preserved muscle thickness and shorter duration of mechanical ventilation. However, ultrasound measurements are operator-dependent and cannot be performed continuously.
Overall, the authors emphasise that monitoring inspiratory effort and respiratory drive is crucial during assisted mechanical ventilation to reduce the risk of P-SILI and diaphragm dysfunction. While oesophageal and transdiaphragmatic pressure measurements remain the reference standards, multiple non-invasive tools offer reliable alternatives when invasive monitoring is not feasible. These indices are generally well correlated with reference measures and with each other. Their routine clinical use may facilitate personalised ventilatory strategies, although further research is needed to refine safe target ranges and validate their impact on patient outcomes.
Source: Annals of Intensive Care
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