Comparison of Dynamic Mechanical Power Formula With Geometric Method in Pressure- and Volume-Controlled Ventilation: A Validation Study

Mechanical power (MP) is an integrative parameter that quantifies the energy delivered to the lungs during mechanical ventilation. It accounts for multiple factors contributing to ventilator-induced lung injury (VILI), including tidal volume, airway pressures, respiratory rate, compliance, and resistance. MP has been shown to correlate strongly with morbidity and mortality in critically ill patients, particularly those with acute respiratory distress syndrome (ARDS).

Several MP calculation formulas have been proposed for volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV). While formulas such as MPrs (Gattinoni) and MPLM (Trinkle) are widely used, many of them require precise measurement of inspiratory resistance (Raw), which is often not readily accessible at the bedside-especially in PCV. To address this, a dynamic mechanical power formula (MPdyn) was developed by Asar et al., which estimates MP without the need for inspiratory resistance. MPdyn has demonstrated good agreement with MPrs in VCV and with MPLM in PCV. However, it has not been directly validated against the geometric method, which remains the gold standard for MP calculation.

The geometric method involves calculating the area enclosed by the pressure-volume (P-V) loop on the ventilator screen. This area represents the energy per breath, which can be multiplied by respiratory rate to yield MP. Modern ventilators, such as the Maquet Servo-U, provide high-resolution graphical displays that allow for precise area calculations using digital image processing.

This prospective observational study was conducted in the intensive care unit of Bakırköy Dr. Sadi Konuk Training and Research Hospital, Istanbul. A total of 37 ARDS patients who were deeply sedated and under controlled mechanical ventilation were enrolled. Each patient was ventilated in both VCV and PCV modes with two different I:E ratios (1:2 and 1:1). For each setting, three screenshots of the P-V loop were captured, resulting in 12 images per patient and 444 images in total.

Geometric mechanical power (MPgeo) was calculated using Python-based image analysis (OpenCV, NumPy, and PIL libraries). The region of interest (ROI) corresponding to the P-V loop was extracted, binarized, and segmented to compute the loop area. This pixel-based area was then scaled using the known tidal volume (TVe) and pressure (Pinsp) values to obtain mechanical energy per breath. MPgeo was calculated as:

MPgeo (J/min) = [E_breath × RR] + [PEEP × TVe × RR × 0.098]

Dynamic mechanical power (MPdyn) was calculated using the following formula:

MPdyn = MVe × [WOBv + (PEEP × 0.098)] where MVe is the expiratory minute volume (L/min), WOBv is the work of breathing per liter (J/L), and PEEP is the end-expiratory pressure (cmH₂O).

Statistical analysis included descriptive statistics, Shapiro-Wilk test for normality, univariable linear regression to assess correlation between MPdyn and MPgeo, and Bland-Altman analysis to evaluate agreement between methods. Paired measurements were analyzed both at the patient-averaged level and for all individual data points.

The primary outcome of the study was the degree of agreement between MPdyn and MPgeo under different ventilator modes (VCV, PCV) and I:E ratios (1:1, 1:2). Secondary analyses included evaluating potential biases and limits of agreement.

This study is expected to validate MPdyn as a reliable, practical alternative to geometric calculations in clinical settings, facilitating routine bedside monitoring of mechanical power without requiring complex measurements or advanced equipment.

Inclusion criteria

• Age ≥ 18 years
• Diagnosis of moderate-to-severe ARDS according to the Berlin criteria
• Receiving mechanical ventilation in volume-controlled (VCV) or pressure-controlled (PCV) mode
• Stable hemodynamics during data collection
• Expected to remain under mechanical ventilation for at least 48 hours
• Written informed consent from legal representative

Exclusion criteria

• Presence of chest tube or pneumothorax
• Known neuromuscular disease affecting respiratory function
• Pregnancy
• Patients with do-not-resuscitate (DNR) or limitation-of-therapy orders
• Incomplete ventilator data or poor-quality screenshots