Bedside Assessment of the Respiratory System During Invasive Mechanical Ventilation

Summary

The article provides an in-depth review of bedside methods for assessing respiratory mechanics in patients receiving invasive mechanical ventilation. It emphasizes physiological principles guiding clinical management and differentiates between extensive and intensive ventilatory parameters. Detailed methods for evaluating passive patients and those making spontaneous respiratory efforts are outlined, aiming to minimize ventilator-induced lung injury (VILI) and diaphragm dysfunction through precise and tailored ventilatory strategies.

Assessment of the inspiratory phase. Volume control with constant (square) flow: the first breath describes the static conditions of an inspiratory hold required to assess driving and resistive pressures. Notably, the conductive pressure equals the airway resistive pressure, indicating the absence of airway closure; the second and third breaths describe dynamic conditions with negative and positive stress indexes, respectively. Pressure control with decelerating flow: both breaths describe the static conditions of an inspiratory hold. However, in the first one the flow reaches zero before the hold, therefore peak and plateau pressures are synonymous, while a resistive decay is appreciated in the second breath as the flow is still positive when the hold is performed. The green “Partitioning” box describes how to convert airway into transpulmonary pressures with the “elastance derived” method using oesophageal pressures. Paw: airway pressure, Ppeak: peak inspiratory pressure, Pres(A): airway resistive pressure, Pres(T): tissue resistive pressure, ∆P: driving pressure, Pplat: plateau pressure, Pcond: conductive pressure, PEEPset: set positive end-expiratory pressure, Pes: oesophageal pressure, Pespeak: peak inspiratory oesophageal pressure, Pesplat: plateau oeasophageal pressure, ∆Pes: driving oesophageal pressure, Ers: respiratory system elastance, Cstat: static respiratory system compliance; Vt: tidal volume; Ecw: chest wall elastance, EL: lung elastance, ER: elastance ratio, PL: inspiratory transpulmonary pressure, ∆PL: driving transpulmonary pressure.

Assessment of the expiratory phase. The first two breaths describe a decremental PEEP trial (from 15 to 5 cm H2O) to calculate the recruitment to inflation ratio in volume control ventilation with constant (square) flow. Note that the second breath shows the presence of airway opening pressure and of intrinsic PEEP, the latter visualised during an expiratory hold. The driving pressure is calculated from the airway opening pressure, being higher than the set and total PEEP. Airway opening pressure can be assessed with three methods: (1) AOP is detected when Pcond is significantly higher than Pres. The AOP value is defined as: AOP = PEEP + (Pcond − Pres); (2) as the change in slope in a low-flow (<10 L/min) pressure–time curve (third breath); (3) as the beginning of inflation during the corresponding low-flow pressure–volume curve. Paw: airway pressure, Vol: volume, ∆Prec: change in PEEP during a decremental PEEP trial; Pres(A): airway resistive pressure, Pcond: conductive pressure, AOP: airway opening pressure, ∆Plow: driving pressure at the lower PEEP, PEEPset: set positive end-expiratory pressure, PEEPi: intrinsic positive end-expiratory pressure, PEEPtot: total positive end-expiratory pressure, Vrel: release volume during a decremental PEEP trial, Vt: tidal volume, Vinfl: PEEP-induced inflation volume, Vrec: PEEP-induced recruited volume; FRC: functional residual capacity, Cstat(low): static respiratory system compliance at the lower PEEP, Crec: compliance of the recruited volume, R/I: recruitment to inflation ratio, LIP: lower inflection point, UIP: upper inflection point; MaxD: maximal distance between inspiratory and expiratory limb of a low-flow pressure–volume loop, Vmax: maximum volume inflated during a low-flow manoeuvre.

Key Points:

Ventilatory Variables (Extensive vs. Intensive): Ventilatory variables are classified as extensive (dependent on lung size, e.g., tidal volume) or intensive (independent of lung size, e.g., driving pressure), guiding clinicians in minimizing lung injury by normalizing ventilatory parameters to lung size.
Inspiratory Phase Assessment: Static conditions during inspiratory holds allow calculation of key parameters such as driving pressure (∆P), plateau pressure, airway resistance, and tissue resistance, which are crucial for setting safe ventilation targets.
Dynamic Conditions and Stress Index: Dynamic inspiratory waveform analysis (stress index) indicates potential tidal recruitment or alveolar overdistension. A stress index around 1 suggests optimal ventilatory conditions without harmful alveolar collapse or hyperinflation.
Expiratory Phase and Airway Closure: Expiratory phase assessment includes detection of intrinsic positive end-expiratory pressure (PEEPi) and airway opening pressure (AOP), influencing correct PEEP settings and preventing unnecessary alveolar collapse or hyperinflation.
Recruitment and Inflation Effects: Distinguishing between PEEP-induced recruited volume (beneficial) and inflation volume (potentially harmful) helps clinicians optimize PEEP levels. Methods include best compliance evaluation, recruitment-to-inflation (R/I) ratio, and low-flow pressure–volume loops.
Mechanical Power (MP): MP, representing the total energy delivered per minute, highlights how respiratory rate and inspiratory flow affect lung injury. High MP, even with low tidal volumes, can significantly increase VILI risk.
Chest Wall Contribution: Differentiating between chest wall and lung mechanics through esophageal pressure measurements helps accurately interpret lung stress and strain, especially important when abnormal chest wall elastance is present.
Neural Respiratory Drive Assessment: Reliable evaluation of respiratory drive includes monitoring respiratory rate, subjective dyspnea assessments, blood gases, and objective ventilator-derived measures like occlusion pressure (P0.1).
Diaphragm Function and Dysfunction: The diaphragm’s function can be assessed clinically and via ultrasound measurements of diaphragmatic thickening fraction. Dysfunction is common in critically ill patients, necessitating targeted assessment and prevention strategies to avoid diaphragm injury.
Total Respiratory Effort and Patient–Ventilator Interaction: Accurately assessing respiratory effort involves measuring muscular pressure (Pmus) invasively (esophageal pressure changes) or estimating it through non-invasive methods (occlusion maneuvers, pressure muscle index), crucial for identifying potentially injurious patient self-inflicted lung injury (P-SILI).

Mechanical power in volume and pressure control ventilation. Note the different shapes of the dynamic pressure–volume curves under volume control with constant (square) flow and pressure control with decelerating flow. The greater resistive work (pink area) in pressure control ventilation is due to the higher initial flows. To highlight this phenomenon, we have assumed here that tissue resistances play a significant role only in pressure control ventilation, although they certainly (but at a lower level) exist in volume control ventilation as well. The green and yellow areas represent the elastic work due to PEEP and driving pressure and are not different in the two modes of ventilation. The sum of the yellow, green, and pink areas describes the inspiratory work, from which the mechanical power is currently calculated. Conversely, the area (pink + green) enclosed by the inspiratory and expiratory limbs (solid black lines) represents the hysteresis area, indicating the dissipated energy that remains in the parenchyma after a whole breath. Please note that the pressure–volume curve describing mechanical power is.

Conclusion

Proper bedside assessment of respiratory mechanics during invasive mechanical ventilation is crucial to preventing lung and diaphragm injury. Accurate measurement and interpretation of ventilatory parameters, appropriate differentiation between lung and chest wall contributions, and effective integration of spontaneous respiratory efforts into ventilatory strategies are key to achieving protective ventilation and optimal patient outcomes.

Discussion Questions:

How can bedside measurements of ventilatory mechanics be practically integrated into daily clinical workflow to reliably guide ventilator?
In what clinical scenarios should invasive measures such as esophageal pressure monitoring be prioritized to optimize ventilatory settings and prevent lung injury?
Given the risks associated with excessive patient effort, what strategies can clinicians employ to effectively balance spontaneous respiratory efforts and ventilatory support in critically ill patients?

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