Assessing breathing effort in mechanical ventilation: physiology and clinical implications

Abstract

Recent studies have shown both beneficial and detrimental effects of patient breathing effort in mechanical ventilation. Quantification of breathing effort may allow the clinician to titrate ventilator support to physiological levels of respiratory muscle activity. In this review we will describe the physiological background and methodological issues of the most frequently used methods to quantify breathing effort, including esophageal pressure measurement, the work of breathing, the pressure-time-product, electromyography and ultrasound. We will also discuss the level of breathing effort that may be considered optimal during mechanical ventilation at different stages of critical illness.

Keywords: Breathing effort; critical illness; diaphragm; mechanical ventilation; monitoring.

Figure 1

Schematic representation of the respiratory system and relevant pressures. Note the single catheter equipped with an esophageal pressure balloon, gastric pressure balloon and an electrode array in between the two balloons. Orange lines represent the phrenic nerves. For calculations of the pressure gradients refer to the text. Pao, airway opening pressure; Paw, airway pressure; Pbs, pressure at body surface; Palv, alveolar pressure; Ppl, pleural pressure; Pes, esophageal pressure; Pga, gastric pressure.

Figure 2

Pressure-based assessment of breathing effort during the inspiratory phase. Dashed lines represent moments of zero flow. (A) Flow waveform; (B) gastric pressure (Pga) tracings; (C) transdiaphragmatic pressure (Pdi), calculated as Pga − Pes. Green hatched area is the pressure-time-product (PTP) of the diaphragm during inspiration. Notice the presence of Pdi before onset of inspiratory flow, a sign of intrinsic PEEP (PEEPi); (D) esophageal pressure (Pes) tracings. The compliance of the chest wall (Ccw), estimated at 4% of VC, has been superimposed on Pes at the onset of the fall in Pes and at onset of inspiratory flow generation (first vertical line), together with the dynamic lung compliance (C_L,dyn). The colored area compromises the total PTP of respiratory muscle pressure (Pmus). The red area is the PTP attributed to PEEPi, the gray area represents elastic PTP and the blue area represents resistive PTP; (E) pressure-volume curve of Pes and lung volume. The Ccw and C_L,dyn intersect at FRC. The red area represents WOB attributed to PEEPi, the gray area represents elastic WOB and the blue area represent resistive WOB. PEEP, positive end-expiratory pressure; FRC, functional residual capacity; VC, vital capacity.