Chest wall mechanics during pressure support ventilation

Abstract

Introduction: During pressure support ventilation (PSV) a part of the breathing pattern is controlled by the patient, and synchronization of respiratory muscle action and the resulting chest wall kinematics is a valid indicator of the patient's adaptation to the ventilator. The aim of the present study was to analyze the effects of different PSV settings on ventilatory pattern, total and compartmental chest wall kinematics and dynamics, muscle pressures and work of breathing in patients with acute lung injury.

Method: In nine patients four different levels of PSV (5, 10, 15 and 25 cmH2O) were randomly applied with the same level of positive end-expiratory pressure (10 cmH2O). Flow, airway opening, and oesophageal and gastric pressures were measured, and volume variations for the entire chest wall, the ribcage and abdominal compartments were recorded by opto-electronic plethysmography. The pressure and the work generated by the diaphragm, rib cage and abdominal muscles were determined using dynamic pressure-volume loops in the various phases of each respiratory cycle: pre-triggering, post-triggering with the patient's effort combining with the action of the ventilator, pressurization and expiration. The complete breathing pattern was measured and correlated with chest wall kinematics and dynamics.

Results: At the various levels of pressure support applied, minute ventilation was constant, with large variations in breathing frequency/ tidal volume ratio. At pressure support levels below 15 cmH2O the following increased: the pressure developed by the inspiratory muscles, the contribution of the rib cage compartment to the total tidal volume, the phase shift between rib cage and abdominal compartments, the post-inspiratory action of the inspiratory rib cage muscles, and the expiratory muscle activity.

Conclusion: During PSV, the ventilatory pattern is very different at different levels of pressure support; in patients with acute lung injury pressure support greater than 10 cmH2O permits homogeneous recruitment of respiratory muscles, with resulting synchronous thoraco-abdominal expansion.

Figure 1

Experimental tracings obtained during a breath from patient receiving PSV (pressure support 5 cmH₂O). Time t₀ is defined as where Pes starts to decrease; t₁ is the onset of inspiratory flow; t₂ is where Pes starts to increase; and t₃ is the end of inspiration. Vab, abdominal volume; Vcw, chest wall volume; Vrc, a, volume of the abdominal rib cage; Vrc, p, volume of the pulmonary rib cage; Paw, airway pressure; Pdi, transdiaphragmatic pressure; Pes, oesophageal pressure; Pga, gastric pressure.

Figure 2

Relationship between V_t and respiratory rate. Shown is the relationship between V_t and respiratory rate (f) in the patients at different levels of pressure support: 5 cmH₂O (closed circles), 10 cmH₂O (open circles), 15 cmH₂O (closed squares) and 25 cmH₂O (open squares). The straight lines represent isopleths of different values of f/V_t (20, 40, 60, 80 and 100 l^-1·min^-1). The curved line is the fitting of data points by the following equation: f = K/V_t (where K = 7.9385 ± 0.4324). PS, pressure support; Vt, tidal volume.

Figure 3

Relationships between pressure support levels and duration of the various phases of inspiration. The phases (phase 1 [closed circles], phase 2 [open circles] and phase 3 [closed triangles]) are defined in the text. Data are expressed as mean ± standard error of the mean. **P < 0.01, ***P < 0.001, versus pressure support = 5 cmH₂O. °P < 0.05, versus pressure support = 10 cmH₂O.

Figure 4

Pressure-volume dynamic relationship of the total and compartment chest wall. (a) Change in oesophageal pressure (ΔPes) versus chest wall volume changes (ΔVcw). (b) Upper panel: changes in oesophageal pressure (ΔPes) versus pulmonary rib cage volume changes (ΔVrc, p); averaged loops. Middle panel: changes in transdiaphragmatic pressure (ΔPdi) versus abdominal volume changes (ΔVab); averaged loops. Lower panel: changes in gastric pressure (ΔPga) versus abdominal volume changes (ΔVab); Each point represents the mean ± standard error of the mean (i.e. the average of all patients at the different times [see definition in Figure 1]): The loops refer to the different levels of pressure support: 5 cmH₂O (solid thick line), 10 cmH₂O (dased thick line), 15 cmH₂O (solid thin line) and 25 cmH₂O (dashed thin line). The arrows indicate the direction of the loops. The symbols in (b) are t₀ (closed circles), t₁ (open squares), t₂ (open triangles) and t₃ (open circles).

Figure 5

Relationship between pressure support level and muscle pressure, chest wall volume distribution and synchronization. (a) Mean ± standard error of the mean (SEM) values of transdiaphragmatic pressure (Pdi; closed symbols) and rib cage muscle pressure (Prcm; open symbols) at different levels of pressure support. **P < 0.01,***P < 0.001, versus pressure support at 5 cmH₂O. °P < 0.01, versus pressure support at 10 cmH₂O. P < 0.05, versus Prcm. (b) Mean ± SEM values of percentage contribution to tidal volume of abdomen (Vab; closed symbols) and rib cage (Vrc; open symbols) at different levels of pressure support. *P < 0.05, versus Pressure support at 5 cmH₂O. (c) Mean ± SEM values of absolute values of phase shift between rib cage (RC) and abdomen (AB) at different levels of pressure support. **P < 0.01, versus pressure support at 25 cmH₂O. °°P < 0.01 versus pressure support at 15 cmH₂O.