Vent Waveforms

Vent Waveforms

The Paw waveform The interactions between a ventilator and a relaxed intubated patient can be modeled as a piston connected to a tube (flow-resistive element) and balloon (elastic element). Accordingly, at any instant in time (t), the pressure at the tube inlet reflects the sum of a resistive pressure (Pres) and an elastic pressure (Pel) [1]. Pres is determined by the product of tube resistance with V̇, while Pel is determined by the product of balloon elastance (a measure of balloon stiffness) with volume [1]. In this model, the resistive element reflects the properties of the intubated airways, while the elastic element reflects those of lungs and chest wall. When applied to volume preset ventilation with constant inspiratory V̇ and a short post-inflation pause, the resulting Paw tracing has three distinct components: (1) an initial step change proportional to Pres; (2) a ramp that reflects the increase in Pel as the lungs fill to their end-inflation volume; and (3) a sudden decay from a pressure maximum (Ppeak) to a plateau (Pplat) that reflects the elastic recoil (Pel) of the relaxed respiratory system at the volume at end-inflation. Since in this example flow is held constant throughout inflation, Pres must remain constant unless flow resistance changes volume and time. Consequently, the initial step change in Paw and its decay from Ppeak to Pplat are of similar magnitude. Fig. 1a demonstrates these features. Since, in pneumatic systems, there are invariable delays in the pressure and flow transients, in practice the step changes in pressure are never as sudden as they are depicted in Fig. 1a [2]. Nevertheless, the amplitude of transients can be easily estimated by extrapolating the tracing relative to the slope of the pressure ramp. Finally, while the principles that govern the interactions between pressure, volume and flow apply to all modes of mechanical ventilation, the specific pressure waveforms depicted in Fig. 1 refer only to constant flow inflation (square wave) and look very different when other flow profiles (e.g., decelerating, sine wave) are used. Our use of square wave profiles in Fig. 1 should not be interpreted as an endorsement of a specific mode, but rather as the most convenient means to present this information. Fig. 1  Schematic illustration of the Paw profile with time during constant-flow, volume-cycle ventilation. a Passive respiratory system with normal elastance and resistance. Work to overcome the resistive forces is represented by the black shaded area, and the gray shaded area represents the work to overcome the elastic forces. b Up-sloping of the Paw tracing representing increased respiratory system elastance. c Paw tracing in the presence of inadvertent PEEP. d scalloping of the Paw tracing generated by a large patient effort (Paw airway pressure, Pel elastic pressure, Ppeak pressure maximum, Pplat pressure plateau, PEEPi inadvertent PEEP, Pres resistive pressure)

The tracing in Fig. 1b differs in several important respects: the Paw ramp is steeper and it is nonlinear with respect to time. Since V̇ is constant the nonlinearity between Paw and t means that the relationship between Paw and V must be nonlinear as well. Assuming identical ventilator settings as in Fig. 1a the increased steepness of the ramp and its convexity to the time axis indicates a stiffening of the respiratory system with volume and time and suggests that the lungs may be overinflated to volumes near or exceeding their capacity. At the bedside, such an observation should raise concern for injurious ventilator settings [2].

The tracing in Fig. 1c is characterized by a larger-than-expected initial step change in Paw that exceeds the peak-to-plateau pressure difference. In an otherwise relaxed patient, such an observation should raise suspicion for dynamic hyperinflation and inadvertent PEEP (PEEPi). If Pel at end-expiration is greater than Paw at that time (i.e., PEEPi is present), then gas will flow in the expiratory direction. The step change in Paw during the subsequent inflation will therefore not only reflect Pres but also PEEPi that must be overcome to reverse flow at the tube entrance [1]. Tracings like the one in Fig. 1c should therefore alert the clinician to the presence of dynamic hyperinflation and provide an estimate of the extrinsic PEEP necessary to minimize the associated work of breathing. PEEPi is invariably associated with a sudden transient in expiratory flow prior to ventilator-assisted lung inflation [3]. However, this flow transient need not be associated with dynamic hyperinflation, because it is also seen in patients with increased respiratory effort and active expiration.

The tracing in Fig. 1d represents a significant departure from relaxation patters. There is no initial step change in Paw; the ramp is nonlinear, and the end-inspiratory pressure plateau is lower than expected. This tracing suggests that the inspiratory muscles are active throughout machine inflation and that their work represents a considerable fraction of the work performed on the respiratory system. This pattern should alert clinicians to the presence of a potentially fatiguing load.

References

1. de Chazal I, Hubmayr RD (2003) Novel aspects of pulmonary mechanics in intensive care. Br J Anaesth 91: 81–91   2. Grasso S, Terragni P, Mascia L, Fanelli V, Quintel M, Herrmann P, Hedenstierna G, Slutsky AS, Ranieri VM (2004) Airway pressure-time curve profile (stress index) detects tidal recruitment/hyperinflation in experimental acute lung injury. Crit Care Med 32:1018–1027   3. Brochard L (2002) Intrinsic (or auto-) PEEP during controlled mechanical ventilation. Intensive Care Med. 28:1376–1378

(Intens Care Med 2006;32:658)

Static PV Curves

review (Crit Care 2000;4:91)

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