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\t \t\tThe Paw waveform \t \t\t \t\t\tThe interactions between a\u00a0ventilator and a\u00a0relaxed intubated \t\t\tpatient can be modeled as a\u00a0piston connected to a\u00a0tube \t\t\t(flow-resistive element) and balloon (elastic element). Accordingly, \t\t\tat any instant in time (t<\/i>), the pressure at the tube inlet \t\t\treflects the sum of a\u00a0resistive pressure (Pres<\/i>) and an \t\t\telastic pressure (Pel<\/i>)\u00a0[1<\/a><\/cite>]. \t\t\tPres<\/i> is determined by the product of tube resistance with \t\t\tV\u0307<\/i>, while Pel<\/i> is determined by the product of balloon \t\t\telastance (a\u00a0measure of balloon stiffness) with volume\u00a0[1<\/a><\/cite>]. \t\t\tIn this model, the resistive element reflects the properties of the \t\t\tintubated airways, while the elastic element reflects those of lungs \t\t\tand chest wall. When applied to volume preset ventilation with \t\t\tconstant inspiratory V\u0307<\/i> and a\u00a0short post-inflation pause, the \t\t\tresulting Paw<\/i> tracing has three distinct components: (1) an \t\t\tinitial step change proportional to Pres<\/i>; (2) a\u00a0ramp that \t\t\treflects the increase in Pel<\/i> as the lungs fill to their \t\t\tend-inflation volume; and (3) a\u00a0sudden decay from a\u00a0pressure maximum \t\t\t(Ppeak<\/i>) to a\u00a0plateau (Pplat<\/i>) that reflects the elastic \t\t\trecoil (Pel<\/i>) of the relaxed respiratory system at the volume \t\t\tat end-inflation. Since in this example flow is held constant \t\t\tthroughout inflation, Pres<\/i> must remain constant unless flow \t\t\tresistance changes volume and time. Consequently, the initial step \t\t\tchange in Paw<\/i> and its decay from Ppeak<\/i> to Pplat<\/i> \t\t\tare of similar magnitude. Fig.\u00a01<\/a>a demonstrates \t\t\tthese features. Since, in pneumatic systems, there are invariable \t\t\tdelays in the pressure and flow transients, in practice the step \t\t\tchanges in pressure are never as sudden as they are depicted in \t\t\tFig.\u00a01<\/a>a\u00a0[2<\/a><\/cite>]. \t\t\tNevertheless, the amplitude of transients can be easily estimated by \t\t\textrapolating the tracing relative to the slope of the pressure \t\t\tramp. Finally, while the principles that govern the interactions \t\t\tbetween pressure, volume and flow apply to all modes of mechanical \t\t\tventilation, the specific pressure waveforms depicted in Fig.\u00a01<\/a> \t\t\trefer only to constant flow inflation (square wave) and look very \t\t\tdifferent when other flow profiles (e.g., decelerating, sine wave) \t\t\tare used. Our use of square wave profiles in Fig.\u00a01<\/a> \t\t\tshould not be interpreted as an endorsement of a\u00a0specific mode, but \t\t\trather as the most convenient means to present this information. \t\t\t \t\t\t\t \t\t\t \t\t\t\tFig.\u00a01\u00a0 Schematic illustration of \t\t\t\tthe Paw profile with time during constant-flow, volume-cycle \t\t\t\tventilation. a<\/b> Passive respiratory system with normal \t\t\t\telastance and resistance. Work to overcome the resistive forces \t\t\t\tis represented by the black shaded area<\/i>, and the gray \t\t\t\tshaded area<\/i> represents the work to overcome the elastic \t\t\t\tforces. b<\/b> Up-sloping of the Paw<\/i> tracing<\/i> \t\t\t\trepresenting increased respiratory system elastance. c<\/b> \t\t\t\tPaw tracing<\/i> in the presence of inadvertent PEEP. d<\/b> \t\t\t\tscalloping of the Paw<\/i> tracing<\/i> generated by a large \t\t\t\tpatient effort (Paw<\/i> airway pressure, Pel<\/i> elastic \t\t\t\tpressure, Ppeak<\/i> pressure maximum, Pplat<\/i> pressure \t\t\t\tplateau, PEEPi<\/i> inadvertent PEEP, Pres<\/i> resistive \t\t\t\tpressure) \t\t\t \t\t\t \t \t<\/p>\n The tracing in Fig.\u00a01<\/a>b differs in several \timportant respects: the Paw<\/i> ramp is steeper and it is nonlinear with \trespect to time. Since V\u0307<\/i> is constant the nonlinearity between Paw<\/i> \tand t<\/i> means that the relationship between Paw<\/i> and V<\/i> \tmust be nonlinear as well. Assuming identical ventilator settings as in \tFig.\u00a01<\/a>a\u00a0the increased steepness of the ramp and its \tconvexity to the time axis indicates a\u00a0stiffening of the respiratory system \twith volume and time and suggests that the lungs may be overinflated to \tvolumes near or exceeding their capacity. At the bedside, such an \tobservation should raise concern for injurious ventilator settings\u00a0[2<\/a><\/cite>]. \t<\/p>\n The tracing in Fig.\u00a01<\/a>c is characterized by \ta\u00a0larger-than-expected initial step change in Paw<\/i> that exceeds the \tpeak-to-plateau pressure difference. In an otherwise relaxed patient, such \tan observation should raise suspicion for dynamic hyperinflation and \tinadvertent PEEP (PEEPi<\/i>). If Pel<\/i> at end-expiration is greater \tthan Paw<\/i> at that time (i.e., PEEPi<\/i> is present), then gas will \tflow in the expiratory direction. The step change in Paw<\/i> during the \tsubsequent inflation will therefore not only reflect Pres<\/i> but also \tPEEPi<\/i> that must be overcome to reverse flow at the tube entrance\u00a0[1<\/a><\/cite>]. \tTracings like the one in Fig.\u00a01<\/a>c should therefore alert \tthe clinician to the presence of dynamic hyperinflation and provide an \testimate of the extrinsic PEEP necessary to minimize the associated work of \tbreathing. PEEPi<\/i> is invariably associated with a\u00a0sudden transient in \texpiratory flow prior to ventilator-assisted lung inflation\u00a0[3<\/a><\/cite>]. \tHowever, this flow transient need not be associated with dynamic \thyperinflation, because it is also seen in patients with increased \trespiratory effort and active expiration. <\/p>\n