Question

Explain airway resistance related to mechanical ventilation settings

Answer 1

Respiratory mechanics refers to the expression of lung function through measures of pressure and flow. From these measurements, a variety of derived indices can be determined, such as volume, compliance, resistance, and work of breathing. Plateau pressure is a measure of end-inspiratory distending pressure. It has become increasingly appreciated that end-inspiratory transpulmonary pressure (stress) might be a better indicator of the potential for lung injury than plateau pressure alone. This has resulted in a resurgence of interest in the use of esophageal manometry in mechanically ventilated patients. End-expiratory transpulmonary pressure might also be useful to guide the setting of PEEP to counterbalance the collapsing effects of the chest wall. The shape of the pressure-time curve might also be useful to guide the setting of PEEP (stress index). This has focused interest in the roles of stress and strain to assess the potential for lung injury during mechanical ventilation. This paper covers both basic and advanced respiratory mechanics during mechanical ventilation.

auto-PEEPchest wallcomplianceesophageal pressuremechanical ventilationplateau pressureresistancerespiratory mechanicswork of breathing

Introduction

Respiratory mechanics refers to the expression of lung function through measures of pressure and flow.1,2 From these measurements, a variety of derived indices can be determined, such as volume, compliance, resistance, and work of breathing (WOB). Waveforms are derived when one of the parameters of respiratory mechanics is plotted as a function of time or as a function of one of the other parameters. This produces scalar tracings of pressure-time, flow-time, and volume-time graphics, as well as flow-volume and pressure-volume (P-V) loops. All current-generation positive-pressure ventilators provide some monitoring of pulmonary mechanics and graphics in real time at the bedside. When interpreting these measurements, it is important to remember that bedside monitoring of mechanics and graphics during positive-pressure ventilation portrays the lungs as a single compartment and assumes a linear response over the range of tidal volume (VT). Although this is a physiologic oversimplification, the information nonetheless is useful to evaluate lung function, assess response to therapy, and optimize mechanical ventilator support. An evaluation of respiratory mechanics allows the best available evidence to be individualized to the patient. By necessity, any discussion of respiratory mechanics involves mathematics. Fortunately, much of the mathematics is basic algebra, and for the most part, I will stick to that in this paper.

Pressure

Airway Pressure

Airway pressure is measured universally during mechanical ventilation. Pressure is measured ideally at the proximal airway, but most ventilators do not because proximal airway pressure monitoring exposes the sensor to secretions and carries other technical issues.3 Alternatively, the ventilator can measure pressure proximal to the expiratory valve during the inspiratory phase to approximate inspiratory proximal airway pressure, and it can measure pressure distal to the inspiratory valve during the expiratory phase to approximate expiratory proximal airway pressure. Because flow in the expiratory limb is zero during the inspiratory phase and flow in the inspiratory limb is zero during the expiratory phase, pressures measured in this manner should approximate proximal airway pressure.

Airway pressure is typically displayed on the ventilator screen as a function of time. The shape of the airway pressure waveform is determined by flow and VT from the ventilator, lung mechanics, and any active breathing efforts of the patient.

Equation of Motion

Airway pressure is predicted mathematically by the equation of motion:

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(1)

where Pvent is the proximal airway pressure applied by the ventilator, Pmus is the pressure generated by the patient's inspiratory muscles, VT is tidal volume, CRS is respiratory system compliance, Raw is airway resistance, V̇I is inspiratory flow, PEEP is the PEEP set on the ventilator, and PEEPi is intrinsic PEEP (auto-PEEP). The inertance variable, representing the effect of inertia, is assumed to be low and thus disregarded.

Raw and CRS can be obtained by fitting the equation of motion to P, V, and V̇ with a multiple linear regression analysis, called linear least-squares fitting.4 This approach is incorporated into the software of some ventilators, allowing display of Raw, CRS, and auto-PEEP without the need for inspiratory and expiratory pause maneuvers. P, V, and V̇ are digitized at 100 Hz, allowing Raw and CRS be calculated from 100 or more equations per breath. This method can be applied during the whole breathing cycle or only in the inspiratory or expiratory phase, although restricting the analysis to the inspiratory phase may be more appropriate in patients with COPD who have flow limitation. The least-squares fitting method assumes that Pmus is zero and is thus less valid if the patient is actively breathing. An important methodological issue is that the least-squares fitting approach uses a single linear model that does not take into account changes of Raw and CRS with lung volume, and it also neglects flow turbulence and inertial forces.

Alveolar Pressure

During volume control ventilation, alveolar pressure (Palv) at any time during inspiration is determined by the volume delivered and CRS: Palv = V/CRS + PEEP. For pressure control ventilation, Palv at any time after the initiation of inspiration is:

Palv = ΔP × (1 − e−t/τ) + PEEP, where ΔP is the pressure applied to the airway above PEEP, e is the base of the natural logarithm, t is the elapsed time after initiation of the inspiratory phase, and τ is the time constant.

Plateau Pressure

Due to Raw, proximal airway pressure will always be greater than Palv during inspiration if flow is present. Palv is estimated with an end-inspiratory hold maneuver. Plateau pressure (Pplat) is measured during mechanical ventilation by applying an end-inspiratory breath-hold for 0.5–2 s, during which pressure equilibrates throughout the system, so the pressure measured at the proximal airway approximates the Palv. With rapid airway occlusion at the end of inspiration, flow drops to zero, and the proximal airway pressure immediately decreases to a lower level (the pressure at zero flow [Pz]). Raw and end-inspiratory flow determine the difference between peak inspiratory pressure (PIP) and Pz. During airway occlusion, pressure further declines to reach a plateau (Pplat). The difference between Pz and Pplat is determined by time constant heterogeneity within the lungs (ie, pendelluft) and the viscoelastic behavior of the stress relaxation of the pulmonary tissues. Measurement of Pplat is valid only during passive inflation of the lungs, but not during active breathing. During pressure control ventilation, the flow might decrease to zero at the end of the inspiratory phase; if this occurs, PIP and Pplat are equal.

Pplat is determined by VT and CRS during full ventilatory support: Pplat = VT/CRS. A high Pplat indicates risk of alveolar over-distention. Pplat should ideally be kept at ≤ 30 cm H2O,5 with some evidence suggesting that Pplat should be targeted to < 25 cm H2O in patients with ARDS.6,7 This assumes that chest-wall compliance (CCW) is normal. A high Pplat may be safe (and necessary) if CCW is decreased.

A method has been described that uses the expiratory time constant (τE) to provide real-time determinations of Pplat without the need for an end-inspiratory pause maneuver.8 Using this approach, τE is estimated from the slope of the passive expiratory flow curve between 0.1 and 0.5 s. Pplat is then calculated as:

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(2)

This approach has the advantage of being able to be used in spontaneous breathing modes such as pressure support, but has the disadvantage of requiring a computerized algorithm to make the necessary calculations.

Auto-PEEP

Incomplete emptying of the lungs occurs if the expiratory phase is terminated prematurely. The pressure produced by this trapped gas is called auto-PEEP, intrinsic PEEP, or occult PEEP. Auto-PEEP increases end-expiratory lung volume and thus causes dynamic hyperinflation.9,10

Auto-PEEP is measured by applying an end-expiratory pause for 0.5–2 s . The pressure measured at the end of this maneuver in excess of the PEEP set on the ventilator is defined as auto-PEEP. For a valid measurement, the patient must be relaxed and breathing in synchrony with the ventilator, as active breathing invalidates the measurement. The end-expiratory pause method can underestimate auto-PEEP when some airways close during exhalation, as may occur during ventilation of the lungs of patients with severe asthma In spontaneously breathing patients, measurement of esophageal pressure (Pes) can be used to determine auto-PEEP.

Auto-PEEP is a function of ventilator settings (VT and expiratory time [TE]) and lung function (Raw and lung compliance [CL]): auto-PEEP = VT/(CRS × (eKx × TE − 1), where Kx is the inverse of the τE (1/τ). Note that auto-PEEP is increased with increased resistance and compliance, increased breathing frequency or increased inspiratory time (TI; both decrease TE), and increased VT. Clinically, auto-PEEP can be decreased by decreasing minute ventilation (rate or VT), increasing TE (decreasing rate or TI), or decreasing Raw (eg, bronchodilator administration).

Mean Airway Pressure

Mean airway pressure (P̄aw) is determined by PIP, the fraction of time devoted to the inspiratory phase (TI/Ttot, where Ttot is total respiratory cycle time), and PEEP. For constant flow-volume ventilation, in which the airway pressure waveform is triangular, P̄aw can be calculated as: P̄aw = 0.5 × (PIP − PEEP) × (TI/Ttot) + PEEP. During pressure ventilation, in which the airway pressure waveform is rectangular, P̄aw can be estimated as: P̄aw = (PIP − PEEP) × (TI/Ttot) + PEEP. The mean Palv may be different than P̄aw if the inspiratory airway resistance (RI) and expiratory airway resistance (RE) are different, which is often the case in lung disease: mean Palv = P̄aw + (V̇E/60) × (RE − RI), where V̇E is expiratory flow.

Esophageal Pressure

Pleural pressure (Ppl) cannot be easily measured directly. The traditional approach to assess Ppl is the use of an esophageal balloon,11–18 which consists of a thin catheter with multiple small holes in the distal 5–7 cm of its length. A 10-cm-long balloon is placed over the distal end of the catheter to prevent the holes in the catheter from being occluded by esophageal tissue and secretions, and the balloon is inflated with a small amount of air (0.5 mL). The proximal end of the catheter is attached to a pressure transducer.

The catheter is inserted orally or nasally to ∼35–40 cm from the airway opening. Correct positioning of the esophageal balloon is necessary to ensure accurate Pes measurements. After the balloon is inflated and the pressure is measured, the Pes waveform should be compared to the airway pressure waveform. If they appear similar in pressure and shape, the catheter is likely in the trachea and should be removed. If the catheter is in the esophagus, cardiac oscillations should be visible on the Pes waveform, indicating that the balloon is positioned in the lower third of the esophagus directly behind the heart (Fig. 5). Some clinicians use a technique in which the catheter is intentionally inserted into the stomach, air is added to the balloon, and the catheter is then withdrawn until cardiac oscillations are observed.The classic technique used to validate the balloon's position requires the patient to perform static Valsalva and Müller maneuvers with the glottis open. In patients unable to cooperate, changes in Pes and airway pressure are assessed during a gentle push on the abdomen with the airway occluded. Airway occlusion is accomplished using the expiratory pause control on the ventilator. When changes in Pes are equal to airway pressure, it is assumed that transmission of Ppl to Pes is unimpeded, and Pes accurately reflects Ppl. A chest radiograph can also be used to validate correct positioning 19 but this is usually not necessary.There are potential sources of error in the use of Pes to estimate Ppl.12,20 It is important to appreciate that the Pes estimates Ppl mid-thorax. The Ppl is more negative in the non-dependent thorax and more positive in the dependent thorax. The weight of the heart can bias the Pes by as much as 5 cm H2O.16 The results of Guérin and Richard21 suggest that referencing absolute Pes values to those obtained at the relaxation volume of the respiratory system might improve the customization of the correction of Pes based on the physiologic and individual context, rather than using an invariant value of 5 cm H2O.

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