A typical airway pressure waveform during volume-controlled ventilation (VCV) is shown in Figure 30-1. With VCV, pressure increases as volume is delivered. If a constant flow pattern is chosen, there should be a constant increase in pressure during inspiration. With a descending ramp flow pattern, the inspiratory pressure waveform will be more rectangular. PIP varies with resistance, flow, tidal volume, respiratory system compliance, and PEEP.
A typical airway pressure waveform during volume ventilation.
An end-inspiratory pause of sufficient duration (0.5-2 seconds) allows equilibration between proximal airway pressure and alveolar pressure (Palv). This measurement should be made on a single breath and removed immediately to prevent the development of auto-PEEP. During the end-inspiratory pause, there is no flow and a pressure plateau develops as proximal airway pressure equilibrates with Palv (Figure 30-2). The pressure during the inspiratory pause is the Pplat and represents peak Palv. Because it reflects Palv, Pplat should usually be kept at less than 30 cm H2O and always should be kept as low as possible.
Plateau pressure is determined using an end-inspiratory pause.
The difference between PIP and Pplat is due to the resistive properties of the system (eg, pulmonary airways, artificial airway), and the difference between Pplat and total PEEP is due to respiratory system compliance. The measurement of Pplat is valid only if the patient is passively ventilated—active breathing invalidates the measurement. The measurement is also not valid if leaks are present.
During pressure-controlled ventilation (PCV), PIP and peak Palv may be equal due to the flow waveform with this mode of ventilation (Figure 30-3). With PCV, flow decreases during inspiration and is often followed by a period of zero flow at end inspiration. During this period of no flow, proximal airway pressure should be equal to Palv. If flow does not reach zero before the end of inspiration during PCV, an end-inspiratory pause maneuver is needed to determine Pplat. With all other factors held constant (eg, tidal volume, compliance, PEEP), Palv is identical for volume- and pressure-control ventilation. Because lung injury is related to peak Palv (ie, Pplat), the importance of the decrease in PIP that occurs when changing from volume to pressure ventilation is questionable.
Typical airway flow waveforms for pressure-controlled ventilation with low resistance and low compliance (eg, acute respiratory distress syndrome [ARDS]), and with high resistance and high compliance (eg, chronic obstructive pulmonary disease [COPD]).
An end-expiratory pause is used to determine auto-PEEP (Figure 30-4). This method is valid only if the patient is not spontaneously breathing and there are no system leaks (eg, circuit leak or bronchopleural fistula). For patients who are triggering the ventilator, an esophageal balloon catheter is needed to measure auto-PEEP. During the end-expiratory pause, there is equilibration between end-expiratory pressure and proximal airway pressure. Auto-PEEP is the difference between set PEEP and total PEEP measured with this maneuver. All current-generation ventilators have the capability of measuring auto-PEEP using an end-exhalation pause maneuver.
Auto-PEEP is determined using an end-expiratory pause.
Auto-PEEP is determined by the tidal volume, respiratory system compliance, airways resistance, and expiratory time:
where Ke = 1/(RE × C), e is the base of the natural logarithm, TE is expiratory time, RE is expiratory airways resistance, and C is respiratory system compliance. Because set PEEP may counterbalance auto-PEEP, the presence of auto-PEEP is most appropriately measured with no PEEP set on the ventilator. Auto-PEEP causes dynamic hyperinflation, hemodynamic instability, ventilation-perfusion mismatch, and difficulty triggering the ventilator.
Measurements of auto-PEEP and Pplat reflect average alveolar pressures. Because of the inhomogeneity of the lungs with disease, some lung units have an auto-PEEP (or Pplat) higher or lower than that measured. This is of particular concern with measures of auto-PEEP due to airway closure during exhalation (Figure 30-5).
The effect of airway closure on measurement of auto-PEEP. Although the auto-PEEP is high in some lung units, the level measured is only that in lung units where the airway remains open at end exhalation.
Many of the desired and deleterious effects of mechanical ventilation are related by mean airway pressure (P̄aw). Factors affecting mean airway pressure are PIP, PEEP, I:E, and inspiratory pressure waveform. During PCV, the inspiratory pressure waveform is rectangular and P̄aw is estimated as:
where TI is inspiratory time and TT is total cycle time. For example, with a PIP of 25 cm H2O, PEEP of 10 cm H2O, TI of 1 second, rate 20/min (TI/TT = 0.33), P̄aw is 15 cm H2O. During constant-flow volume ventilation, the inspiratory pressure waveform is triangular, and P̄aw can be estimated as:
For example, with a PIP of 25 cm H2O, PEEP 5 cm H2O, TI 1.0 seconds, rate 30/min (TI/TT = 0.5), P̄aw is 15 cm H2O. Many current-generation microprocessor ventilators display P̄aw from integration of the airway pressure waveform. Because expiratory resistance is usually greater than inspiratory resistance, Paw is not equivalent to Palv. The difference between mean alveolar pressure (P̄aw) and P̄aw is estimated by the following relationship:
The difference between Pplat and total PEEP is determined by the compliance of the lungs and chest wall. Thus, compliance can be calculated as:
The VT used in this equation is the actual tidal volume delivered to the patient, and it should be corrected for the effects of volume compressed in the ventilator circuit. PEEP should include any auto-PEEP that is present. Pplat should be determined from an end-inspiratory breath-hold that is long enough to produce equilibration between proximal airway pressure and alveolar pressure. Normal respiratory system compliance is 100 mL/cm H2O and should be greater than 50 mL/cm H2O in mechanically ventilated patients. Causes of a decrease in compliance in mechanically ventilated patients are listed in Table 30-1.
Table 30-1Causes of decreased compliance and increased resistance in mechanically ventilated patients ||Download (.pdf) Table 30-1 Causes of decreased compliance and increased resistance in mechanically ventilated patients
|Compliance ||Resistance |
Congestive heart failure
Low lung volume
Chest wall effects:
Chest wall deformity
Small endotracheal tube
Low lung volume
Compliance can be used to determine the best PEEP setting. The optimal level of PEEP is associated with the highest compliance. Low lung volume (insufficient PEEP) and overdistention (too much PEEP) are associated with a lower compliance than best PEEP.
The difference between PIP and Pplat is determined by inspiratory resistance and end-inspiratory flow. During constant-flow volume ventilation, inspiratory resistance can be calculated as:
where V̇i is the inspiratory flow. Expiratory resistance can be estimated from the time constant (τ) of the lung: RE = τ/C (Figure 30-6). Expiratory resistance can also be estimated as RE = (Pplat – PEEP)/V̇e, where V̇e is peak expiratory flow. Causes of increased resistance during mechanical ventilation are listed in Table 30-1. Inspiratory resistance is less than expiratory resistance due to the increased diameter of airways during inspiration. Normal airways resistance is 1 to 2 cm H2O/L/s and should be less than 10 cm H2O/L/s in intubated mechanically ventilated patients.
Use of the tidal volume waveform to measure time constant (τ) and calculate expiratory airways resistance.
Least Squares Fitting Method
This method allows dynamic estimation of respiratory mechanics without the need for measurement of Pplat through use of the equation of motion:
If the respiratory muscles are relaxed (Pmus = 0), and if many measures of Pvent (airway pressure), volume, and flow are made during inspiration, it is possible to calculate resistance, compliance, and auto-PEEP using an iterative least squares fitting method. This is the method used by ventilators that display resistance and compliance on every breath without flow interruption. Because it assumes that the respiratory muscles are relaxed, this method becomes less accurate during spontaneous breathing modes. If resistance, compliance, and auto-PEEP are known, it is possible to calculate Pmus.
Inspiratory work-of-breathing performed by the ventilator can be estimated during constant-flow passive inflation of the lungs by the following calculation:
For example, if PIP = 30 cm H2O, Pplat = 25 cm H2O, and tidal volume = 0.4 L, then W = 0.07 kg·m, or 0.18 kg·m/L. The units for work-of-breathing are kilogram-meter (kg·m) or joules (J); 0.1 kg·m = 1.0 J. Work-of-breathing is often normalized to the tidal volume (work/L). Normal work-of-breathing is ≈ 0.5 J/L. Work-of-breathing will increase with an increase in resistance, a decrease in compliance, or an increase in tidal volume. Although work-of-breathing is not commonly calculated, it is reasonable to expect that ventilator libetration will be difficult with a high work-of-breathing.
Power of breathing is the rate at which work is done, and may be a better assessment of respiratory muscle loads than work of breathing per breath because it is a measure over time, (normal adult power of breathing is 4-8 J/min). Work-of-breathing in spontaneously breathing patients has traditionally required use of an esophageal balloon catheter. However, the use of an artificial neural network may allow power of breathing, and hence work-of-breathing is calculated noninvasively without the need for an esophageal balloon catheter.