This chapter will discuss the principles of mechanical ventilation, indications, modes of mechanical ventilation, weaning and spontaneous breathing trials, tracheostomy, complications of mechanical ventilation, special situations, and noninvasive mechanical ventilation.
Proximal Airway Pressure, Alveolar Pressure, and Plateau Pressure
Proximal airway pressure applied by the ventilator (Pvent) is approximated during the expiratory phase in the inspiratory limb when flow is 0 and during the inspiratory phase in the expiratory limb when the flow is 0.1 It uses the following equation2:
Pvent + Pmus = VT/CRS + Raw × VI + PEEP + iPEEP + Inertance
In this equation, CRS is respiratory system compliance, PEEP is peak end-expiratory pressure, iPEEP is intrinsic PEEP, Pmus is pressure from patient’s inspiratory muscles, Pvent is proximal airway pressure from the ventilator, Raw is airway resistance, VI is the inspiratory volume, inertance (cm H2O L−1 s2) is pressure difference to cause change in rate of change in volume flow rate in time, and VT is tidal volume. Alveolar pressure (PA) during inspiration in volume control ventilation is V/CRS + PEEP, and during inspiration in pressure control ventilation is ΔP × (1 − e−t/T) + PEEP.1 In this equation, t is the elapsed time after initiation of inspiration, and T is the time constant.
Due to Raw, the presence of flow causes proximal airway pressure to be greater than PA. Plateau pressure (Pplat) is determined by applying an end-inspiratory breath hold for 0.5 to 2 seconds, where the pressure equilibrates when the flow is 0. It is calculated via Pplat = VT/CRS during passive inflation and via Pplat = (VT × PIP) − (VT × PEEP)/(VT + [TE × VI]) in spontaneous breathing modes.1 In this equation, PIP is peak inspiratory pressure and TE is the expiratory time constant. During end-inspiratory breath hold, the Pplat approximates PA. Ideally, Pplat is less than 30 cm H2O (Fig. 12-1).1
The relationship of pressure over time.
Understanding the relationship between peak and plateau pressures can help troubleshoot mechanically ventilated patients. PIP without increase in plateau pressure suggests an increase in airway resistance from a kinked or blocked endotracheal (ET) tube, bronchospasm, or increased secretions.3 Increased PIP with increased plateau pressure suggests decreased compliance such as extrathoracic compression, bronchial intubation, atelectasis, pulmonary edema, pneumothorax, and hyperinflation.3 Decreasing PIP, low tidal volumes, gurgling sounds, and stridor can indicate cuff leak.