MANAGING VENTILATORS SETTINGS
The primary function of mechanical ventilation is to provide respiratory support while treating the underlying process that caused respiratory failure. Safe oxygenation and ventilation can be accomplished in either a volume- or pressure-targeted mode. The chosen mode is often based on provider preference and experience. Patient outcomes are unlikely to be changed by the mode of ventilation.1,2
In a volume-targeted mode (e.g., volume control; Figure 29B-1), the ventilator provides an inspiratory flow over time to target a set volume. To avoid ventilator-induced injury, multiple trials have shown that limiting the tidal volume improves patient outcomes compared to using the larger volumes more common in the past (often ≥10 mL/kg to start).3-6 One approach is to start volume-targeted ventilation at 6 mL/kg ideal body weight for patients with or at risk for acute respiratory distress syndrome and 6 to 8 mL/kg ideal body weight for other patients. The definition of acute respiratory distress syndrome is provided in Table 29B-1. Those at risk for acute respiratory distress syndrome have pneumonia, sepsis, trauma, pancreatitis, or any shock state.7 To choose ventilation settings, calculate ideal body weight using the patient’s height, an important caveat in obese patients in whom harm is risked if tidal volumes choice is based on actual body weight.
Normal ventilator waveforms for volume control with a square inspiratory waveform flow. Yellow is the pressure waveform, green is the flow waveform, and blue is the volume waveform. This figure is an example of volume control with a square inspiratory waveform where the tidal volume of 600 mL is delivered over 1 second. PEEP = positive end-expiratory pressure.
TABLE 29B-1Acute Respiratory Distress Syndrome Definition (Berlin Definition)8 |Favorite Table|Download (.pdf) TABLE 29B-1 Acute Respiratory Distress Syndrome Definition (Berlin Definition)8
Onset within 1 week of a known clinical insult or new or worsening respiratory symptoms
Bilateral opacities on chest imaging not fully explained by lobar/lung collapse or nodules
Respiratory failure not fully explained by cardiac failure or volume overload
PaO2/FIO2 ratio ≤300 mm Hg with PEEP ≥5 cm H2O
Mild PaO2/FIO2 200–300 mm Hg
Moderate PaO2/FIO2 100–200 mm Hg
Severe PaO2/FIO2 ≤100 mm Hg
In volume control mode, the airway pressures depend on the tidal volume plus the size and compliance of the respiratory system. Peak pressures are the highest pressures seen ...