5.1 Ventilator Modes & Settings

Choosing Invasive vs. Noninvasive Support

Mechanical ventilation exists on a spectrum from a nasal cannula to a fully controlled endotracheal circuit. The CCRN expects you to match the device to the physiology and, critically, to know when noninvasive support is unsafe.

Noninvasive ventilation (NIV) avoids an artificial airway. Continuous positive airway pressure (CPAP) delivers one constant pressure throughout the breath; it splints open alveoli and is best for pure oxygenation problems and cardiogenic pulmonary edema and obstructive sleep apnea. Bilevel positive airway pressure (BiPAP) delivers a higher inspiratory pressure (IPAP) and a lower expiratory pressure (EPAP). The difference between them (the pressure support) unloads the diaphragm and augments ventilation, which is why BiPAP is first-line for an awake COPD patient in acute hypercapnic respiratory failure who is still protecting the airway. High-flow nasal cannula (HFNC) delivers heated, humidified gas up to 60 L/min at an FiO2 up to 1.0; it washes out anatomic dead space and generates a small amount of positive pressure.

When NIV Is Contraindicated

NIV requires a patient who can protect the airway and cooperate. Intubate instead when there is apnea or an inability to protect the airway, a depressed level of consciousness, hemodynamic instability or shock, copious secretions or high aspiration risk, facial trauma, or failure to improve after a short NIV trial. A common exam trap is offering BiPAP to an obtunded patient who cannot guard the airway; the correct answer is intubation.

Invasive Ventilator Modes

AC/CMV

In assist-control (AC), also called continuous mandatory ventilation, every breath is fully supported to a set tidal volume (volume control) or set pressure (pressure control), whether the patient or the machine triggers it. This gives maximal rest but has a signature hazard: a tachypneic, anxious patient who over-breathes the set rate receives a full breath each time, driving down PaCO2 into respiratory alkalosis and stacking breaths into auto-PEEP.

SIMV and PSV

SIMV delivers a set number of synchronized mandatory breaths; between them the patient breathes spontaneously and receives only the volume they can generate, so it is often paired with pressure support. Pressure support ventilation (PSV) has no backup rate at all — the patient must have an intact respiratory drive, because each breath must be patient-triggered. PSV is the workhorse of weaning. PRVC is a dual-control mode that guarantees a set tidal volume while automatically using the lowest pressure needed, protecting against both under-ventilation and barotrauma.

Core Settings

• Tidal volume (VT): 6-8 mL/kg of ideal (predicted) body weight calculated from height and sex, never actual weight; ARDS uses 6 mL/kg.
• Respiratory rate (RR): typically 12-16; combined with VT it sets minute ventilation and therefore PaCO2.
• FiO2: start at 1.0 after intubation, then titrate DOWN to the lowest level that keeps SpO2 at least 92% (PaO2 at least 60 mmHg) to limit oxygen toxicity.
• PEEP: default 5 cmH2O; raise it to recruit alveoli and improve oxygenation, watching for a fall in blood pressure as raised intrathoracic pressure reduces preload.
• I:E ratio: normally 1:2; prolong expiration (1:3, 1:4) in obstructive disease to prevent air trapping.

Reading Basic Graphics and Adjusting from the ABG

Scalar waveforms plot pressure, flow, and volume against time. On the flow-time curve, expiratory flow that does not return to baseline before the next breath signals auto-PEEP. The plateau pressure, measured with an inspiratory hold, reflects alveolar/compliance pressure; a wide gap between a high peak pressure and a normal plateau points to airway resistance (secretions, bronchospasm, kinked tube).

Adjust the ventilator by dividing gas exchange into two levers. Oxygenation problems (low PaO2 or SpO2) are fixed by raising FiO2 and PEEP. Ventilation problems (high PaCO2, respiratory acidosis) are fixed by raising minute ventilation — increase the rate or the tidal volume. A low PaCO2 with alkalosis means the patient is over-ventilated; lower the rate or tidal volume. Keeping these two levers separate is the single most testable concept in ventilator management.

A Peak-vs-Plateau Worked Example

Suppose a ventilated patient's peak inspiratory pressure jumps from 28 to 45 cmH2O. You perform an inspiratory hold and the plateau pressure is still 24. Because the plateau (which reflects alveolar compliance) is unchanged while the peak (which reflects flow resistance) has climbed, the problem is a resistance issue — think secretions, bronchospasm, biting, or a kinked tube — not stiff lungs. Suction, assess for wheeze, and check the tube. If instead BOTH peak and plateau rose together, suspect a compliance problem such as worsening ARDS, pneumothorax, pulmonary edema, or auto-PEEP. This peak-versus-plateau split is a favorite CCRN discriminator.

Permissive Hypercapnia

When you cap tidal volume at 6 mL/kg to protect the lung in ARDS or severe asthma, minute ventilation falls and PaCO2 rises. Permissive hypercapnia deliberately tolerates that higher PaCO2 (and a pH down to roughly 7.20-7.25) rather than injuring the lung with larger volumes. The correct response to a modestly elevated PaCO2 in a lung-protective strategy is often to accept it, not to chase normal numbers with harmful tidal volumes.

Common mistake: cranking the respiratory rate to normalize CO2 in an obstructive patient. Higher rates shorten expiration and worsen air trapping. Sometimes the right move is a LOWER rate with a longer expiratory time.