4.5 Mechanical Ventilation Initiation and Adjustment

Why Ventilator Items Are High Yield

Mechanical ventilation sits at the center of this section because a setting change can immediately improve or harm gas exchange. The TMC rewards cause-and-effect logic: oxygenation depends mainly on FiO2 and mean airway pressure (PEEP and inspiratory time), while ventilation and PaCO2 depend mainly on minute ventilation (respiratory rate x tidal volume). Begin every item by naming the problem — hypoxemic, hypercapnic, apneic, fatigued, obstructed, noncompliant, asynchronous, or ready for less support — then pick the setting that directly changes it.

Mode Decision Table

Initial Settings and the Lung-Protective Rule

For adults, size tidal volume from ideal body weight (IBW), not actual weight. A general start is 6-8 mL/kg IBW, dropping toward 6 mL/kg when acute respiratory distress syndrome (ARDS) or poor compliance is present. PEEP typically starts near 5 cmH2O and rises if oxygenation demands it. Do not pick a large tidal volume just to clear carbon dioxide quickly — if the plateau pressure is high or ARDS is present, lung protection wins, and permissive hypercapnia is acceptable while the pH stays tolerable (often >7.20-7.25) and oxygenation is managed.

Match the rest to the airway problem: apnea or heavy sedation needs a backup rate; severe work of breathing needs enough support to rest the muscles.

Adjustment Logic

Worked example: pH 7.27, PaCO2 61, PaO2 91, VT 430, rate 10, FiO2 0.40, PEEP 5, plateau 23 cmH2O. The problem is respiratory acidosis; because the plateau is safe and oxygenation is fine, raise the rate to increase minute ventilation rather than touch FiO2.

Peak vs Plateau: The Most-Tested Skill

Peak inspiratory pressure reflects airway resistance plus compliance; plateau pressure (measured during an inspiratory hold with no flow) reflects alveolar pressure and compliance alone.

• High peak, normal plateau = increased airway resistance: secretions, bronchospasm, kinked or bitten tube, water in the circuit.
• High peak, high plateau = reduced compliance or excessive tidal volume: ARDS, pneumothorax, pulmonary edema, abdominal distension, or right mainstem intubation.

Keep plateau at or below 30 cmH2O as the lung-protection ceiling.

Alarms, Waveforms, and Disease-Specific Logic

Low-pressure or low-volume alarms point to a leak, disconnect, cuff problem, or displaced airway; high-pressure alarms demand a resistance-versus-compliance check before blaming the ventilator — assess the patient first, then the circuit. On the flow waveform, expiratory flow that does not return to baseline before the next breath signals air trapping. In ARDS, use 6 mL/kg IBW, keep plateau at or below 30, apply adequate PEEP, and consider prone positioning for refractory hypoxemia.

In asthma or COPD, avoid trapping: a high rate, long inspiratory time, or large tidal volume shortens exhalation and builds intrinsic PEEP, so lengthen expiratory time. In neuromuscular failure, the lungs are often compliant but the patient cannot move enough air or cough — support ventilation, follow vital capacity and negative inspiratory force, and anticipate airway protection. Reassess SpO2, ABG, exhaled VT, peak and plateau pressures, alarms, hemodynamics, and comfort after every change.

Oxygenation Levers in Detail

When FiO2 is already high (above about 0.60) and oxygenation is still poor, the exam wants you to raise mean airway pressure rather than keep climbing FiO2, because prolonged high FiO2 risks absorption atelectasis and oxygen toxicity. PEEP is the primary lever: it recruits collapsed alveoli, raises functional residual capacity, and improves the ventilation-perfusion match, letting you wean FiO2 toward 0.40-0.50.

The cost of PEEP is hemodynamic — it raises intrathoracic pressure, reduces venous return, and can drop cardiac output and blood pressure, so a falling blood pressure after a PEEP increase is a recognized adverse response, not an unrelated event. Excessive PEEP can also overdistend healthy alveoli, raise plateau pressure, and reduce compliance, so titrate to the best oxygenation at an acceptable plateau rather than to a maximum number.

Ventilation Levers and Permissive Hypercapnia

Carbon dioxide clearance is set by minute ventilation, but the two levers differ. Raising the rate increases minute ventilation with little effect on plateau pressure, making it the safer first choice when lung protection matters. Raising tidal volume also lowers PaCO2 but increases plateau pressure and the risk of volutrauma, so it is avoided in ARDS. When low-tidal-volume lung protection causes the PaCO2 to climb, the exam accepts permissive hypercapnia as long as the pH stays roughly above 7.20-7.25; you tolerate the high carbon dioxide to protect the lung rather than injuring alveoli to normalize a number.

Watch for the opposite error too: a low PaCO2 with a high pH signals overventilation, corrected by lowering the rate or tidal volume.

Synchrony and Trigger Problems

Asynchrony lowers comfort and effectiveness. Ineffective triggering (the patient's effort fails to start a breath) suggests auto-PEEP, an insensitive trigger, or weakness; reduce trapping and improve trigger sensitivity. Double triggering (two stacked breaths) suggests the set inspiratory time or tidal volume is too short for the patient's demand. Flow starvation, where the pressure waveform scoops downward during inspiration, means the inspiratory flow is too low for an air-hungry patient; raise the flow or switch to a mode that delivers variable flow.

Before changing a setting, always check for pain, anxiety, leaks, and secretions, because the fix is sometimes a bedside problem rather than a ventilator parameter.