Mechanical Ventilation Basics

Clinical Orientation

A patient is intubated and on the ventilator. The alarms sound. The nurse must quickly assess: Is it the patient, the circuit, or the machine? This chapter covers the essential concepts nurses need for safe bedside care of ventilated patients.

Key Concepts

Positive pressure ventilation: Unlike spontaneous breathing (negative pressure draws air in), the ventilator pushes air in under positive pressure. This reverses normal cardiopulmonary physiology: positive intrathoracic pressure → decreased venous return → decreased preload → potential hypotension. This is why post-intubation hypotension is common.

Modes:

• Assist-Control (AC): Every breath is fully supported (either patient-triggered or machine-delivered). Guarantees set rate and tidal volume.
• SIMV (Synchronized Intermittent Mandatory Ventilation): Set number of mandatory breaths; patient can take spontaneous breaths between them.
• Pressure Support (PS): Patient-triggered breaths are pressure-supported but patient controls rate and tidal volume. Used for weaning.

Settings to know:

• FiO2: Fraction of inspired oxygen (0.21-1.0). Titrate to SpO2/PaO2.
• Tidal volume (Vt): Volume delivered per breath. ARDS: 6 mL/kg PBW. Others: 6-8 mL/kg PBW.
• Respiratory rate: Set minimum breaths per minute.
• PEEP: Positive end-expiratory pressure. Keeps alveoli open at end-expiration. Improves oxygenation. Typical starting: 5 cm H2O.
• Pressure support: Added pressure during spontaneous breaths to overcome circuit resistance.

Alarms:

• High pressure: Coughing, secretions, biting tube, bronchospasm, pneumothorax, kinked circuit, water in tubing.
• Low pressure: Disconnection, leak, cuff leak, extubation.
• Low minute volume: Patient not breathing adequately, disconnection, apnea.
• High respiratory rate: Pain, anxiety, hypoxia, hypercapnia, acidosis.

Nursing Priorities

1. Always assess the PATIENT first when an alarm sounds. Then the circuit, then the ventilator.
2. ETT placement: Confirm with ETCO2 after intubation and with every position change. Secure the tube — note the centimeter marking at the lip.
3. Oral care: Every 2-4 hours with chlorhexidine. Prevents ventilator-associated pneumonia (VAP).
4. Head of bed 30-45 degrees. Reduces aspiration and VAP risk.
5. Sedation vacation / spontaneous breathing trial (SBT): Daily assessment for readiness to wean. Reduces ventilator days.
6. DVT and stress ulcer prophylaxis: Standard for ventilated patients.
7. Suctioning: Only when indicated (visible secretions, coughing, sawtooth waveform, desaturation, increased peak pressures). Preoxygenate before suctioning.
8. Monitor for complications: Hypotension (decreased venous return), barotrauma (pneumothorax), VAP, auto-PEEP (incomplete expiration in COPD/asthma — causes progressive hyperinflation).

Auto-PEEP

In patients with high airway resistance (COPD, asthma) or high minute ventilation, expiration may be incomplete before the next breath → progressive air trapping → increased intrathoracic pressure → decreased venous return → hypotension. If a ventilated patient becomes hypotensive and breath sounds are absent on one side, disconnect from the ventilator briefly — if BP improves, auto-PEEP was the culprit.

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