Basic Ventilatory Parameters
Tidal Volume (Vt)
Definition: Volume of air delivered with each breath.
Normal:
Spontaneous breathing: ~500 mL
Mechanical ventilation: 6–8 mL/kg ideal body weight
Clinical note: Aim Low VT (6 mL/kg) is lung-protective (ARDS).
Respiratory Rate (RR)
Definition: Number of breaths per minute.
Normal adult: 12–20 breaths/min
Clinical note: ↑ RR can help compensate for metabolic acidosis.
Minute Ventilation (Ve)
Definition: Total air moved per minute.
Formula:
Ve=Vt × RR
Normal: 5–8 L/min
Clinical note: Determines PaCO₂.
Oxygenation Parameters
Fraction of Inspired Oxygen (FiO₂)
Definition: Percentage of oxygen in inspired air.
Normal (room air): 21% (0.21)
Ventilator range: 21–100%
Clinical note: Use lowest FiO₂ to keep SpO₂ ≥ 95% (aim 88-92% if indicated in ARDS where PO2>8; pCO2<8 may be advised – ‘permissive hypercapnia’)
Oxygen Saturation (SpO₂)
Definition: % of haemoglobin saturated with oxygen.
Normal: 95–100%
Acceptable in COPD: 88–92% (if documented by medical team)
Partial Pressure of Oxygen (PaO₂)
Definition: Oxygen tension in arterial blood.
Normal: 80–100 mmHg (11 to 13.1KPa)
Pressure Parameters
Peak Inspiratory Pressure (pIP)
Definition: Maximum pressure during inspiration.
Normal: < 30–35 cmH₂O
Affected by: Airway resistance + lung compliance
High PIP suggests: Bronchospasm, secretions, kinked tube
Plateau Pressure (Pplat)
Definition: Pressure in alveoli at end-inspiration (no airflow).
Normal: < 30 cmH₂O
Clinical note: Best indicator of alveolar over distension.
Positive End-Expiratory Pressure (PEEP)
Definition: Pressure maintained at end-expiration.
Normal: 5 cmH₂O
Purpose: Prevents alveolar collapse, improves oxygenation.
High PEEP risks: Barotrauma, hypotension.
Gas Exchange Parameters
Partial Pressure of Carbon Dioxide (PaCO₂)
Definition: CO₂ tension in arterial blood.
Normal: 35–45 mmHg (4.6-6.0KPa)
Clinical note: Inversely proportional to alveolar ventilation.
End-Tidal CO₂ (EtCO₂)
Definition: CO₂ at end of expiration.
Normal: 35–45 mmHg (4.5 to 6.0 KPa)
Uses: Confirms intubation, monitors ventilation, tube position.
5. Lung Mechanics
Compliance
Definition: Lung distendibilty.
Formula:
Compliance= Vt/(Pplat−PEEP)
Normal: 50–100 mL/cmH₂O
Low compliance: ARDS, pulmonary oedema, fibrosis
Airway Resistance
Definition: Resistance to airflow in airways.
Increased in: Asthma, COPD, secretions
Common Ventilation Modes
Volume Control Ventilation (VCV)
Fixed Vt, variable pressure
Pressure Control Ventilation (PCV)
Fixed pressure, variable Vt
SIMV
Mandatory + supported spontaneous breaths
Pressure Support Ventilation (PSV)
Patient-initiated breaths with pressure support
Summary:
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