Physiology of positive-pressure ventilation
- Mechanism
- Exam priority: medium
A ventilator pushes air in with positive pressure. That reverses normal breathing and can squeeze the veins returning blood to the heart.
Also called: Mechanical ventilation, PEEP, Resuscitators and ventilators
What it is
In normal breathing the chest wall and diaphragm lower pleural pressure and air is sucked in. A resuscitator bag or a ventilator does the opposite: it pushes gas into the airway under positive pressure, so airway and pleural pressures rise during inspiration. This explains both the benefits (recruiting alveoli, resting tired muscles) and the costs (less venous return, lung stretch injury, air leaks).
How it works, step by step
- The ventilator pushes gas into the airway; alveolar pressure rises above atmospheric pressure.
- Lungs expand outward against the chest wall, so pleural pressure becomes less negative or positive.
- Higher pleural pressure is transmitted to the right atrium and great veins; the gradient for venous return narrows.
- Less venous return lowers right and then left ventricular filling, so stroke volume and blood pressure fall.
- Expanded alveoli also stretch alveolar capillaries, raising pulmonary vascular resistance and right ventricular afterload.
- Normally, sympathetic reflexes constrict veins and raise mean systemic filling pressure, which partly restores venous return.
- With hypovolemia or blunted reflexes (sedatives), that compensation fails and hypotension follows soon after starting ventilation.
- Expiration is passive: lung and chest-wall recoil push gas out down to the set PEEP.
Equations
- VT
- tidal volume
- Pplat
- plateau pressure during an inspiratory hold
- PEEP
- positive end-expiratory pressure
Units: mL/cmH₂O. Falls when lungs or chest wall are stiff.
- ΔP
- pressure that inflates the lung each breath
A larger tidal volume for a small (stiff or reduced) lung means a higher driving pressure.
- Ppeak
- highest airway pressure in inspiration
A wide gap points to the airway (bronchospasm, secretions, kinked tube); a high plateau points to the lung or chest wall.
Typical values in a ventilated adult (approximate)
| Parameter | Value | Note |
|---|---|---|
| Respiratory system compliance | ≈ 50–100 mL/cmH₂O | much lower in ARDS |
| Plateau pressure linked to more lung injury | > 30 cmH₂O | in ARDS studies |
| Peak − plateau gap | ≈ 5–10 cmH₂O | larger with airway obstruction |
| Pleural pressure at end-expiration, spontaneous breathing | ≈ −5 cmH₂O | becomes positive with high PEEP |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Spontaneous vs positive-pressure inspiration
| Feature | Spontaneous | Positive pressure |
|---|---|---|
| What moves air | Fall in pleural pressure | Rise in airway pressure |
| Pleural pressure in inspiration | More negative | Less negative or positive |
| Venous return in inspiration | Rises | Falls |
| Right ventricular afterload | Little change | Rises with large volumes and PEEP |
Clinical notes
- Hypotension right after intubation: sedatives lower venous tone and positive pressure lowers venous return, which is worst in hypovolemia.
- A sudden rise in plateau pressure with falling blood pressure in a ventilated patient suggests tension pneumothorax.
- Lung-protective ventilation in ARDS uses small tidal volumes because the aerated lung is small ('baby lung').
Test yourself
Think of the answer, then tap the question to see it.
Why does blood pressure often fall when positive-pressure ventilation is started in a bleeding patient?
Positive pressure raises pleural and right atrial pressure, narrowing the gradient for venous return; with low blood volume, reflexes cannot raise filling pressure enough, so output falls.
A ventilated patient's peak pressure rises sharply but the plateau pressure is unchanged. Where is the problem?
In the airways (higher resistance: bronchospasm, secretions, a kinked or bitten tube). Plateau reflects compliance, which has not changed.
Tidal volume 500 mL, plateau 30 cmH₂O, PEEP 10 cmH₂O. What is the static compliance?
500 / (30 − 10) = 25 mL/cmH₂O, well below normal: stiff lungs or chest wall, as in ARDS.
How does PEEP improve oxygenation?
It keeps collapsed alveoli open at end-expiration, raising FRC and reducing intrapulmonary shunt, so more blood meets ventilated alveoli.
Connections
In Pharmacology2
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Ventilator/Ventilator Support - What Is a Ventilator? | NHLBI, NIH (National Heart, Lung, and Blood Institute (NIH), 2022); Acute Respiratory Distress Syndrome (StatPearls (NCBI Bookshelf), 2025); Hypovolemia and Hypovolemic Shock (StatPearls (NCBI Bookshelf), 2025).
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Representative model, for educational purposes only.