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LessonNormal Ventilation Vs. Artificial Ventilation

Normal Ventilation vs. Artificial Ventilation

The Physics of Breathing

The way air enters the lungs during normal breathing is fundamentally different from the way it moves during artificial ventilation.

Understanding this difference is critical, since positive-pressure ventilation (PPV) changes not only airflow but also physical circulation and gastric pressures inside the body.

Negative vs Positive Pressure Ventilation Comparison

Natural (Spontaneous)

Negative Pressure

When the diaphragm contracts downward, the chest cavity expands, creating a vacuum effect that actively pulls air into the lungs. This process is controlled automatically by chemoreceptors sensing CO₂ levels.

Artificial (BVM)

Positive Pressure

Air is artificially pushed into the lungs. This change in direction—from "pulling" to "pushing"—creates several important and potentially dangerous physiological side effects inside the chest cavity.

Physiological Effects of Artificial Ventilation

1. Air Movement

Positive pressure forces air into the lungs, potentially over-stretching delicate alveolar tissue. Clinical Risk: Over-inflation can rupture alveoli, causing severe barotrauma or pneumothorax.

Technique note: If the airway device is inserted too deeply, air may enter only one lung (usually the right), leading to unequal chest rise, low SpO₂, and low EtCO₂.

2. Blood Movement

Normal inspiration assists venous return (blood flowing back to the heart) by creating a vacuum in the chest. Positive pressure reverses this effect.

Each assisted breath increases intrathoracic pressure, which crushes the vena cava and reduces venous return.
Less blood returning to the heart plummets cardiac output and blood pressure.
In hypotensive patients (trauma, sepsis), excessive positive pressure can cause immediate cardiac arrest.

Clinical takeaway: Always monitor perfusion, pulse, and pressure closely when providing PPV!

3. Esophageal Inflation

During normal breathing, the esophagus remains closed to prevent air entry into the stomach. Under positive pressure, air can easily be forced open into the stomach, leading to Gastric distention and extreme vomiting risk.

Pro Tip: Apply gentle cricoid pressure (Sellick’s maneuver) to help keep air out of the stomach and direct flow into the trachea.

Summary Comparison Table

Feature	Natural (Negative)	Artificial (Positive)
Mechanism	Pulled in by diaphragm	Pushed in by an external force (BVM/Vent)
Circulation	Promotes venous return	Reduces venous return and Cardiac Output!
Barotrauma Risk	Minimal	Highly Increased if naturally over-ventilated
Gastric inflation	Rare	Common without strict airway control
Regulator	Autonomic feedback	Provider-controlled