Ventilation

Ventilation is the movement of gas in and out of the lungs — specifically, the process by which fresh gas reaches the alveoli and carbon dioxide is eliminated from the body. Adequate oxygen saturation does not guarantee adequate ventilation; these are independent processes that must be assessed separately.

Ventilation is driven by the respiratory muscles — primarily the diaphragm — creating negative intrathoracic pressure that draws air into the lungs. It is regulated by brainstem respiratory centers responding to CO2 levels, pH, and oxygen tension. When ventilation fails, CO2 accumulates (hypercapnia), leading to respiratory acidosis, altered mental status, and eventually respiratory arrest.

Ventilation is monitored at the bedside through:

• Respiratory rate and pattern — tachypnea, bradypnea, irregular patterns, or apnea
• Tidal volume assessment — visible chest rise, auscultation of breath sounds
• Capnography — the most direct bedside measure of ventilation adequacy, tracking exhaled CO2 over time
• Arterial blood gas (ABG) — PaCO2 provides definitive measurement but requires blood sampling

Ventilation failure can result from:

• Central drive depression — opioids, sedatives, brainstem pathology reducing respiratory drive
• Neuromuscular weakness — myasthenia gravis, Guillain-Barre syndrome, residual paralysis
• Mechanical obstruction — upper-airway obstruction, severe bronchospasm, tension pneumothorax
• Chest wall restriction — obesity, flail chest, massive pleural effusion

The clinical trap is supplemental oxygen masking ventilation failure. A patient on high-flow oxygen may maintain SpO2 of 95% while their CO2 climbs to dangerous levels. Without capnography or serial blood gases, this deterioration is invisible until the patient becomes obtunded or arrests.

Related terms

• Oxygenation — the companion process assessed independently
• Capnography — the primary bedside monitor for ventilation
• Airway — the passage through which ventilation occurs