In thoracic anaesthesia, few techniques test the anesthesiologist’s skill and physiological understanding as much as one-lung ventilation (OLV). The ability to isolate and ventilate a single lung while maintaining oxygenation and carbon dioxide removal from only half the pulmonary surface is both an art and a science. Behind this clinical precision lies a fascinating world of physiological adaptation, vascular responses, and gas exchange dynamics.
The Normal Two-Lung Physiology
Under usual conditions, both lungs share the work of ventilation and perfusion in near harmony. Gravity directs more blood flow to the dependent lung regions, while alveolar ventilation follows a similar pattern.
This balanced ventilation–perfusion (V/Q) matching keeps arterial oxygen levels high and carbon dioxide elimination efficient. Only about 5% of cardiac output is normally shunted past non-ventilated areas of the lung.
The Shift During One-Lung Ventilation
When OLV begins, the anesthesiologist deliberately collapses one lung to create a still surgical field. Suddenly, only the dependent lung (the ventilated one) participates in gas exchange, while the non-dependent lung continues to receive some blood flow despite being unventilated.
This creates an inevitable right-to-left shunt, reducing arterial oxygenation (PaO₂). Yet, the body is far from helpless—several physiological mechanisms come into play to protect oxygen delivery.
- Hypoxic Pulmonary Vasoconstriction (HPV): The Guardian Reflex
HPV is one of nature’s most elegant defence mechanisms. When alveolar oxygen tension falls in the non-ventilated lung, the small pulmonary arteries in that region constrict. This vasoconstriction diverts blood away from poorly oxygenated alveoli toward areas that are better ventilated, thereby reducing shunt and improving arterial oxygenation. Although HPV cannot completely prevent desaturation, it can decrease blood flow to the non-ventilated lung by nearly half. Certain factors—like high volatile anesthetic concentrations, alkalosis, or excessive pulmonary artery pressures—can blunt this response. - Changes in Ventilation–Perfusion Relationships
During OLV:
Ventilation is directed only to the dependent lung.
Perfusion continues in both lungs, though reduced in the non-ventilated one because of HPV. The result is a temporary V/Q mismatch that the anesthesiologist must correct using careful ventilatory and oxygenation strategies. - Lung Mechanics in OLV
The ventilated (dependent) lung faces increased mechanical stress. Because of the lateral decubitus position, the mediastinum and abdominal contents compress the dependent lung, reducing its compliance. Airway pressures rise, and without caution, alveolar overdistension may occur. Thus, lung-protective ventilation strategies are crucial—low tidal volumes (4–6 mL/kg), moderate PEEP, and adequate FiO₂ are used to maintain oxygenation while avoiding volutrauma. - Oxygenation and Shunt Fraction
Normally, the shunt fraction is small (<5%), but during OLV, it may rise to 20–30%. Despite this, most patients maintain acceptable oxygenation because of HPV and appropriate ventilatory management. However, in conditions that increase pulmonary blood flow to the non-ventilated lung—such as high cardiac output or inhibition of HPV—significant hypoxemia can occur. - Carbon Dioxide Elimination
Interestingly, carbon dioxide elimination remains relatively stable during OLV. CO₂ diffuses rapidly across alveolar membranes, and the ventilated lung can usually compensate by slightly increasing minute ventilation. Only in severe hypoventilation does hypercapnia become significant. - The Influence of Patient Position
The lateral decubitus position benefits oxygenation because gravity directs most pulmonary blood flow to the dependent (ventilated) lung. This improves V/Q matching during OLV. In contrast, supine OLV often results in worse oxygenation since perfusion distribution becomes more uniform, leaving more blood flowing through the non-ventilated lung.
Anesthetic Considerations
Anesthetic management during OLV requires constant vigilance and adaptability.
Maintain FiO₂ sufficient for SpO₂ > 90%.
Apply PEEP to the ventilated lung to prevent collapse.
If hypoxemia persists, add CPAP (2–5 cm H₂O) to the non-ventilated lung, if the surgical field allows.
Avoid excessive airway pressures or agents that suppress HPV.
Ensure normovolemia and stable hemodynamics to optimise perfusion.
One-lung ventilation is a remarkable demonstration of physiological resilience. The lungs, heart, and pulmonary vasculature coordinate to sustain life even when one lung rests. For the anesthesiologist, understanding these physiological principles is not merely academic—it’s the foundation of safe thoracic anaesthesia. When science and skill merge, OLV becomes more than a challenge; it becomes a testament to how precisely the human body adapts when guided by informed hands.
Dr.Rabida.C
Specialist-Anaesthesiology
Baby memorial hospital-Kannur