Hypoxia During One-Lung Ventilation

Pathophysiology of Hypoxia During One-Lung Ventilation

• When OLV is initiated, the nondependent lung becomes atelectatic. Alveolar oxygen level decreases, and hypoxemia develops. During OLV, a shunt-like effect may arise from continued perfusion of the nonventilated lung and inadequate expansion of the ventilated or dependent lung while the patient is receiving a high inspired O₂ fraction (FiO₂ = 1.0) or from hemodynamic changes related to the anesthetic management and position during thoracic surgery.
• Hypoxemia during OLV is caused by venous admixture through shunts and areas of low V/Q gas exchanging units. Based on animal studies, the maximal HPV response during OLV decreases blood flow to the nondependent lung by 50%. During OLV, the estimated venous admixture is usually 20−25% of total cardiac output. Therefore, patients who are well oxygenated during two-lung ventilation will have a PaO₂ of approximately 350−400 mmHg and, when converted to OLV, will have a PaO2 of approximately 150−200 mmHg while receiving FiO₂ of 1.0.

HPV

• With the patient in a lateral decubitus position when both lungs are being ventilated, the proportion of the pulmonary blood flow is distributed as follows: The dependent (nonoperative) lung receives approximately 60% of the pulmonary blood flow (more perfusion), whereas the nondependent lung (operative lung) receives 40% of the total pulmonary blood flow. When OLV is instituted, the nondependent lung becomes atelectatic. Because the alveolar O₂ decreases, there is a response to HPV in which increased pulmonary vascular resistance diverts blood flow towards the dependent lung.
• HPV is a biphasic reaction with an early response that usually starts within seconds and reaches a peak between 20−30 minutes during OLV, followed by a delayed response during the next 2 hours when the maximal vasoconstrictor response is achieved. Throughout this time, the PaO₂ will gradually increase. However, during repeated episodes of OLV and re-expansion of the nondependent lung, the HPV reflex appears to be augmented. During these subsequent periods of OLV, oxygenation is thought to improve because the blood flow in the reinflated lung has not returned to baseline before the subsequent hypoxic challenge.⁴
• Figure 1 displays the physiology of OLV. HPV is an autoregulatory reflex mechanism that reduces pulmonary blood flow through the nondependent lung by 40−50% and diverts part of the blood flow to the dependent lung. Factors that will modify the HPV response during OLV include hypotension, hypocapnia, hypothermia, the presence and severity of chronic obstructive pulmonary disease (COPD), and to a minimal degree, the use of vasodilators, vasoconstrictors, and anesthetic agents.
• HPV is inhibited by several anesthetic agents.⁴
  • All volatile anesthetics (halothane, sevoflurane, isoflurane, and desflurane) inhibit HPV in a dose-dependent fashion.
  • Inhaled nitric oxide (NO) inhibits HPV secondary to its pulmonary vasodilatory effects.
  • Nitroprusside and nitroglycerin are NO donors and inhibit HPV.
  • Phosphodiesterase inhibitors such as sildenafil impede the breakdown of cyclic GMP, resulting in more NO and vasodilation.
  • Calcium channel blockers inhibit HPV.
  • Angiotensin-converting enzyme inhibitors also inhibit HPV.
  • Prostacyclin is a potent pulmonary vasodilator.
• Of note, propofol has not been shown to inhibit HPV. The effects of nitric oxide on HPV are not clear.⁴

Predictors of Hypoxemia

Factors that predict which patients are most likely to develop hypoxemia during OLV include:

• Side of the surgery: right-sided surgery (right lung collapse) and left-sided ventilation increases the risk of hypoxia because the right lung is approximately 10% larger than the left lung.
• Pulmonary function test is the percentage of forced expiratory volume in one second [FEV₁]. An inverse correlation exists between FEV₁ and PaO₂.
• Low intraoperative PaO₂, decreased values of PaO₂ intraoperatively during two-lung ventilation in a lateral position while the patient is receiving 100% oxygen.
• Perfusing of the lungs: lung perfusion studies have demonstrated that the nonventilated and collapsed lung is more impaired in patients presenting for pneumonectomy than in patients who present with solitary lung nodes undergoing video-assisted thoracoscopic surgery.
• Body mass index (BMI): patients with a BMI greater than 30kg/m² who require OLV develop more episodes of intraoperative and postoperative hypoxemia when compared with nonobese patients.
• Previous lobectomy on the contralateral side increases the risk of hypoxia because 25% of the lung function has been affected by previous lobectomy.
• Position of the patient during surgery: during OLV in a lateral decubitus position, gravity augments the redistribution of perfusion to the ventilated or dependent lung, improving and maintaining ventilation-perfusion (V/Q) matching.
• Table 1 lists the factors that increase the risk of hypoxemia during OLV.

Management

• If hypoxia occurs during OLV, any nonurgent surgical procedure must be stopped, FiO₂ should be increased to 1.0, and two-lung ventilation restored.
• After hypoxia is treated, a fiberoptic bronchoscopic exam should be performed to ensure that the DLT or bronchial blocker is in optimal position.
• Alveolar recruitment maneuvers should be used to improve arterial oxygenation. Lung recruitment is a ventilator maneuver aimed to reverse atelectasis by means of a brief, controlled increase in the airway pressures with expansion of the lungs. This maneuver will improve arterial oxygenation, decrease pulmonary shunt and dead space with adequate levels of positive end-expiratory pressure (PEEP).
• Application of continuous positive airway pressure (CPAP) to the nondependent lung has been suggested in the deflation phase of tidal volume (VT) breath. CPAP is thought to improve oxygenation by a passive mechanism (uptake of oxygen by the alveoli with continuous oxygen administration).
• Apneic oxygen insufflation with application of 3L of O₂ via a suction catheter to the nonventilated side has improved oxygenation during OLV.⁵
• Other recommended maneuvers to manage hypoxia during OLV are listed in Table 2.