Oxygenation during one-lung ventilation

Mechanism of hypoxia during one lung ventilation

• In the lateral decubitus position the dependent lung is under ventilated as it is compressed by the compression of the abdominal contents and the weight of the mediastinum. The nondependent lung is relatively over ventilated secondary to increased compliance as the corresponding hemithorax is opened.
• There is also a difference in perfusion in the lateral decubitus position. Perfusion is higher in the dependent lung secondary to the effects of gravity. This mismatch of ventilation and perfusion contributes to hypoxia while in the lateral decubitus position.
• During one lung ventilation in the lateral decubitus position there is mixing of unoxygenated blood from the collapsed nondependent lung with oxygenated blood from the still-ventilated depended lung widening the P(A-a) gradient and leading do hypoxemia.

Factors Affecting oxygenation during one lung ventilation

• The degree of HPV (hypoxic pulmonary vasoconstriction). HPV is an adaptive mechanism unique to the pulmonary circulation that allows redirection of blood flow to alveoli with higher oxygen tension, thereby reducing ventilation/perfusion mismatch. HPV has been shown to be of greatest benefit when 30-70% of the lung is made hypoxic.
• Factors known to inhibit HPV and worsen right-to-left shunting and oxygenation include : (1) most systemic vasodilators (nitroglycerin, nitroprusside, dobutamine, calcium channel antagonists, beta-2 receptor agonists), (2) inhalational volatile anesthetics, (3) very high or very low pulmonary artery pressures, (4) hypocapnia, (5) high or very low mixed venous PO₂, (6) pulmonary infection.
• Factors that decrease blood flow to the ventilated lung can counteract the effects of HPV and worsen oxygenation by indirectly directing blood to the collapsed unventilated lung: (1) high mean airway pressures in the ventilated lung secondary to high levels of PEEP, hyperventilation, or high peak inspiratory pressures, (2) a low FIO₂ leading to HPV in the ventilated lung, (3) vasoconstrictors (dopamine, epinephrine, phenylephrine) that will vasoconstric normoxic vessels greater than hypoxic ones, (4) intrinsic PEEP from inadequate expiratory times.

Mechanisms to improve oxygenation during one lung ventilation

• Adequate FIO₂. An FIO₂ of 1.0 has been shown to help protect against hypoxemia and is associated with PaO₂ values from 150-250 mm Hg during OLV. A high FIO₂ also promotes vasodilation in the dependent, ventilated lung to accept blood flow redistribution from the hypoxic nondependent lung. However, high FIO₂ may contribute to absorption atelectasis, oxygen toxicity, and bleomycin induced injury. So maintaining an adequate but not excessive FIO₂ is recommended.
• Most advise protective lung ventilation strategies with smaller tidal volumes of 6-8 ml/kg. Very high tidal volumes can lead to increased peak and mean airway pressures and PVR that may shunt blood flow to the nonventilated lung and may also contribute to barotrauma. However, very low tidal volumes may cause decreased FRC and atelectasis in the dependent lung which should also be avoided.
• Adequate ventilation. The respiratory rate should be adjusted to maintain a normal PaCO₂ of 40 mm Hg. This may require increasing the respiratory rate if tidal volumes are reduced. Hypocarbia can cause vasodilation that would inhibit HPV. Hypercarbia can cause increases in PVR to the worsening perfusion to the ventilated lung.

Interventions once a patient undergoing OLV develops hypoxia:

• Increase FIO₂.
• Recheck positioning of double lumen tube for correct placement using fiberoptic bronchoscope.
• Suction both lumens of double lumen tube for secretions or mucous plugs
• May need to notify surgeons to stop and return to two-lung ventilation. Or at least ask for periodic reinflation/recruitment maneuvers of the nondependent collapsed lung.
• Consider CPAP (5-10 cm H₂O) to the collapsed, nondependent lung. This maintains patency of the nondependent alveoli allowing gas exchange to occur and will divert blood away from the collapsed lung.
• Consider PEEP (5-10 cm H₂O) to the ventilated, dependent lung. This can increase FRC and improve gas exchange in the dependent lung. However, high levels can increase PVR and shunt blood flow to the nondependent lung.
• Ultimately may need to ligate or clamp the ipsilateral pulmonary artery (i.e. during pneumonectomy) so that all blood flow directed to ventilated lung.
• Last resort (as in patient undergoing a lung transplant) may need to go on cardiopulmonary bypass to improve oxygenation.