Intraoperative Hypoxia During Thoracic Surgery Tarek Ashoor

Objectives Shunting and its significance. Alveolar dead space. Physiology of LDP. HPV and the factors affecting it. Causes of hypoxia in one lung ventilation. How to manage them.

Introduction Shunting is : Shunting is simply the passage of venous blood (Venous admixture) to the left side of the heart. So What?

Introduction (cont.) The venous admixture causes dilution of the P a O 2 in the arterial blood ending in

Introduction (cont.) The venous admixture causes dilution of the P a O 2 in the arterial blood ending in Hypoxia

Introduction (cont.) This occur physiologically due to: –Thebesian veins of the heart –The pulmonary bronchial veins –Mediastinal and pleural veins Accounting for normal A-aD02, mmHg

Introduction (cont.) atelectatic lung (or part of it).Transpulmonary shunt occur due to continued perfusion of the atelectatic lung (or part of it). Perfused Non-ventilated part of the lung

Introduction (cont.) Dead space: Space in the respiratory tract that doesn ’ t share in gas exchange. This accounts for the normal difference between P a CO 2 and ETCO 2 (5 mmHg).

Introduction (cont.) Alveolar dead space: Parts in the lungs that are ventilated but not perfused. Ex: Pulmonary embolism

V-Q relationships in the anesthetized, open- chest and paralyzed patients in LDP

V-Q relationships in the anesthetized, open- chest and paralyzed patients in LDP (cont.)

Physiology of the LDP Upright LDP, lateral decubitusUpright LDP, lateral decubitus

Physiology of OLV The principle physiologic change of OLV is the redistribution of lung perfusion between the ventilated (dependent) and blocked (nondependent) lung Many factors contribute to the lung perfusion, the major determinants of them are hypoxic pulmonary vasoconstriction, and gravity.

HPV HPV, a local response of pulmonary artery smooth muscle, decreases blood flow to the area of lung where a low alveolar oxygen pressure is sensed. HPV aids in keeping a normal V/Q relationship by diversion of blood from underventilated areas. HPV is graded and limited, of greatest benefit when 30% to 70% of the lung is made hypoxic. But effective only when there are normoxic areas of the lung available to receive the diverted blood flow

Two-lung Ventilation and OLV

Factors Affecting Regional HPV

HPV is inhibited directly by volatile anesthetics (not N20), vasodilators (NTG, SNP, dobutamine, many ß 2-agonist), increased PVR (MS, MI, PE) and hypocapnia HPV is indirectly inhibited by PEEP, vasoconstrictor drugs (Epi, dopa) by preferentially constrict normoxic lung vessels

Hypoxemia in OLV Causes of hypoxemia in OLV: Causes of hypoxemia in OLV: –Mechanical failure of 0 2 supply or airway blockade –Hypoventilation –Factors that decrease Sv0 2 (  CO,  0 2 consumption)

Hypoxemia in OLV If severe hypoxemia occurs:If severe hypoxemia occurs: - -Am I using FiO2= 1? -Is my tube in correct position? -Is the tube clear (no secretions) -Am I using vasodilator?

Hypoxemia in OLV If severe hypoxemia occurs:If severe hypoxemia occurs: After asking those Questions consider: –CPAP (5-10 cm H 2 O, 5 L/min) to nondependent lung, most effective –PEEP (5-10 cm H 2 O) to dependent lung, least effective –Intermittent two-lung ventilation –Clamp pulmonary artery.

Right Robert Shaw – FOB Internal View from Tracheal Lumen

Left Robert Shaw – FOB Internal View