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 : So What? 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 PaO2 in the arterial blood ending in

Introduction (cont.) The venous admixture causes dilution of the PaO2 in the arterial blood ending in Hypoxia

Accounting for normal A-aD02, 10-15 mmHg 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, 10-15 mmHg

Introduction (cont.) Perfused Non-ventilated part of the lung 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 PaCO2 and ETCO2 (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 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

Factors Affecting Regional HPV HPV is inhibited directly by volatile anesthetics (not N20), vasodilators (NTG, SNP, dobutamine, many ß2-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: Mechanical failure of 02 supply or airway blockade Hypoventilation Factors that decrease Sv02 (CO, 02 consumption)

Hypoxemia in OLV 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: After asking those Questions consider: CPAP (5-10 cm H2O, 5 L/min) to nondependent lung, most effective PEEP (5-10 cm H2O) 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

Broncho-Cath CPAP System

Rich Man’s* CPAP *Guageguided CPAP system *Permits measuring actual pressure applied Adjust to 5-10 cmsH2O

POOR MAN’sCPAP (DLETT) 1 = BABYSAFEUnit 2 = Attached to surgical DLETT lumen 3 = O2 tubing to aux. O2port on anesthesia machine 4 = adjust flow so bag is just full(not quantitative)

CPAP with Arndt 1 = BABYSAFE system 2 = special connector (in kit) for Arndt CPAP administration through blocker lumen 3 = adjuster valve 4 = standard anesthesia circuit

X = Don’t place tight sealed catheter in endotracheal tube to try and deliver CPAP!!! It can lead to ………………. →

1 - Mediastinal Air 2 -Pneumothorax on side opposite sugery

Questions The increase in alveolar PCO2 decrease alveolar PO2 Pulmonary embolism increase the difference between the PaCO2 and ED CO2. Shunting cause mainly hypercarbia Pulmonary oedema may occur in the nondependent lung during single lung ventilation.

Questions(cont.) Application of CPAP to the nondependent lung is the least effective way to guard against hypoxia during single lung ventilation. The use of vasodilator is the appropriate way to manage hypertension during single lung ventilation. Valvular lesions of the heart have no impact on PO2 during single lung ventilation.

Questions(cont.) HPV is an all or non reflex. Decrease in FiO2 than 1% is important to guard against absorption collapse in the ventilated lung during single lung ventilation. Patients under single lung ventilation should receive below average IV fluids.

Questions(cont.) Single lung ventilation cause 50% shunting. High dose of inhalational anaesthetic is appropriate in controlling hypertension during single lung ventilation.

Questions(cont.) Hypotension increase the alveolar dead space. Physiological shunting accounts for the normal difference between the alveolar and the pulmonary end capillary PO2.

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