Intraoperative
Hypoxia During
Thoracic Surgery
Tarek Ashoor
Objectives
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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 PaO2 in the arterial blood
ending in
Introduction (cont.)
The venous admixture causes dilution
of the PaO2 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, 10-15 mmHg
Introduction (cont.)
• 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, openchest and paralyzed patients in LDP
V-Q relationships in the anesthetized, openchest 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