Intraoperative
Hypoxia During
Thoracic Surgery
Tarek Ashoor
Objectives
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•
•
•
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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
Broncho-Cath CPAP
System
Rich Man’s* CPAP
*Guageguided
CPAP system
*Permits
measuring
actual
pressure
applied
Adjust to 510 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.
•THANKS