The document discusses hypoxemia as a potential complication in anesthesia, emphasizing the role of hypoxic pulmonary vasoconstriction (HPV) in managing oxygen exchange during general anesthesia. It outlines the historical observations related to pulmonary oxygen exchange and the effects of various anesthetic agents on HPV. Additionally, it highlights that HPV is crucial in minimizing ventilation/perfusion mismatch, particularly during one-lung ventilation (OLV) scenarios, and addresses factors that can inhibit or enhance HPV efficacy.

1. By,
Dr. Arun Kumar MD
dept of Anesthesiology
YMC.
Moderator: Dr. Harish Hegde MD

2.  Hypoxemia resulting from respiratory
depression, airway obs, circulatory collapse is a
potential complication for any anesthetic
technique.
 However its also noted that during GA., an
additional cause of hypoxemia is noted i.e.
detioriation in the pulmonary exchange of
oxygen.

3.  First modern observation by Bradford and
Dean in 1894.
 Von Euler and Liljestrand 1946 recognized it as
the adaptive mechanism.
 They studied changes in the pulmonary
circulation of the cat in response to changes in
hypoxic mixtures.
 Relationship between PaO2 and hypoxic
segment of the lung was studied in animal
model (dogs).

4.  When hypoxic segments were small
(<30%), HPV had little effect on PaO2, because
shunt was small.
 When most lung is hypoxic no significant
normoxic region is seen to divert flow.
 When the lung is partially hypoxic (30-70%) as
in OLV, large difference between PaO2 to be
expected with normal HPV and in its absence.
 HPV brings the PaO2 from dangerously low
levels to acceptable levels.

5.  Current description is:
 The induction and maintenance of general
anesthesia results in.,
 Decrease in resting lung volume hence FRC
and the compliance of the lung and chest wall
reduces.
 These changes leads to atelectasis in the
dependant area of the lung.
 Pulmn shunting and perfusion of low V/Q
regions increases.

6.  HPV reduces blood flow to both atelectasis and
the low V/Q regions.
 Hence reducing the effects of the abnormalities
during the gas exchange.

7.  Redox theory.
 Pulmonary vascular smooth muscle
cells(pvsmc) and type 1 cells of the carotid
body.
 Hypoxia causes inhibition of outward
potassium current. Thus causing membrane
depol. and calcium entry through voltage gated
calcium channels.

8.  The general view is that the inhalational agents
inhibit HPV and intravenous agents do not.
 The studies done on the same, none are with
consistent results.
 But it is evident from these studies that both
inhalational and intravenous agents inhibit, but
inhibition is less in intravenous agents than in
inhalational.

9.  It is Observed that ketamine- no reduction in
FRC(adults), impaired O2 exchange(children).
 Thiopentone- (sheep, dogs) not assoc with dec
FRC, atelectasis or abnormal oxygen exchange.

10.  Inhibition:
 Trauma, vasodialator drugs(
nitroprusside, nitroglycerin, nitric
oxide, isoprenaline).
 Vasoconstrictors: which constrict the pulmn
vasculature (noradr, dopamine, histamine)
reduces the effectiveness of HPV.
 Indirect inhibitors: mitral stenosis, volume
overload, hypothermia, thromboembolism, larg
e hypoxic lung segment.

11.  Potentiator: Almitrine a respiratory stimulant
drug.
 Found to improve PaO2 in patients with COPD
and to have this effect in the absence of ventilatory
stimulation.
 It is tested in combination with nitric oxide to be
used in OLV.
 But till date the drug is not marketed outside of
france.

12.  Hypoxic pulmonary vasoconstriction is a
physiological, protective process occurring in a
non physiological settings.
 Helps in minimising the V/Q mismatch in a
hypoxic lung.
 It is abolished by factors causing pulmonary
vasodialatation, and also reduced by
vasoconstrictors.
 It is a process which makes OLV possible.

13.  During GA hypoxemia occurs due to atelectasis
or inhibition of HPV. Without these two events,
There wouldn’t have been increased shunt, and
hypoxemia would be a problem in every
patients, and OLV be difficult to manage
without additional measures to reduce blood
flow to collapse lung.
If hypoxemia occurs and extra pulm causes ruled
out, then its most probable cause is impairment
of HPV.

14.  What is hypoxia and hypoxemia?
 Why in OLV there is no saturation drop,
whereas in endobronchial intubation there is?
 Mechanism of HPV.
 V/Q mismatch.
 Factors affecting HPV.