Respiratory physiology - Hypoxic pulmonary vasoconstriction

Question 1

What is the fundamental difference between the physiology of the systemic and pulmonary circulations?

Answer 1

Hypoxic pulmonary vasoconstriction

Question 2

What is the purpose of hypoxic pulmonary vasoconstriction

Answer 2

In the fetus:
In utero, in the fetus, HPV prevents blood flow through the unventilated lungs promoting fetal circulation to and from the placenta instead.

In adults:
It helps to match regional ventilation and perfusion although it has little effect in healthy lungs

Question 3

Does HPV diminish with Age

Answer 3

In some animal studies but not shown in humans

Question 4

What effect does hypercapnoea and acidosis have on the pulmonary vasculature

Answer 4

High CO2 and low pH cause increased H+ concentration –> pulmonary vasoconstriction –> increased pulmonary vascular resistance

Question 5

How long does HPV take

How long does pulmonary vascoconstriction associated with hypercapnoea and acidosis take?

Answer 5

HPV is biphasic
Phase 1 –> active within seconds and peaks 15 mins

When moderate hypoxia is sustained for 30 minutes
Phase 2 –> further increase in PVR with peak at 2 hours

Hypercapnoea and acidosis related pulmonary vasoconstriction peaks at 4 hours

Question 6

What is the predominant stimulus for HPV

Answer 6

Low Alveolar PAO2

Small contribution from PaO2 in the alveolar capillaries

Question 7

Name drugs that augment HPV

Answer 7

1. B2 agonists - salbutamol

2. alpha 1 agonists - phenylephrine / noradrenalin

Question 8

Name non-anaesthetic drugs that attenuate HPV

Answer 8

1. Inhaled NO
2. Phosphodiesterase inhibitors
3. NO donors: nitroprusside and nitroglycerine
4. Prostacylin
5. CCB
6. ACE:I

Question 9

Name anaesthetic drugs that inhibit HPV

Answer 9

1. Volatile agents (halothane most)

Iso, sevo, des –> very mild impairment HPV (dose dependent)

Question 10

Name anaesthetic drugs that do not inhibit HPV

Answer 10

1. Intravenous anaesthetic agents

Question 11

Why is one lung ventilation (OLV) performed?

Answer 11

To facilitate surgical access in the chest during Lung, mediastinal and intrathoracic oesophageal surgery

Question 12

Define hypoxaemia during One Lung Ventilation (OLV)

In what % of OLV cases is hypoxaemia observed

Answer 12

SaO2 < 90% with an FiO2 of 1.0

5% of cases

Question 13

Why has the incidence of hypoxaemia during one lung ventilation improved in recent times

Answer 13

1. Fibreoptic scope assisted placement of double lumen endotracheal tubes and bronchial blockers
2. Improved anaesthetic agents and techniques that cause less inhibition of HPV during OLV