Intro to Hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction, a seldom heard phenomenon but very effective physiologic property which helps lungs utilise ventilation to the maximum

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

2. INTRODUCTION 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. INTRO… CONTD. 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. Findings 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. PATHOPHYSIOLOGY 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. Pathophysiology contd.. 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. Molecular mech. 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. Effects of anesthetics 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. Other factors 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 , large 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. TAKE HOME…. Msg.. 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. Conclusion 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. Possible questions 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.