Physics and Anesthesiology

Ventilation: Perfusion Concepts:

13Attribution: color illustrations and design below: from Lecture 8. Lung Dynamics by Dr. M. Ludwig, McGill University (http://www.mmi.mcgill.ca/Unit2/Ludwig/lect21ventilationperfusion.htm)

 

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  • Normal case:  The continuous (solid) lines in the above graph refer to the case when alveolar and arterial pressure pressures rise in a coordinated way with the limit being the inspired partial pressure.  That is, without ventilation/perfusion abnormalities, the alveolar (PA or PEnd Tidalor PET and arterial Pa) anesthetic partial pressures increase together (solid lines)

  • By contrast, is 50% of cardiac output is shunted only one lung, the consequent effect depends on blood: gas solubilities of the anesthetics.  Generally, with 50% shunting, the rate of rise of PET is increased and the rate of rise of arterial partial pressure (Pa) is reduced

    • An example of a very soluble anesthetic in this analysis is ether whereas a representative of a sparingly soluble anesthetic is cyclopropane. although these anesthetics are not in use clinically, they are helpful in illustrating the influence of the particular agent solubility on the effects of shunting.

  • With one lung hyperventilated (secondary to endotracheal intubation for example), some effect will be seen on FA/FI for the sparingly soluble agent (cyclopropane). Because of the poor solubility, which is very limiting, the slight increase in FA/FI is not sufficient to compensate for the loss of potential anesthetic uptake from the other lung; consequently, the rate of rise in arterial anesthetic tension is significantly slowed.  

    • On the other hand, if one examines a very soluble anesthetic, represented here by ether, a slight increase in FA/FI is again observed; however, in this case there is sufficient capacity in the blood to compensate for the loss of uptake by the non-ventilated lung.  Therefore, the rate and extent of rise in arterial gas tension are not significantly affected.

      • Cyclopropane; blood:gas partition coefficient = 0.46; MAC = 9.2%

      • Nitrous oxide blood:gas partition coefficient = 0.47; MAC = 110%

      • Diethylether: blood:gas partition coefficient = 12; MAC = 1.92%

      • Halothane blood:gas partition coefficient = 2.5; MAC = 0.75%

  • The above graph represents a simulation; however, experimental data comparing the rate of anesthesia tension rise in the presence in absence of endotracheal intubation dogs reveals comparable findings:(see below graph)  [Figure from: Eger II, E.I., "Uptake and Distribution" in Anesthesia 5th edition vol. 1 (Miller, R.D. editor; Cucchiara, R.F., Miller, Jr., E.D., Reves, J.G., Roizen, M.F. and Savarese, J.J., consulting editors) Churchill Livingstone, Philadelphia, 2000, pp 83 (reference 10); 15original citation:Stoelting,.R.K., Longnecker, D.E. Effects of right-to-left shunt on rate of increase in arterial anesthetic concentration.  Anesthesiology 6: 352-356, 1972.

  • As expected, the rate of rise for cyclopropane (Cyclopropane; blood:gas partition coefficient = 0.46,very sparingly soluble), is delayed compared to control; for halothane (Fluothane), C, an effect is also observed, but not as prominent.  And for the soluble agent, methoxyflurane, ; blood:gas partition coefficient = 15, no appreciable effect on the rate of rise was noted. 

  • One clinically-relevant conclusion might be that, if ventilation perfusion mismatching exists, a greater delay in anesthesia would be observed for the relatively sparingly soluble agents such as desflurane (Suprane), sevoflurane (Sevorane, Ultane) and nitrous oxide by contrast to other agents such as isoflurane (Forane) or halothane (Fluothane).

 

 

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