How to read a blood-gas distribution curve

A blood gas distribution curve reveals the relationship of the partial pressure of of carbon dioxide versus the partial pressure of oxygen.

In order to properly read a blood gas distribution curve, one should understand underlying princinples of ventilation-perfusion relationships. Ventilation-perfusion matching is a very important concept because the ratio of ventilation to perfusion is an essential factor in determing gas exchange. Essentially, when the ventilation of a region of the lung is high, the blood flow to that region should also be high. This usually occurs at the base of the lung. Conversely, when ventilation to a region of the lung is low, blood flow to that region should also be low. This usuallly occurs at the apex of the lung.

Most importantly, one should be familiar with the respiratory exchange ratio R. When there is no carbon dioxide present in the inspired gas (which is a good assumption to make since the concentration of carbon dioxide in the inspired air is often neglible), R equals the rate of carbon dioxide ouput divided by the rate of oxygen intake. The respiratory exchange ratio is also equal to

     PECO2= The partial pressure of carbon dioxide in expired air,
     PIO2 = The partial pressure of oxygen in inpired air,
     PEO2 = The partial pressure of oxygen in expired air,
     FIO2 = The fractional concentration of oxygen in inspired dry air.

Once an R value has been determined, one can now graph the partial pressure of carbon dioxide with respect to the partial pressure of oxygen using the equation:

where PA represents the concentration of a gas in alveolar space. One can then draw several different lines by changing the value of R. The points where these blood and gas R lines intersect make up the distribution curve. Assume momentarily that there is no inequality between ventilation and perfusion. The blood partial pressures of carbon dioxide and oxygen are then given by the same point as the gas partial pressures. This point is known as the ideal point. At the ideal point, PO2=100 mmHg and PCO2=40 mmHg. As R increases or decreases, the ventilation-perfusion inequality becomes more pronounced. The partial pressure of these gases in the mixed capillary blood(point a) than deviate from the partial pressures the mixed alveolar gas (point A). The alveolar-arterial O2 difference is then given by the x value of A minus the x value of point a.

As you follow the distribution curve from point i to point v, the ventilation perfusion ratio decreases. As you follow the distribution curve from point i to point I, the ventilation perfusion ratio increases.

Good luck on the midterm! :)

Sample problems:

Here are some sample questions that might appear on exams to test your proficiency in reading blood gas distribution curves:

1) Diagram 1 is a blood gas distribution curve.

Which point on the diagram represent repesents the highest ventilation perfusion ratio: point 1, 2, or 3?

2) If point 1 occurs at 60 mmHg of oxygen, what is the ideal alveolar-arterial oxygen difference?

Answers:

1) Point 3.

2) 40 mmHg.

Subtract the partial pressure of oxygen at point 1 from the ideal alveolar oxygen partial pressure to get 40 mmHg.

For more information on this topic, please refer to West , pages 53, 54, 171.

Also, check out the following links that may be helpful:

• University of Arizona: Respiration Lecture #49 . Good for practicing reading V/Q curves.
• Loyola: Concept of Ventiation/Perfusion Matching
• And, as always, if you have questions, or if you can suggest a helpful link, please e-mail me.

No!
Please don't make me go to any of those pages!
I want to go somewhere completely different!

This page was written by Mary Alice Kalpakian, a student in this course.

BME 403 Pages maintained by the T.A., Douglas Miles.