All You Really Need to Know to Interpret Arterial Blood Gases
Lawrence Martin, M.D.

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You are scuba diving off Grand Cayman island. You enter the water with an 80 cu. ft. tank of air pressurized to 3000 psi. The water temperature is a balmy 82 degrees. You and your dive buddy descend to 132 feet and maneuver at that depth over a sandy bottom. As the two of you cruise in a westerly direction, you espy a large object swimming only 30 meters away, and coming toward you. Sure enough, it is the infamous Carcharodon carcharias, at least 16 feet long, one of the largest sharks ever seen in this part of the world. You are thrilled and scared to death at the same time. The Great White comes to within 5 meters, then veers to the left and passes by.

Whew! At that moment your heart is pounding and your blood is racing. In fact, at that moment your cardiac output is 7 liters/minute. You check your air gauge: it is down to down to 1200 psi. You are still at 132 feet depth; it is time to make an ascent and your buddy agrees. Fortunately, being a trained scuba diver, you have managed to maintain normal alveolar ventilation and acid-base status during this encounter. You are also fortunate because your respiratory system, including your hemoglobin content, is normal.

Assuming that your body is metabolizing a normal 25% of the oxygen that is delivered to the tissues, before you begin ascent what is your PO2 in the following two blood vessels?

a) Aorta
b) Main pulmonary artery

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Lawrence Martin, M.D.