A DIFFICULT BLOOD GAS QUESTION
SCUBA DIVER MEETS GREAT WHITE
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,
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?
b) Main pulmonary artery
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Forward any comments to:
Lawrence Martin, M.D.