Odd QBank Question about tourniquets and acid-base

[CharleyVCU1988]

24-yo man having anterior cruciate ligament repair with hamstring graft. Tourniquet inflated to 300 mmHg as a method of decreasing blood loss. Patient stable intraoperatively and the procedure lasts just over 60 mins. What type of acid-base disturbance is expected with deflation of the tourniquet?

A - resp alkalosis
B - resp acidosis
C - metabolic acidosis
D - mixed acidosis

picked C, got told I was wrong and it was D. Part of the explanation given by the Qbank is:

"The respiratory acidosis will occur in the extremity as carbon dioxide levels continue to rise, however this will not occur in isolation. Respiratory acidosis will be evident when the tourniquet is deflated and a transient rise in end tidal CO2 is seen. A metabolic acidosis will occur in the extremity as oxygen is consumed, and as anaerobic metabolism becomes the predominant form of ATP production. As lactic acidosis in the extremity accumulates the acidosis will worsen."

What gives? I thought respiratory depression (acutely) comes from things like airway obstruction, centralized failure to ventilate from CNS, drug induced depression, or neuromuscular disease.

What am I missing?

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[pgg]

The respiratory acidosis doesn't come from respiratory depression. The extremity uses up its oxygen but the CO2 has nowhere to go until the tourniquet is released. That CO2 gets back to the rest of the body when the tourniquet goes down, and that's what causes the resp acidosis.

 

[CharleyVCU1988]

I guess the better question is, am I approaching the criteria for defining respiratory acidosis all wrong?

 

[deleted171991]

Respiratory acidosis is defined as (acidemia associated with) an increase in pCO2. Regardless of cause (99% ventilation-related, unless compensatory). It's secondary to either increased production or decreased elimination of CO2.

Anyway, tricky question. This is like saying that IV sodium bicarbonate gives (transitory) respiratory acidosis. I wouldn't worry about it. Some wiseguy author wanted to feel smarter than he actually is; this question is "statistically significant", but not clinically, if you get my drift.

I have never had to hyperventilate a patient after tourniquet deflation to compensate for this so important acidosis, and that's the only reason we should even mention it.