Physiology Questions for the RRT Exam — Practice Free

A P50 of 32 mmHg (Normal ~27 mmHg) indicates a RIGHT SHIFT. This means hemoglobin has a decreased affinity for oxygen, which facilitates easier unloading of O2 at the tissue level. Common causes include Hypercapnia, Acidosis, Hyperthermia, and increased 2,3-DPG. NBRC Exam Tip: Use the mnemonic CADET, face Right! (CO2, Acid, DPG, Exercise, Temperature) to remember factors that shift the curve to the right. While a right shift improves unloading, a severe shift can make it harder for Hgb to pick up oxygen in the lungs if PaO2 is low.

The Bohr effect describes the impact of CO2 and pH on Hb-O2 affinity. At the systemic tissues, high CO2 levels and the resulting decrease in pH (acidosis) cause a right shift in the oxyhemoglobin dissociation curve. This decreases Hgb's affinity for O2, promoting oxygen unloading to the metabolically active tissues. NBRC Distractor: Do not confuse this with the Haldane effect. The Bohr effect happens at the TISSUES (CO2 affecting O2), while the Haldane effect happens at the LUNGS (O2 affecting CO2 transport).

Normal CvO2 is 12-15 vol%. A low CvO2 (10 vol%) suggests that either oxygen delivery (DO2) is inadequate (due to low cardiac output, low Hb, or low PaO2) or tissue oxygen consumption (VO2) has significantly increased (fever, shivering, sepsis). The tissues are forced to extract a higher percentage of oxygen from the blood to meet metabolic demands. NBRC Tip: CvO2 is the best indicator of the adequacy of tissue perfusion and the balance between supply and demand.

The patient has Methemoglobinemia. Benzocaine oxidizes the iron in hemoglobin from the ferrous (Fe2+) to the ferric (Fe3+) state. MetHb cannot bind oxygen and causes a Left Shift of the remaining normal hemoglobin, further hindering O2 release. SpO2 typically locks around 85% regardless of FiO2. NBRC Strategy: Suspect this when there is a saturation gap (calculated SaO2 from ABG is much higher than measured SpO2). Treatment of choice is Methylene Blue.

VO2 = Qt x [C(a-v)O2] x 10. Calculation: 6 L/min x (18 - 13) x 10 = 6 x 5 x 10 = 300 mL/min. The factor of 10 is used to convert vol% (mL/100mL) to mL/L. Normal VO2 is approximately 250 mL/min. NBRC Exam Tip: If the question asks for VO2 in L/min, the answer would be 0.30 L/min. Always check the units required in the multiple-choice options. This calculation helps determine if a patient's metabolic rate is elevated (e.g., in sepsis or hyperthermia).