American Board of Surgery Qualifying Exam (ABS QE) Practice Test

Which of the following indicates a right shift in the oxygen dissociation curve?

1. High PCO2 and low pH

2. Low PCO2 and high pH

3. Low temperature and high DPG

4. High temperature and low PCO2

The correct answer is: High PCO2 and low pH

A right shift in the oxygen dissociation curve indicates that hemoglobin's affinity for oxygen has decreased, meaning that oxygen is more readily released to the tissues. This shift can occur in response to certain physiological conditions. High levels of carbon dioxide (PCO2) and low pH are significant factors that contribute to this rightward shift. When PCO2 is elevated, it leads to an increase in hydrogen ions (H+) in the blood, causing a drop in pH (increased acidity). This phenomenon is part of the Bohr effect, where increased CO2 and lowered pH enhance the release of oxygen from hemoglobin, facilitating oxygen delivery to metabolically active tissues that require it most during periods of increased activity or stress. The other options do not support a right shift in the curve. Low PCO2 and high pH would suggest an increase in hemoglobin's affinity for oxygen, moving the curve to the left. Similarly, low temperature typically stabilizes hemoglobin’s conformation favorably for oxygen binding, which also promotes a leftward shift, while high temperature and low PCO2 do not correspond to the conditions typically associated with a right shift.