An increase in which of the following parameters will shift the O2 dissociation curve to the left?

Oxygen dissociation curve demonstrates the percentage of haemoglobin bound with oxygen (saturation) as the oxygen concentration increases. It represents the affinity of haemoglobin for oxygen. A shift of the curve to the left indicates increased affinity of hemoglobin for oxygen. Three impoant factors which affect the oxygen hemoglobin dissociation curve are pH, temperature and the concentration of 2,3 bisphosphoglycerate (2,3, BPG). Fall in temperature, low levels of 2,3 BPG and rise in pH cause shift of the curve to left. Fall in Pco2 result in an increase in pH and shift of the curve to left. Ref: Textbook of Medical Physiology By Guyton, 10th Edition, Page 466; Ganong's Review of Medical Physiology, 23rd Edition, Page 667-9.