The interpretation of blood gas data relies on certain standard variables being in place. Apart from atmospheric pressure (which everybody always assumes is 760 mmHg), theother most important variable is temperature. Temperature changes the physicochemical properties of water, influencing solubility of gases and the autoionisation of water into H3O+ and OH-.
Influence of temperature on pH and gas solubility
In brief summary:
- PaO2 drops by 5mmHg for every degree below 37°C.
- PaCO2 drops by 2mmHg for every degree below 37°C.
- pH increases by 0.015 for every degree below 37°C.
Influence of temperature on ABG interpretation
Two different approaches exist regarding the interpretation of ABG results from a hypothermic patient.
- Warm (or correct) all samples to 37°C, no matter how cold the patient
- We have no normal reference ranges for hypothermic pH PaO2 and PaCO2
- Normal ranges at 37°C don't apply to hypothermic samples
- Cellular physiology remains the same- at all temperatures intracellular pH remains at pN- the normal pH of neutrality, required for cellular function. The reason for this is that protein buffering of intracellular pH (via imidazole histidine residues) is also temperature dependent, and changes in parallel with body temperature. (Some considerable detail is offered on the physics and chemistry underlying these issues in the chapter on neutrality and the influence of temperature and pressure on pH.)
- Correct all samples to the patient's body temperature
- This makes all the samples appear alkalotic
- You are then tempted to hypoventilate or add CO2.
- The addition of CO2 counteracts the increased solubility and decreased partial pressure of CO2 at low temperature
- The added CO2 counteracts the hypothermic leftward shift of the oxygen dissociation curve, resulting in better oxygen delivery
- Increased CO2 improved cerebral blood flow by vasodilating the cerebral vessels.
In practical terms:
- The lower the temperature, the higher the gas solubility.
- The higher the solubility, the lower the partial pressure. This is Henry's Law - dissolved gas and free gas are in a temperature-dependent equilibrium. (a great deal more detail on this matter is available in the chapter on partial pressure and the solubility of gases).
- Warming a hypothermic blood sample to 37°C releases more gas, and the partial pressures will appear higher than they actually are in the hypothermic patient
- However: we always interpret results at 37°C, using the alpha-stat approach.
- At any temperature, an uncorrected pH of 7.4 and a PCO2 of 40 mm Hg represent normal acid-base balance.
Apocrypha, speculation, digression
Of the two abovementioned approaches, the alpha-stat approach is generally held to be superior. This may an arbitrary or accidental decision by a group of cardiac anaesthetists. In truth, there is no clear advantage to either approach, and one or the other may be preferred in certain circumstances.
The following chapters offer additional reading material on this topic: