- Blood pH is normally maintained at 7.36-7.44
- pH is a logarithmic scale
- A change in pH of 0.3 units is equivalent to a doubling of hydrogen ion concentration
- pH is maintained by biological buffering mechanisms involving
- Proteins
- Bicarbonate
- Haemoglobin
- The relationship between serum pH and bicarbonate concentration is simple
- Described by the Henderson-Hasselbach equation
- Compensatory mechanisms exist to compensate for changes in pH
Blood gas analysis
- Blood gas analyser measures
- Partial pressure oxygen
- Partial pressure carbon dioxide
- pH
- Other variables are derived using Henderson-Hasselbach equation
Variables derived from a blood gas analyser
| Variable |
Normal value |
| Temperature |
37 degrees |
| pH |
7.36 - 7.44 |
| Partial pressure CO2 (pCO2) |
4.6 - 5.6 kPa |
| Partial pressure O2 (pO2) |
10.0 - 13.3 kPa |
| Bicarbonate |
22 -26 mmol/l |
| Total carbon dioxide |
24 - 28 mmol/l |
| Standard bicarbonate (SBC) |
22 - 26 mmol/l |
| Base excess (BE) |
-2 to +2 mmol/l |
| Standard base excess (SBE) |
-3 to +3 mmol/l |
| Oxygen saturation |
>95% |
| Haemoglobin |
11.5 - 16.5 g/dl |
Definitions
- Acidosis = a rise serum in hydrogen iron concentration or fall in pH
- Alkalosis = a reduction in hydrogen iron concentration or rise in pH
- Respiratory acidosis = a fall in pH due to a rise in partial pressure of carbon dioxide
- Respiratory alkalosis = a rise in pH due to a fall in partial pressure of carbon dioxide
- Metabolic acidosis = a fall in pH due to a metabolic cause
- Metabolic alkalosis = a rise in pH due to a metabolic cause
Anion gap
- Is the sum of the positive and negative charges in the plasma
- Cations = sodium and potassium
- Anions = chloride, bicarbonate
- Difference between the two is the anion gap
- If metabolic acidosis is due to anion excess the anion gap is increased
- If metabolic acidosis is due to bicarbonate loss the anion gap is normal
- Lactic acidosis and renal failure are associated with an increased anion gap
Interpretation of results
- Interpret results with knowledge of the patients clinical condition
- Check for the consistency within the blood gas sample
- Look at the pH for the primary acid-base disorder
- Assess a respiratory component by looking at the partial pressure of carbon dioxide
- Assess the metabolic component by looking at the BE or SBE
- Calculate the anion gap
Bibliography
Grocott M P, Stephens R, Hamilton M A. What are you going to do about the base
deficit? Hosp Med 2002; 63: 701.
Kellum J A. Determinates of pH in health and disease. Crit Care 2000; 4: 6-14. |