C. Osmolar gap

The difference between calculated plasma osmolality and the measured osmolality is called the osmolar gap. Normally, the gap is less than 20 mosmo/kg H2O. If it is raised, then it denotes presence of unaccounted ions.

Causes of increased osmolar gap

•Ethanol

•Isopropyl alcohol

•Methanol, glycine, glycerol

•Ethylene glycol

Step 6: Look for alerts to mixed acid–base disturbances

•Absence of compensation

•Longstanding pulmonary or renal disease

•Excessive compensation

•Respiratory assistance

•Settings conducive to mixed disturbances

Step 7: If there is some discrepancy between ABG values and clinical condition of the patient, do validity check to authenticate the report

A.H+ = 24 × PCO2/HCO3.

Place the value of PCO2 and HCO3 and calculate H+.

B.Calculate H+ from pH as seen on ABG. At pH of 7.4, H+ concentration is 40.

For every 0.1 ↓ in pH, multiply H+ concentration sequentially by 1.25. For every 0.1 ↑ in pH, multiply H+ concentration sequentially by 0.8. Last two digits of pH = H+.

C.Match the H+ concentration by two methods: A and B. If it is matching, ABG is valid.

If it is not matching, recheck the ABG.

Suggested Reading

1.Ghosh AK. Diagnosing acid–base disorders. J Assoc Phys India. 2006;54:720–4.

The article provides a stepwise approach for evaluation of acid–base disorder.

2.Kellum JA. Clinical review: reunification of acid–base physiology. Crit Care. 2005;9:500–7.

It has been emphasized that both quantitative and traditional approaches can be combined for bedside assessment of acid–base status.

3.Story DA, Morimatsu H, Bellomo R. Strong ions, weak acids and base excess: a simplified Fencl-Stewart approach to clinical acid-base disorders. Br J Anaesth. 2004;92:54–60.

This article provides a simplified equation for calculation of sodium chloride effect and albumin effect on base excess.

4.Fencl V, Jabor A, Kazda A, et al. (2000) Diagnosis of metabolic acid-base disturbances in critically ill patients. Am J Resp Crit Care Med 162:2246–51.

Comprehensive description of acid–base disorder.

5.Gluck SL. Acid-base. Lancet. 1998;352:474–9.

6.Haber RJ. A practical approach to acid-base disorders. West J Med. 1991;155:146–51.

Websites

1.www.acidbase.org

2.www.uchc.edu

3.www.lakesidepress.com

4.www.merck.com