Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 102

Most blood results in medicine are relative easy to make sense of; there’s one value, and it’s either high or low. When faced with the results of an arterial blood gas measurement, the clinician has to handle six different values, which must be drawn together and interpreted as one. The inexperienced often find their attention skipping from one number to the next, declaring each as either high or low, before becoming utterly confused and giving up. A simple stepwise system for interpreting blood results would help. The following algorithm is easy to follow and will make sense of most of the results you’ll come across in clinical practice.

1 Look at the pH. Decide whether this is an acidosis or alkalosis. Once that fact is determined, don’t be diverted from it after reviewing the other values. It won’t change. If the pH is in the normal range, note whether it is erring towards one end of the range or the other. If there is a compensated abnormality, the position of the pH within the range may indicate the nature of the primary disturbance. Remember that physiological compensatory mechanisms don’t overcompensate.

2 Look at the PCO 2 . Ask whether the PCO2 is contributing to or attempting to compensate for the abnormality identified in the pH. That will allow you to know whether the primary disturbance is respiratory (contributing) or metabolic (attempting to compensate).

3 Look at the bicarbonate. I would suggest either the sHCO3 – or base excess. In the case of a primary metabolic problem, the bicarbonate may hold no surprises. In a metabolic acidosis, it will be low; in a metabolic alkalosis, high. In the case of a primary respiratory problem, the bicarbonate may be normal (suggesting the problem is acute, having not had time to change), attempting to correct the respiratory effect on the pH (suggesting the problem is chronic) or compounding the problem (suggesting a mixed disturbance).

4 Look at the PO 2 .Knowing the inspired partial pressure of oxygen, ask whether the PO2 is what you’d expect given the level of ventilation (i.e. given the PCO2) or lower than expected. This may be difficult to gauge, in which case the alveolar gas equation should be applied (see Chapter 1). You can then determine whether type I respiratory failure, type II respiratory failure or both are present.