Step 4: Is there appropriate compensation for the primary disturbance? Usually, compensation does not return the pH to normal 7. If the observed compensation is not the expected compensation, it is likely that more than one acid-base disorder is present.
Step 6: If an increased anion gap is present, assess the relationship between the increase in the anion gap and the decrease in [HCO 3 -]. This ratio should be between 1. Table 1 : Characteristics of acid-base disturbances. Table 2 : Selected etiologies of respiratory acidosis.
Table 3 : Selected etiologies of respiratory alkalosis. Table 4 : Selected causes of metabolic alkalosis. Table 5 : Selected etiologies of metabolic acidosis. Table 6 : Selected mixed and complex acid-base disturbances. The American Thoracic Society improves global health by advancing research, patient care, and public health in pulmonary disease, critical illness, and sleep disorders.
Tobacco Control Tuberculosis Washington Letter. However, because bicarbonate is the greater part of the base buffer, for most practical interpretations, BE provides essentially the same information as bicarbonate. The major advantage of BE is that its normal range is really easy to remember.
If one has established that problem is respiratory, then the BE can tell us something of the duration of the problem. If, for example, in a respiratory acidosis, the sHCO 3 has shown no sign of responding still within the normal range , the probable explanation is that there has not yet been time to respond ie the problem is an acute respiratory acidosis.
A respiratory acidosis with a low sHCO 3 would indicate a combined respiratory and metabolic -acidosis. Remember that one cannot live for long with pH outside of the normal range. An abnormal pH means there has to be an acute component to the problem. It is sometimes thought that type 2 respiratory failure is simply a more severe version of type 1.
However, this is not the case. Type 1 and type 2 respiratory failures are due to entirely different mechanisms. Type 2 respiratory failure is extremely an issue of ventilation, that is, the business of pumping air in and out of the lungs. When underventilation occurs, for what ever reason eg muscular weakness or opiate overdose , the P a CO 2 will increase the definition of underventilation and P a O2 must decrease even if the lungs are perfectly healthy.
Type 2 respiratory failure results from underventilation, which can occur even in the context of healthy lungs. In such circumstances, oxygen delivered to the lungs by ventilation is handled inefficiently and P a O 2 falls. However, provided that overall ventilation is normal, P a CO 2 is maintained. When P a O 2 is low yet P a CO 2 normal, type 1 respiratory failure is present, and such a result implies lung or pulmonary -vascular disease. Type 1 and type 2 respiratory failure can occur simultaneously.
Indeed, the combination is common in severe chronic obstructive pulmonary disease, for example. Given that the two conditions result from entirely different mechanisms, with implications for treatment, one should be able to distinguish between them. When the only derangement is P a O 2 , clearly the failure is type 1.
However, when the P a CO 2 is high, one has to work out whether the low P a O 2 can be accounted for by underventilation alone or whether there is an additional type 1 problem ie whether there is anything wrong with the lungs.
To do this, one needs to measure the alveolar—arterial gradient, that is, the difference between the alveolar partial pressure of oxygen P A O 2 and the P a O 2. In healthy young adults, the difference should be less than 2 kPa.
If the patient is older, breathing higher concentrations of O 2 or over ventilating, then the gap can widen, although in healthy patients this would not usually be expected to be greater than 4 kPa. If the alveolar—arterial gradient is higher than it should be, then a type 1 respiratory failure is present.
ABG interpretation is not difficult. Break down the task into steps and do them in order. For a more detailed review of arterial blood gas interpretation, see Ref 1. Box 1 provides an example of a patient presenting with breathlessness, where ABGs form an important diagnostic test. National Center for Biotechnology Information , U. Journal List Clin Med Lond v. Clin Med Lond. Graham P Burns , consultant respiratory physician A. Author information Article notes Copyright and License information Disclaimer.
Email: ku. Key points Most doctors struggle with arterial blood gas ABG interpretation ABG interpretation is easy Break it down into steps The first priority for the respiratory system is pH If partial pressure of carbon dioxide pCO2 goes down, partial pressure of oxygen pO2 should go up. The respiratory system — oxygenation vs pH In health, we are driven to take our next breath by the arterial partial pressure of carbon dioxide P a CO 2 , which is intimately linked to pH. Respiratory and metabolic systems — the speed of response The respiratory system can respond quickly to a metabolic derangement, with changes occurring to the blood gases within seconds to minutes.
What is the base excess? What does the base picture tell us? Box 1.
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