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Respiratory Assessment and Monitoring 343

TABLE 13.6 Arterial blood gas findings for acid–base disturbances

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pH PaCO 2 (mmHg) HCO 3 (mmHg)
Respiratory acidosis
Uncompensated <7.36 >45 Within normal limits
Partially compensated <7.36 >45 >32
Fully compensated Within normal limits >45 >32
Respiratory alkalosis
Uncompensated >7.44 <35 Within normal limits
Partially compensated >7.44 <35 <22
Fully compensated Within normal limits <35 <22
Metabolic acidosis
Uncompensated <7.36 Within normal limits <22
Partially compensated <7.36 <35 <22
Fully compensated Within normal limits <35 <22
Metabolic alkalosis
Uncompensated >7.44 Within normal limits >32
Partially compensated >7.44 >45 >32
Fully compensated Within normal limits >45 >32

therapy. Base excess is an additional parameter mea- outside of normal limits but not enough to bring pH
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sured as part of the ABG report and it reflects the excess back to within normal limits
(or deficit) of base to acid in the blood. A positive figure ● in a non-compensated state, the pH will be outside
indicates a base excess (more base than acid; i.e. alkalosis normal limits, and the primary disruption (either CO 2
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if >+3); a negative figure indicates a base deficit (more or HCO 3 ) will also be outside normal limits while
acid than base i.e. acidosis if >−3). If the base excess is the remaining parameter has not compensated for this
+2 mmol/L, then removal of 2 mmol of base per litre of derangement and has stayed within normal limits.
blood is required to return the pH to 7.4. If the base
excess is −2 mmol/L (i.e. a base deficit), then 2 mmol of It can be difficult to differentiate the patient’s primary
base per litre of blood needs to be added to have a pH problem from their compensatory response. As a quick
of 7.4. Understanding this concept is useful as it can guide, if the CO 2 is moving in the opposite direction to
determine how much treatment is necessary to restore a pH, then the primary disruption is respiratory; if the
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patient’s pH to normal. 49,51 HCO 3 is moving in the same direction as pH, the disrup-
tion is metabolic. Table 13.6 provides a guide to ABG
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The final step of interpretation is to examine the pH, CO 2 findings for each acid–base disorder. Other parameters
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and HCO 3 levels collectively to determine if the patient measured on the ABG sample, such as lactate, electrolytes,
has fully compensated or partially compensated the haemoglobin and glucose, are also considered in deter-
primary dysfunction, or is in an uncompensated state. mining patient status.
With the respiratory system regulating the acid (CO 2 ) and
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the metabolic system regulating the base (HCO 3 ), resto- Oxygen Tension Derived Indices
ration of normal acid–base balance and homeostasis is
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possible. The ability of the body to achieve this deter- The alveolar-arterial gradient is a marker of intrapul-
mines whether the imbalance is fully compensated (pH monary shunting (i.e. blood flowing past collapsed areas
returned to normal), partially compensated (pH outside of alveoli not involved in gas exchange). The index is
of normal limits) or uncompensated. To assess compen- calculated as PAO 2 − PaO 2 (PAO 2 is the partial pressure
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sation, pH, CO 2 and HCO 3 are examined in the context of oxygen in the alveoli). PAO 2 is determined by a
of a patient’s clinical presentation: complex equation, the alveolar gas equation. PAO 2 and
PaO 2 are equal when perfusion and ventilation are
● in a fully compensated state, the pH is returned to perfectly matched. The gradient increases with age but
within normal limits, but the other two parameters a value of 5–15 is normal up until approximately middle
will be outside normal limits as the body has success- age. Despite questions about its clinical usefulness,
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fully manipulated CO 2 and HCO 3 levels to restore pH particularly in the critically ill, it is used in clinical
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● in a partially compensated state, the pH is not within practice as a trending tool to track intrapulmonary
normal limits, and the other parameters will also be shunting. Simply put, the larger the gradient between