CHAPTER 3 Blood Gas and Acid-Base Analysis
1 What are the normal arterial blood gas values in a healthy patient breathing room air at sea level?
pH | 7.36–7.44 |
PaCO2 | 33–44 mm Hg |
PaO2 | 75–105 mm Hg |
HCO3 | 20–26 mmol/L |
Base deficit | +3 to −3 mmol/L |
SaO2 | 95%–97% |
2 What information does arterial blood gas provide about the patient?
Arterial blood gas (ABG) provides an assessment of the following:
Oxygenation (PaO2). The PaO2 is the amount of oxygen dissolved in the blood and therefore provides initial information on the efficiency of oxygenation.
Ventilation (PaCO2). The adequacy of ventilation is inversely proportional to the PaCO2 so that, when ventilation increases, PaCO2 decreases, and when ventilation decreases, PaCO2 increases.
Acid-base status (pH, , and base deficit [BD]). A plasma pH of >7.4 indicates alkalemia, and a pH of <7.35 indicates acidemia. Despite a normal pH, an underlying acidosis or alkalosis may still be present.
Oxygenation and ventilation were discussed in Chapter 2 and acid-base status will be the area of focus for this chapter.
5 What are the common acid-base disorders and their compensation?
Primary Disorder | Primary Disturbance | Primary Compensation |
---|---|---|
Respiratory acidosis | ↑ PaCO2 | ↑ HCO3 |
Respiratory alkalosis | ↓ PaCO2 | ↓ HCO3 |
Metabolic acidosis | ↓ HCO3 | ↓ PaCO2 |
Metabolic alkalosis | ↑ HCO3 | ↑ PaCO2 |
* Primary compensation for metabolic disorders is achieved rapidly through respiratory control of CO2, whereas primary compensation for respiratory disorders is achieved more slowly as the kidneys excrete or absorb acid and bicarbonate. Mixed acid-base disorders are common.
6 How do you calculate the degree of compensation?
Primary Disorder | Rule |
---|---|
Respiratory acidosis (acute) | increases 0.1 × (PaCO2 − 40) pH decreases 0.008 × (PaCO2 − 40) |
Respiratory acidosis (chronic) | increases 0.4 × (PaCO2 − 40) |
Respiratory alkalosis (acute) | decreases 0.2 × (40 − PaCO2) pH increases 0.008 × (40 − PaCO2) |
Respiratory alkalosis (chronic) | decreases 0.4 × (40 − PaCO2) |
Metabolic acidosis | PaCO2
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