section name header

Information

(see also ABG Values; Base Excess/Deficit; SVO2 Monitoring)

Table 3.1

Table 3.2

Respiratory Acidosis (↓pH, ↑PaCO2)

Respiratory acidosis is related to stroke, drug overdose, aspiration, pneumonia, acute respiratory distress syndrome (ARDS), cardiac arrest, chronic obstructive pulmonary disease (COPD), hypoventilation, and neuromuscular disorders.

Treatment: Aggressive chest PT, suction. Increase respiratory rate, increase tidal volume.

Respiratory Alkalosis (↑pH, ↓PaCO2)

Respiratory alkalosis is related to anxiety, fear, head trauma, brain tumor, hepatic insufficiency, fever, mechanical overventilation, pulmonary embolism, and thyrotoxicosis.

Treatment: Sedation, support, breathe in paper bag for attack of hyperventilation. Decrease respiratory rate, decrease tidal volume.

Metabolic Acidosis (↓pH, ↓HCO3−)

Metabolic acidosis is related to renal failure, diarrhea, TPN, acetazolamide (Diamox) (diuretic that prevents carbonic acid formation), ketoacidosis, and lactic acidosis (caused by bicarbonate loss or excess acids in extracellular fluid).

Treatment: Treat underlying cause, monitor intake/output and dysrhythmias, protect against infection.

Metabolic Alkalosis (↑pH, ↑HCO3−)

Metabolic alkalosis is related to volume depletion (loss of H+, Cl−, K+ from vomiting or diarrhea, gastric suction, or diuretic therapy). Hint: al-K+-low-sis means potassium value is low when the patient is alkalotic.

Treatment: Treat underlying cause, monitor intake/output, potassium replacement therapy.

Figure 3.3

Acute Decompensation vs. Chronic Compensation

There are rules that are consistent in ABG analysis:

  • The pH will never “overcorrect.” For example, if the imbalance is primary acidosis, the body will attempt to correct by raising the pH, but it will not increase >7.39 (7.40) even when fully compensated. Conversely, if alkalosis is the issue, the pH will correct to 7.41 (7.40) but not less than this value.
  • If the imbalance is acute, the compensatory system will not have engaged, resulting in a compensatory value that is within normal range and a pH out of range. For example, if the imbalance is metabolic acidosis, the pH will be low, the HCO3 will be low, and the PaCO2 will be decreasing to help compensate, but will still be within normal range. The ABG may look something like this: pH 7.30, HCO3 17, PaCO2 37.
  • If the imbalance is chronic, the compensatory system will be fully engaged, resulting in a pH that is within normal range, and the compensating system will be out of range. For example, a chronic COPD patient may have an ABG that looks like this:

pH 7.39, HCO3 32, PaCO2 56

ABG Sampling

Blood gas samples can be either arterial, venous, or mixed:

  • Arterial blood gas (ABG) is sampled via an arterial puncture or arterial line. The sample reflects fully oxygenated blood, with higher PO2 and O2 saturation percent than venous blood gases. ABGs are often used to assist with optimizing oxygen/ventilator therapy and trending acid-base balance. If the patient does not have an arterial line, a venous blood gas may be a better option, based on patient condition.
  • Venous blood gas (VBG) is sampled via a venous puncture or central venous catheter. The sample represents blood returning to the heart after oxygenating tissues. VBGs are used to evaluate acid-base balance and assess the body's ability to meet the oxygen supply demand of tissues
  • Mixed venous ABGs refer to a blood sample that indicates a complete mixing of the blood, that is, blood returned from the extremities to the right ventricle. This may sometimes be sampled from a central venous line, although even in the superior vena cava or right atrium, the mixing is incomplete.
Key Point

Often VBGs and mixed venous blood gases are used interchangeably, but a true mixed venous sample must come from the distal port of a pulmonary artery catheter. Mixed venous sampling provides more accurate information than a VBG, but both are beneficial when monitoring the trended values.

Table 3.3