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Arterial blood gases The University of Manchester | W.L. Gan | 2012

Normal PaO2 Normal pH Normal PaCO2 Normal HCO3

80 - 100 mmHg 7.35 - 7.45 35 - 45 mmHg 22 - 26 mEq/L

pH PaCO2 HCO3

Respiratory acidosis normal

Respiratory alkalosis normal

Metabolic acidosis normal

Metabolic alkalosis normal

** same direction - metabolic

** opposite direction - respiratory

Partially compensated states

pH PaCO2 HCO3

Respiratory acidosis

Respiratory alkalosis

Metabolic acidosis

Metabolic alkalosis

Fully compensated states

pH PaCO2 HCO3

Respiratory acidosis normal but < 7.40

Respiratory alkalosis normal but > 7.40

Metabolic acidosis normal but < 7.40

Metabolic alkalosis normal but > 7.40

Respiratory buffer response

CO2 is carried in the blood to the lung where excess CO2 combines with water to form

carbonic acid. The blood pH changes with carbonic acid level. This triggers the lung to

increase or decrease the rate and depth of ventilation until the appropriate amount of

CO2 has been re established. The correction starts within 1 to 3 minutes.

Renal buffer response

As the blood pH decreases, the kidney will compensate by retaining HCO3 and as pH

rises, the kidney will excrete HCO3 through urine. It takes days to hours to correct.

Mixed respiratory and metabolic acidosis - cardiac arrest

pH PaCO2 HCO3

Type I respiratory failure Type II respiratory failure

PaO2 < 8kPa with normal or low PaCO2 PaO2 < 8kPa with PaCO2 > 6.0kPa

Pneumonia, pulmonary edema, PE, asthma, COPD, asthma, fibrosis, sleep apnea,

emphysema, fibrosing alveolitis, ARDS sedative drugs, CNS trauma, MG, GBS,

flail chest, kyphoscoliosis

Treat underlying cause. Oxygen (35-60%) Treat underlying cause. Controlled oxygen.

Assisted ventilation if PaO2