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Acid-Base Analysis W. Rose See Marieb & Hoehn 9 th ed., Chapter 26 . Normal Arterial Blood Gas pH = 7.35-7.45 P a CO 2 = 35-45 mmHg HCO 3 - = 22-26 mEq /L P a O 2 = 80-100 mmHg We will skip base excess and anion gap. Abnormal Blood Gases Arterial pH<7.35: Acidosis

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Acid-Base Analysis W. Rose See Marieb & Hoehn 9 th ed., Chapter 26

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Acid-Base Analysis

W. Rose

See Marieb & Hoehn 9th ed., Chapter 26


Normal Arterial Blood Gas

pH = 7.35-7.45

PaCO2 = 35-45 mmHg

HCO3- = 22-26 mEq/L

PaO2 = 80-100 mmHg

We will skip base excess and anion gap


Abnormal Blood Gases

Arterial pH<7.35: Acidosis

Arterial pH >7.45: Alkalosis


Arterial pH < 7.35: Acidosis

Is it respiratory, metabolic, or both?

If PaCO2 abnormally high (>45 mmHg), acidosis has a respiratory cause. PaO2 likely to be low (<80 mmHg).

If HCO3abnormally low (>22 mEq/L), acidosis has a metaboliccause.


Arterial pH > 7.45: Alkalosis

Is it respiratory, metabolic, or both?

If PaCO2 abnormally low (<35 mmHg), alkalosis has a respiratory cause (hyperventilation). PaO2 likely to be high normal (95-100 mmHg).

If HCO3abnormally high (>26 mEq/L), alkalosis has a metabolic cause (gain of strong acid, or loss of base).


Arterial blood gas diagram


Compensation

There is immediate but limited compensation from the bicarbonate buffer system of blood.

Lungs can compensate (in minutes) for a metabolic disturbance. Increased breathing to compensate for metabolic acidosis is a more robust and reliable response than decreased breathing in response to metabolic alkalosis.

Kidneys can compensate (in hours to days) for a respiratory disturbance. Renal compensation for respiratory acidosis is slow but may be nearly complete after 3-4 days; renal compensation for respiratory alkalosis is slow and incomplete.


Compensation

Abnormally low PaCO2 (<35 mmHg) in

metabolic acidosis indicates respiratory compensation (hyperventilation).

Pure (uncompensated) metabolic acidosis seldom seen

since respiratory system compensates quickly.

PaO2is often high normal (95-100 mmHg)

when there is respiratory compensation.

Compensated (or chronic) metabolic acidosis

pH=7.25, PaCO2=25 mmHg, HCO3=10 mEq/L

There’s respiratory compensation

It’s acidosis

The acidosis is metabolic


Compensation

Abnormally high PaCO2 (>45 mmHg) in

metabolic alkalosis indicates respiratory compensation (hypoventilation).

PaO2 may be normal or slightly below normal (<=80 mmHg)

Uncompensated metabolic alkalosis

pH=7.58, PaCO2=44 mmHg, HCO3=40 mEq/L

Compensated metabolic alkalosis

pH=7.50, PaCO2=55 mmHg, HCO3=40 mEq/L

There’s respiratory compensation

It’s alkalosis

The alkalosis is metabolic


Compensation

Abnormally high HCO3 (>26 mEq/L) in

respiratory acidosis indicates renal compensation.

Uncompensated (or acute) respiratory acidosis

pH=7.25, PaCO2=60 mmHg, HCO3=25 mEq/L

Compensated (or chronic) respiratory acidosis

pH=7.35, PaCO2=60 mmHg, HCO3=32 mEq/L

Abnormally high – this person isn’t getting rid of CO2

Near normal

Higher than normal – kidneys must be compensating for the high CO2


Compensation

Abnormally low HCO3(<22 mEq/L) in

respiratory alkalosis indicates renal compensation.

Compensated (or chronic) respiratory alkalosis

pH=7.44, PaCO2=26 mmHg, HCO3=17 mEq/L, PaO2=53 mmHg, pneumonia

It’s alkalosis

The alkalosis is not metabolic – quite the opposite

The alkalosis is respiratory


Sources

'Acid-base pHysiology' , K. Brandis, http://www.anaesthesiaMCQ.com

GlobalRPh: Arterial blood gases. http://www.globalrph.com/abg_analysis.htm


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