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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 9 th ed chapter 26

Acid-Base Analysis

W. Rose

See Marieb & Hoehn 9th ed., Chapter 26


Acid base analysis w rose see marieb hoehn 9 th 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

Abnormal Blood Gases

Arterial pH<7.35: Acidosis

Arterial pH >7.45: Alkalosis


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

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.


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

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).


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

Arterial blood gas diagram


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

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.


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

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


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

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


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

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


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

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


Acid base analysis w rose see marieb hoehn 9 th ed chapter 26

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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