ABG Interpretation
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Source: http://www.aliem.com/paucis-verbis-card-abg-interpretation/

ABG Interpretation

Haber RJ. A practical approach to acid-base disorders. West J Med 1991; 155:146-51.

 

RULES OF THUMB:

1. Look at pH. Whichever side of 7.4 the pH is on, the process that caused it to shift to that side is the primary abnormality.

2. Calculate the anion gap. If AG≥20, there is a primary metabolic acidosis regardless of pH or serum bicarbonate concentration

3. Calculate the excess anion gap = total anion gap – 12 + measured bicarb.

* If sum is > 30, then metabolic alkalosis.

* If sum is < 23, then nongap metabolic acidosis

 

* Respiratory compensation occurs almost immediately in response to metab disorders

* Metab compensation occurs over 3-5 days in response to respiratory disorders

 

Primary Respiratory Alkalosis (eg. 7.50 / 29 / 80 / 22)

 

Anxiety

Hypoxia

Lung disease w/ or w/o hypoxia

CNS disease

Drug use (ASA, catechol, progesterone)

Pregnancy

Sepsis

Hepatic encephalopathy

Mechanical ventilation

 

 

Primary Respiratory Acidosis

eg. 7.25 / 60 / 80 / 26 = acute b/c no bicarbonate compensation

eg. 7.34 / 60 / 80 / 31 = chronic b/c bicarbonate compensation

 

CNS depression

Neuromuscular disorder

Acute airway obstruction

Severe pneumonia or pulm edema

Impaired lung motion (PTX)

Thoracic cage injury – flail chest

Ventilator dysfunction

 

 

Primary Metabolic Alkalosis (eg. 7.50 / 48 / 80 / 36)

* If Urine Cl is low: Vomiting, NG suction, diuretic use in past, post-hypercapnia

* If Urine Cl is normal-high: Excess mineralocorticoid activity, current diuretic use, excess alkali administration, refeeding alkalosis

 

Primary Metabolic Acidosis (7.20 / 21 / 80 / 8)

* Nonanion gap: GI bicarb loss (diarrhea, ureteral diversion), renal bicarb loss (RTA, early renal failure, carbonic anhydrase inhibitors, aldosterone inhibitors), HCl administration, post-hypocapnia

* Anion Gap:  MUDPILES

 

Pearl: If anion gap>20 exists, there is a primary metabolic acidosis.

 

Pearl: If anion gap exists, calculate the excess anion gap to determine if an underlying metabolic alkalosis or nongap metabolic acidosis exists.

 

Examples

7.4 / 40 / 80 / 24, Na 145, Cl 100 --> Metab acidosis + metab alkalosis

7.5 / 20 / 80 / 15, Na 145, Cl 100 --> Resp alkalosis+ metab acidosis+ metab alkalosis

7.1 / 50 / 80 / 15, Na 145, Cl 100 --> Resp acidosis+ metab acidosis+ metab alkalosis

7.15 / 15 / 80 / 5, Na 140, Cl 110 --> Metab acidosis gap AND nongap