# interpretation of arterial blood gases (abgs)

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ABGANALYSIS & ACID-BASE DISORDERS

Dr. Haseeb Ahmed2008-98ZMU

STEPS FOR ABG ANALYSIS1. What is the pH? Acidemic or Alkalemic?2. What is the primary disorder present?3. Is there appropriate compensation?4. Is the compensation acute or chronic?5. Is there an anion gap?6. If there is a AG, what is the delta gap?7. What is the differential for the clinical

processes?

STEP 1: Look at the pH: is the blood acidemic or alkalemic?

Variable Normal Range

pH - 7.37 - 7.43

pCO2 - 35-45 -

Bicarbonate - 22-26

NORMAL VALUES

STEP 2: WHAT IS THE PRIMARY DISORDER?

What disorder is present?

pH pCO2 or HCO3

Respiratory Acidosis

pH low pCO2 high

Metabolic Acidosis pH low HCO3 low

Respiratory Alkalosis

pH high pCO2 low

Metabolic Alkalosis pH high HCO3 high

STEP 3: IS THERE APPROPRIATE COMPENSATION? Respiratory Acidosis

Acute: for every 10 increase in pCO2 -> HCO3 increases by 1

Also know for every acute increase of 10 in pCO2 there is a decrease of 0.08 in pH.

Chronic: for every 10 increase in pCO2 -> HCO3 increases by 4

Also know for every chronic increase of 10 in pCO2 there is a decrease of 0.03 in pH

Respiratory Alkalosis Acute: for every 10 decrease in pCO2 -> HCO3

decreases by 2 Chronic: for every 10 decrease in pCO2 -> HCO3

decreases by 5

STEP 3: IS THERE APPROPRIATE COMPENSATION?

Metabolic Acidosis

Winter’s formula: pCO2 = 1.5[HCO3] + 8 ± 2 MEMORIZE

Winter’s formula calculates the expected pCO2 in the setting of metabolic acidosis. If the serum pCO2 > expected pCO2 then there is additional respiratory acidosis in which the etiology needs to also be determined.

Metabolic Alkalosis

For every 10 increase in HCO3 -> pCO2 increases by 6

STEP 4: CALCULATE THE ANION GAP AG = Na – Cl – HCO3 (normal 12 ± 2) AG corrected = AG + 2.5[4 – albumin] If anion gap is greater than 20, a metabolic

acidosis is always present regardless of the pH or serum bicarbonate concentration because the body is not able to physically generate such a large anion gap via purely compensatory mechanisms (i.e. even in the setting of chronic respiratory alkalosis). Therefore, there must be a primary metabolic disorder present.

Differential for Anion Gap Metabolic Acidosis - MUDPILERSMethanolUremiaDiabetic ketoacidosis, starvation ketoacidosis, EtOH ketoacidosisParaldehydeINH, iron toxicityLactic acidosisEthylene glycolRhabdomyolysisSalicylates

STEP 5: CALCULATE THE DELTA GAP

Only need to calculate delta gap (excess anion gap) when there is an anion gap present to determine additional hidden metabolic disorders (nongap metabolic acidosis or metabolic alkalosis)

Delta gap = AG – 12 + HCO3 (normal 22-26)

If delta gap > 26 -> additional metabolic alkalosis

If delta gap < 22 -> additional nongap metabolic acidosis

If delta gap 22 – 26 -> no additional metabolic disorders

THE DELTA GAP Delta gap is equivalent to excess anion gap. The

principle behind this formulation is that for each mMol of acid titrated by the carbonic acid buffer system, 1 mMol of bicarbonate is consumed as water and carbon dioxide and 1 mMol of sodium salt of acid is formed. Therefore, each mMol decrease in bicarbonate is accompanied by a mMol increase in the anion gap.

Delta bicarb = Delta anion gap. The sum of the new (excess) anion gap and the remaining (measured) bicarbonate should be equal to a normal bicarbonate concentration. If the sum of the excess anion gap and the measured bicarbonate value exceeds the normal bicarbonate concentration, then an additional metabolic alkalosis must be present. If the sum is less than normal, there must be an additional nongap metabolic acidosis. If the delta gap is equal to expected, there is no additional metabolic disorders.

NONGAP METABOLIC ACIDOSIS

Causes of nongap metabolic acidosis - DURHAMDiarrhea, ileostomy, colostomy, enteric fistulasUreteral diversions or pancreatic fistulasRTA type I or IV, early renal failureHyperailmentation, hydrochloric acid administrationAcetazolamide, Addison’sMiscellaneous – post-hypocapnia, toulene, sevelamer, cholestyramine ingestion

For nongap metabolic acidosis, calculate the urine anion gap

UAG = UNA + UK – UCL

If UAG>0: renal problemIf UAG<0: nonrenal problem (most commonly GI)

In working kidneys: HCl + NH3 ↔ NH4CL, urine chloride increases, UAG <0.

METABOLIC ALKALOSIS Calculate the urinary chloride to differentiate saline responsive

vs saline resistant Must be off diuretics in order to interpret urine chlorideSaline responsive UCL<10

Saline-resistant UCL >10

Vomiting If hypertensive: Cushings, Conn’s, RAS, renal failure with alkali administartion

NG suction If not hypertensive: severe hypokalemia, hypomagnesemia, Bartter’s, Gittelman’s Syndrome

Over-diuresis Exogenous corticosteroid administration

Post-hypercapnia

Causes of Respiratory AlkalosisAnxiety, pain, feverHypoxia, CHFLung disease with or without hypoxia – pulmonary embolus, reactive airway, pneumoniaCNS diseasesDrug use – salicylates, catecholamines, progesteronePregnancySepsis, hypotensionHepatic encephalopathy, liver failureMechanical ventilationHypothyroidismHigh altitude

Causes of respiratory acidosisCNS depression – sedatives, narcotics, CVANeuromuscular disorders – acute or chronicAcute airway obstruction – foreign body, tumor, reactive airwaySevere pneumonia, pulmonary edema, pleural effusionChest cavity problems – hemothorax, pneumothorax, flail chestChronic lung disease – obstructive or restrictiveCentral hypoventilation, OSA

CASE 165yo M with CKD presenting with nausea, diarrhea and

acute respiratory distress. ABG 7.23/17/235 on 50% VM

Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.1

CASE 1 ANSWER Primary metabolic acidosis – gap of 19

(uremia/renal failure causing gap met acidosis), delta gap 14 -> additional non gap metabolic acidosis (diarrhea). Winter’s formula 18 -> no additional respiratory acidosis.

CASE 260yo M with COPD on steroids presenting with

worsening SOB, hypoxia, and hypotensionABG 7.38/54/

Na 134/ Cl 77/ HCO3= 33

CASE 2 ANSWER Primary respiratory acidosis with

chronic metabolic compensation (COPD). Gap = 24. Gap metabolic acidosis (sepsis). Delta gap = 35 -> additional metabolic alkalosis from exogenous steroids. Triple disorder

CASE 328yo F who is 28 weeks pregnant, diabetic, previous

alcoholic who recently stopped insulin and started binge drinkingABG 7.60/21/

Na 136/ Cl 80/ HCO3 19

CASE 3 ANSWER Primary respiratory alkalosis with acute

metabolic compensation (pregnancy, anxiety). Gap = 37 Gap metabolic acidosis (DKA, alcoholic ketoacidosis). Delta gap 34 -> additional metabolic alkalosis (vomiting). Another triple ripple

CASE 417yo F with a history of depression is brought in

altered to the ED by her mother, who reports finding multiple empty medication bottles around

her.ABG 7.50/20/

Na 140/ Cl 103/ HCO3 15

CASE 4 ANSWER Primary respiratory alkalosis with

chronic metabolic compensation (hyperventilation). Gap = 22. Gap metabolic acidosis (salicylates). Delta gap 25 -> no additional metabolic disorders. ASA overdose

CASE 5A 45yo F with Type 1 Diabetes is admitted with a

gastroenteritis, hyperglycemia and confusion.ABG 7.10/50/102

BMP Na 145/Cl 100 / HCO3 15

CASE 5 ANSWER Primary respiratory acidosis (obtunded)

with gap metabolic acidosis , gap = 30 (DKA). Delta gap 33 -> additional nongap metabolic alkalosis (vomiting).