arterial blood gas...

Arterial Blood GasInterpretation

Felicia Ratnaraj

Internal Medicine PGY 3

PM report

What is ABG?

�Measures:� The oxygen tension (Pao2),

�Carbon dioxide tension (Paco2), �Acidity (pH),

�Oxyhemoglobin saturation (Sao2), �Bicarbonate (HCO3)

�Helpful in critically ill patients

Normal values

�pH – 7.35 to 7.45�PaCO2 – 35 to 45 mmHg (4.7 to 6 kPa)�HCO3 – 21 to 27 mEq/L

Step 1:

�Analyze the primary process? �Acidosis (pH<7.35) or

�Alkalosis (pH>7.45)

Step 2: Metabolic, Respiratory, or BOTH?

�Metabolic, Respiratory or BOTH?

�Look at pCO2 à if acidosis >45, if alkalosis <35

�Look at HCO3 à if acidosis <21, if alkalosis >27

Step 3, part A:

�Check ANION GAP:�Anion Gap = Na – (HCO3 + Cl)

�Normal is 8-12 �If high, then AGMA

Correct for albumin (expected AG will fall by about 2.5 meq/L for every 1 g/dL (10 g/L) reduction in the serum albumin concentration)� Corrected AG = AG + (0.25 X (40-albumin) (albumin

expressed in g/L)

MethanolUremia

DKAParaldehyde

IsoniazidLactic Acidosis

EthanolRenal Failure

Salicylate

Step 3, part B:

�DELTA GAP: �When there is an increase in AG, there is fall in HCO3 à represented by

delta AG / Delta HCO3 ration

�Delta Gap = Measured anion gap – Normal anion gap (10) Normal [HCO3-] (24) – Measured [HCO3-]

�<1 à high AG and normal AG acidosis

�1-2 à Pure anion gap acidosis

�>2 à high AG acidosis and concurrent metabolic alkalosis

Step 4, part A

�COMPENSATION: �If respiratory disorder, determine whether acute or chronic

(evaluate compensation) �Acute respiratory Acidosis è 10 PaCO2 = 1 HCO3

�Chronic Respiratory Acidosis è 10 PaCO2 = 3 HCO3

�Acute Respiratory Alkalosis è 10 ¯ PaCO2 = 2 ¯ HCO3

�Chronic Respiratory Alkalosis è 10 ¯ PaCO2 = 4 ¯ HCO3

if HCO3 > expected, concomitant metabolic alkalosis and if < expected, concomitant metabolic acidosis

Step 4, part B

�CONCOMITANT DISORDER: �Metabolic Acidosis (WINTER’s FORMULA):

�expected PaCO2 = 1.5 * (HCO3) + 8 +/-2

�Metabolic alkalosis: � Expected PaCo2 = (0.7 * HCO3) +21 +/- 2 OR

� Respiratory compensation to metabolic alkalosis should raise the PCO2 by about 0.7 mmHg for every 1 meq/L elevation in the serum HCO3 concentration

if pCO2 > expected, concomitant respiratory

acidosis and if < expected, concomitant respiratory

alkalosis

Case 1

�A 44 year old moderately dehydrated man was admitted with a two day history of acute severe diarrhea. Electrolyte results: Na+ 134, K+ 2.9, Cl- 108, HCO3- 16, BUN 31, Cr 1.5.

�ABG: �pH 7.31�pCO2 33 mmHg�HCO3 16 �pO2 93 mmHg

�What is the acid base disorder?

Thought???

�1. primary process?

�2. Metabolic/respiratory?

�3. Anion gap? Delta gap?

�4 Compensation & Concomitant disorder?

Na+ 134, K+ 2.9, Cl- 108, HCO3- 16, BUN 31, Cr 1.5.

pH 7.31pCO2 33 mmHgHCO3 16 pO2 93 mmHg

Thought???

� 1. primary process? Acidosis

� 2. Metabolic/respiratory? Metabolic acidosis

� 3. Anion gap? Delta gap? 134- (108+16) = 10

� 4 Compensation & Concomitant disorder? Winter formula � Expected PaCo2 = 1.5x(16) + 8 +/- 2 = 32 +/- 2 == > actual PCO2 falls within

the estimated range, we can deduce that the compensation is adequate and there is no seperate respiratory disorder present

� Normal anion gap metabolic acidosis

Na+ 134, K+ 2.9, Cl- 108, HCO3- 16, BUN 31, Cr 1.5.

pH 7.31pCO2 33 mmHgHCO3 16 pO2 93 mmHg

Case 2

�A 60 year-old man with amyotrophic lateral sclerosis is brought into clinic by his family who are concerned that he is more somnolent than normal. On further history, they report that he has been having problems with morning headaches and does not feel very refreshed when he wakes up.

�An arterial blood gas is performed and reveals: �pH 7.37, �PCO2 57, �PO2 70, �HCO - 32. 3

Thought???




�4. Compensation & Concomitant disorder?

pH 7.37, PCO2 57, PO2 70, HCO - 32. 3

Thought???

� 1. primary process? More on the acidotic side

� 2. Metabolic/respiratory? Respiratory acidosis

� 3. Anion gap? Delta gap? Not given

� 4. Acute or Chronic? There is a increase in HCo3 by 8 and increase of CO2 by 17. So likely chronic issue Compensation: expected HCO3 = 24 + 6 = 30 which is > , so compensatory metabolic alkalosis

� Chronic respiratory acidosis with a compensatory metabolic alkalosis

pH 7.37, PCO2 57, PO2 70, HCO - 32. 3

Case 3

�A 45 year-old woman with a history of inhalant abuse presents to the emergency room complaining of dyspnea. She has an SpO2 of 99% on room air and is obviously tachypneic on exam with what appears to be Kussmaul’s respirations. A room air arterial blood gas is performed and reveals: pH 6.95, PCO 9, PO 128, HCO - 2. A chemistry panel revealed sodium of 130, chloride 98, HCO - 2.

Thought???





pH 6.95, PCO 9, PO 128, HCO 2. Na of 130, chloride 98, HCO 2.

Thought???

� 1. primary process? Acidosis

� 2. Metabolic/respiratory? Metabolic acidosis

� 3. Anion gap? Delta gap? 130– (98+2) = 30, Anion gap metabolic acidosisDelta gap: (30-10) / (24-2) = 20/22 = .9 è normal AG acidosis

� 4. Compensation & Concomitant disorder? (winter’s formula)� Expected PaCO2 = 1.5 (2) + 8 +/- 2 = 11+/- 2 à concomitant respiratory alkalosis

� Combined elevated anion gap and non-gap metabolic acidosis with compensatory respiratory alkalosis

pH 6.95, PCO 9, PO 128, HCO 2. Na of 130, chloride 98, HCO 2.

Case 4

�80 year old female presents to ER with tinnitus, confusion and unsteady gait. She has dry heaves and V for several days. She has been having progressive decline in the preceding 2 weeks. Hx of HTN and OA. Meds are lisinopril, HCTZ, and aspirin.

�Labs: Na 142, K 3.2, Cl 100, Bicarb 20�pH 7.56, PCO2 22

Thought???





Labs: Na 142, K 3.2, Cl 100, Bicarb 20pH 7.56, PCO2 22

Thought???

� 1. primary process? Alkalosis

� 2. Metabolic/respiratory? Respiratory alkalosis

� 3. Anion gap? Delta gap? Yes. 142 – (100+20) = 22 à AGMADelta gap = 20-10 / 24 – 20 = 10/4

� 4. Compensation & Concomitant disorder? Expected compensation in chronic resp alkalosis is decrease in bicarb by 4 for

each 10 decrease in pCO2. Expected bicarb is 14-16. Measured bicarb is greater at 20 which suggests coexistance of metabolic alkalosis

Respiratory alkalosis, increased anion gap metabolic acidosis, and metabolic alkalosis

Labs: Na 142, K 3.2, Cl 100, Bicarb 20pH 7.56, PCO2 22

Case 5

�A 65 year-old man comes w/ severe nausea and weakness. H of peptic ulcer disease and has been having similar pain for the past two weeks. Over the week, he has been drinking significant quantities of milk and consuming large quantities of TUMS (calcium carbonate). On his initial laboratory studies, he is found to have a calcium level of 11.5 mg/dL, a creatinine of 1.4 and bicarbonate of 35. The resident working in the ER decides to draw a room air arterial blood gas, which reveals: pH 7.45, PCO2 49, PO 68, HCO - 34. On his chemistry panel, the sodium is 139, chloride 95, HCO - 34

Thought???





Serum bicarbonate of 35. pH 7.45, PCO2 49, PO 68, HCO – 34sodium is 139, chloride 95, HCO - 34

Thought???

� 1. primary process? Alkalosis

� 2. Metabolic/respiratory? Metabolic alkalosis + respiratory acidosis (high CO2)

� 3. Anion gap? Delta gap? 139-(95+34) = 10

� 4. Compensation & Concomitant disorder? � Exp PaCO2 = 0.7 (35) + 21 +/-2 = 45.5 +/- 2

� Primary metabolic alkalosis with compensatory respiratory acidosis

Serum bicarbonate of 35. pH 7.45, PCO2 49, PO 68, HCO – 34sodium is 139, chloride 95, HCO - 34

Case 6

�A 24 year-old woman is found down in Pioneer Square by some bystanders. The medics are called and, upon arrival, find her with an oxygen saturation of 88% on room air and pinpoint pupils on exam. She is brought into the Harborview ER where a room air arterial blood gas is performed and reveals: �pH 7.25, �PCO 60, �PO 65, �HCO - 26, Base Excess 1. �On his chemistry panel, her sodium is 137, chloride 100, bicarbonate 27.

Thought???




�4 Compensation & Concomitant disorder?

pH 7.25, PCO 60, PO 65, HCO - 26, Base Excess 1. On his chemistry panel, her sodium is 137, chloride 100, bicarbonate 27.

Thought???

� 1. primary process? Acidotic

� 2. Metabolic/respiratory? Respiratory acidosis

� 3. Anion gap? Delta gap? 10, no anion gap

� 4. Resp Acute or chronic? Acute

� 5. Concomitant disorder? none� Acute, uncompensated respiratory acidosis leading to acidemia

pH 7.25, PCO 60, PO 65, HCO - 26, Base Excess 1. On his chemistry panel, her sodium is 137, chloride 100, bicarbonate 27.

Case 7

�A 57 year-old woman presents with 2 days of fevers, dyspnea and a cough productive of rust-colored sputum. Her room air oxygen saturation in the emergency room is found to be 85% and the intern decides to obtain a room air arterial blood gas while they are waiting for the chest x-ray to be done. The blood gas reveals: pH 7.54, PCO 25, PO 65, HCO - 22, Base excess -1.

Thought???





pH 7.54, PCO 25, PO 65, HCO - 22, Base excess -1.

Thought???

� 1. primary process? alkalosis

� 2. Metabolic/respiratory? Respiratory alkalosis

� 3. Anion gap? Delta gap? Not given

� 4. Compensation & Concomitant disorder? � Acute or chronic ? Acute � No metabolic acidosis

� Acute, uncompensated respiratory alkalosis

pH 7.54, PCO 25, PO 65, HCO - 22, Base excess -1.

References

�UpToDate�http://fitsweb.uchc.edu/student/selectives/TimurGraham/

Cases.html�The ICU book �MKSAP

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