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INTERPRETATION OF ABG

DR. ARVIND KHAREASSOCIATE PROFESSOR

DEPARTMENT OF ANESTHESIOLOGYJLNMS MEDICAL COLLEGE AND

ATTACHED HOSPITALSAJMER

TERMINOLOGY

Acidemia : Blood pH < 7.35

Alkalemia : Blood pH > 7.45

Acidosis : a physiologic process that tends to cause acidemia

Alkalosis : a physiologic process that tends to cause alkalemia

pH 7.35 - 7.45PaCO2 35 - 45 mm Hg

PaO2 70 - 100 mm Hg **

SaO2 93 - 98%

HCO3¯ 22 - 26 mEq/L

Base excess -2.0 to 2.0 mEq/L

* At sea level, breathing ambient air** Age-dependent

Normal Arterial Blood Gas Values*

Primary & Secondary Acid – Base Disorder

Acid Base Disorder Primary Change Secondary Change

Respiratory Acidosis PaCO2 HCO3-

Alkalosis PCO2 HCO3-

Metabolic Acidosis HCO3- PCO2

Alkalosis HCO3- PCO2

Changes In Actual HCO3- Levels

Respiratory acidosis

Acute: ↑0.1 mmol of HCO3- per 1 mm Hg ↑ Pco2 or HCO3- = 24 + 0.1 X Δ PCO2

Chronic: ↑ 0.35 mmol of HCO3- per 1 mm Hg ↑ Pco2 or HCO3- = 24 + 0.35 X Δ PCO2

Changes In Actual HCO3- Levels

Respiratory alkalosis

Acute: ↓ 0.2 mmol of HCO3- per 1 mm Hg ↓ Pco2 or HCO3- = 24 – 0.2X Δ PCO2

Chronic: ↓ 0.4 mmol of HCO3- per 1 mm Hg ↓ Pco2 or HCO3- = 24 – 0.4X Δ PCO2

CHANGES IN PACO2

Metabolic acidosis

ΔPCO2 = 1.3 X Δ HCO3-

Or

PCO2 = (1.5 X HCO3-) + 8 ± 2

CHANGES IN PACO2

Metabolic alkalosis

ΔPCO2 = 0.6 X Δ HCO3-

Or

PCO2 = (0.7 X HCO3-) + 21 ± 2

• Acid – base interpretation is based on three variables - pH, PCO2 & HCO3

-

• If outside normal range, it is abnormal

Normal range

pH 7.35 to 7.45

PaCO2 35 to 45 mm of Hg

HCO3- 22 to 26 mEq/L

STEP 1: LOOK AT PH

If pH ‹ 7.35 - acidosis

If pH › 7.45 - alkalosis

STEP 2: IS THE PRIMARY DISTURBANCE METABOLIC OR RESPIRATORY?

Look at Pco2 and HCO3-

Primary Change Acid Base Disorder Secondary Change

PaCO2 (› 45mmhg ) Respiratory Acidosis HCO3-

PCO2 (‹35mmhg) Alkalosis HCO3-

HCO3-(‹22mmol/L) Metabolic Acidosis PCO2

HCO3-(›26mmol/L) Alkalosis PCO2

STEP 3

If primary respiratory disorder

Acute: Expected pH= 0.008 × ∆ Pco2

Chronic: Expected pH= 0.003 × ∆ Pco2

STEP 4: IS THERE APPROPRIATE COMPENSATION FOR THE PRIMARY DISTURBANCE?

Respiratory acidosis

Ac : ↑0.1 mmol of HCO3- per 1 mm Hg ↑ Pco2

or HCO3- = 24 + 0.1 X Δ

PCO2

Chr : ↑ 0.35 mmol of HCO3- per 1 mm Hg ↑ Pco2

or HCO3- = 24 + 0.35 X Δ

PCO2

Expected

change in

HCO3-

STEP 4: CONT….

Respiratory alkalosisAcute: ↓ 0.2 mmol of HCO3- per 1 mm Hg ↓ Pco2

or HCO3- = 24 – 0.2X Δ PCO2

Chronic: ↓ 0.4 mmol of HCO3-per 1 mm

Hg ↓ Pco2 or HCO3- = 24 – 0.4X Δ PCO2

Expected

change in

HCO3-

STEP 4:CONT…..

Metabolic acidosis

ΔPCO2 = 1.3 X Δ HCO3-or

PCO2 =(1.5 X HCO3-) + 8 ± 2

Metabolic alkalosis

ΔPCO2 = 0.6 X Δ HCO3-or

PCO2 = (0.7 X HCO3-) + 21 ± 2

Expected

Change

in

pCO2

DIAGNOSIS OF ACID BASE DISTURBANCES: STEP-BY-STEP ANALYSIS

ABG :

pH

HCO3

PCO2

Obtain a minimum diagnosis

Electrolyte (Na+, K+,Cl- and HCO3)

Calculate the Anion gap

K+

SO2

Hb. Complete the diagnosis Match with the clinical diagnosis

and treat accordingly

Step-5 : Calculate the Anion gap

a. Check the anion gap (AG) :AG =Na+ -(HCO3 +

Cl- )(normal = 12 ±

2)

Elevated AG = Acidosis

DIAGNOSIS OF ACID BASE DISTURBANCES: STEP-BY-STEP ANALYSIS

Step 5 :b.Compare fall in HCO3 with increase in plasma anion

gap. i) In high AG metabolic acidosis,

rise in the plasma AG (AG - 12) matches with fall in serum HCO3 ( 24 – HCO3)

(Rise in AG = Fall in HCO3)

ii) If increase in AG exceeds the fall in HCO3 (Rise in AG > Fall in HCO3), it suggests co-existing metabolic alkalosis.

iii) If increase in AG is lesser than the fall of HCO3 (Rise in AG < Fall in HCO3), it suggests loss of HCO3 (diarrhoea) causing non-AG metabolic acidosis.

STEP 5: CALCULATE THE ANION GAP

AG = Na+ – (Cl- + HCO3-)=Normal value is 12 ± 2

Compare the rise in AG with the decrease in HCO3-is Useful to identify additional or hidden metabolic disorders

1. change(rise) in AG = the decrease in HCO3= simple metabolic acidosis

2.change(rise) in AG < the decrease in HCO3= hidden metabolic alkalosis,

3.change(rise) in AG > the decrease in HCO3= hidden non AG acidosis

STEP 6 : CONT……

Actual PCO2 = Expected PCO2 respiratory alkolosis

Actual PCO2 > Expected PCO2 hidden non

AG acidosis

Actual PCO2 < Expected PCO2 hidden metabolic alkalosis

Q. A 19 years old boy brought in emergency with history of injecting some drug. O/E respiration shallow 8/mt, responded to painful stimuli.

Arterial blood gases shows:

pH 7.14

PaO2 86 mm of Hg

PaCO2 72 mm of Hg

ABC 26 mmol/L

BE 1.4 mmol/L

pH 7.14 Acidosis

PaCO2 72 mm of Hg Respiratory

ABC 26 mmol/L

0.008 X (72-40) = 0.256 7.4 – 0.256 = 7.144

0.003 X (72-40) = 0.096 7.4 – 0.096 = 7.304

HCO3- = 24 + 0.1 X Δ PCO2 = 24+ 0.1(72-40) = 27.2

Uncompensated

Uncompensated Acute Respiratory Acidosis

Acute Not Chr.

Q. A 67 years old male with H/O cough and respiratory distress and chronic smoking came in emergency :


pH 7.34

PaO2 76 mm of Hg

PaCO2 60 mm of Hg

ABC 31 mmol/L

BE +4 mmol/L

pH 7.34 Acidosis


ABC 31 mmol/L

0.008 X (60-40) = 0.16 7.4 – 0.16 = 7.24

0.003 X (60-40) = 0.06 7.4 – 0.06 = 7.34

HCO3- = 24 + 0.35 X Δ PCO2 = 24+ 0.35(60-40) = 31

Compensated

Compensated Chronic Respiratory Acidosis

Not Acute Chronic

Q. A 18 years old girl was admitted in the hospital after an argument with her boy friend. She denied taking any medication. On examination chest clear, respiratory rate 34 / mt. Blood gas estimation shows :

pH 7.51

PaO2 82 mm of Hg

PaCO2 26 mm of Hg

ABC 24 mmol/L

BE -1 mmol/L

pH 7.51 Alkalosis


ABC 24 mmol/L

0.008 X (40-26) = 0.112 7.4 + 0.112 = 7.512

0.003 X (40-26) = 0.042 7.4 + 0.042 = 7.442

HCO3- = 24 – 0.2X Δ PCO2 = 24- 0.2(40-26) = 21.2

Uncompensated

Uncompensated Acute Respiratory Alkalosis

Acute Not Chr.

Q. A 52 years old man brought in emergency with history of cough and pleuritic chest pain for few days increasingly becoming breathless:


pH 7.47

PaO2 67 mm of Hg

PaCO2 14 mm of Hg

ABC 15 mmol/L

BE -8.2 mmol/L

pH 7.47 Alkalosis


ABC 15 mmol/L

0.008 X (40-14) = 0.208 7.4 + 0.208 = 7.608

0.003 X (40-14) = 0.078 7.4 + 0.078 = 7.478

HCO3- = 24 – 0.4X Δ PCO2 = 24- 0.4(40-14) = 13.6

Uncompensated

Uncompensated Chronic Respiratory Alkalosis

Not Acute Chronic

Q.60 year old admitted with gluteal abscess, known diabetic

pH 7.18PCO2 18HCO3- 9Na 138K 4.1Cl 110AG 23.1

pH <7.4 acidosis Metabolic low HCO3-

Expected PCO2 = 1.5 X HCO3- + 8 = 1.5 X 9 + 8 = 21.5

Actual PCO2 (18) < Expected PCO2 (21.5) respiratory alkalosis

Diag : Metabolic acidosis with respiratory alkalosis

Increased AG (23) metabolic acidosis

Clinical Diag : Diabetic ketoacidosis with sepsis

Q.50 – year old with CRF, smoker

pH 7.1PCO2 50HCO3- 15Na 140K 5Cl 105AG 23


Expected PCO2 = 1.5 X 15 + 8 = 30.5

Actual PCO2 (50) > Expected PCO2 (30.5) respiratory acidosis

Diag : Metabolic acidosis with respiratory acidosis


Clinical Diag : CRF with COPD and increased AG metabolic acidosis

Q.35 – year old, collapsed on physical exertion was brought to hospital

pH 6.99PCO2 34HCO3- 8Na 141K 6Cl 105AG 28


Expected PCO2 = 1.5 X 8 + 8 = 20

Actual PCO2 (34) > Expected PCO2 (20) respiratory acidosis

Diag : Metabolic acidosis and respiratory acidosis with hyperkalemia


Clinical Diag : Cardiorespiratory arrest

Q.70 – kg healthy male had acute airway obstruction during induction of anaesthesia

pH 7.1PCO2 70HCO3- 21

pH <7.4 acidosis Respiratory high PaCO2 (>40)

Acute hypercapnia – HCO3- increases 0.1 mmol for every increase in 1 mm Hg of PCO2 above 40 mm Hg

HCO3- should be 3 mmol above 24, if PCO2 only is changed

Predicted HCO3- is 27, but actual is 21 deficit of 6 mmol/ L

Combined respiratory and metabolic acidosis

Q.56 – year old male patient, with COPD having resting PCO2 of 70 sustains perioperative MI, BP is 80/60 mm Hg, is sweating, cool and clammy

pH 7.1PCO2 70HCO3- 21

pH <7.4 acidosis Respiratory high PaCO2(>40)

Chronic hypercapnia – HCO3- increases 0.4 mmol for every increase in 1 mm Hg of PCO2 above 40 mm Hg

Predicted HCO3- is 36, but actual is 21 deficit of 15 mmol/ L

Combined respiratory and metabolic acidosis

Metabolic component much greater than previous case

Step 1: Acidemic, alkalemic, or normal?

Step 2: Is the primary disturbance respiratory or metabolic?

Step 3: For a primary respiratory disturbance, is it acute or chronic?

Step 4: For a metabolic disturbance, is the respiratory system compensating OK?

Step 5: For a metabolic acidosis, is there an increased anion gap?

Step 6: For an increased anion gap metabolic acidosis, are there other derangements?

SUMMARY

Useful Formulas for Acid Base Interpretations

Metabolic Acidosis Expected PaCO2 = (1.5 × HCO3) + (8±2)

PRIMARY DISORDER EXPECTED RESULTS

Metabolic Alkalosis Expected PaCO2 = (0.7 × HCO3) + (21±2)

Acute RespiratoryAcidosis

Acute RespiratoryAlkalosis

Chronic RespiratoryAcidosis

Chronic RespiratoryAlkalosis

DpH = 0.003 × DPaCO2

Expected pH = 7.40 + [0.003 × (40 – PaCO2)]


Expected pH = 7.40 – [0.003 × (PaCO2 – 40)]


Expected pH = 7.40 + [0.008 × (40 – PaCO2)]


Expected pH = 7.40 - [0.008 × (PaCO2 – 40)]

ABG analysis is the stethoscope of ICU and is an integral part of management of critical care patients, however, one should always remember that

‘Treat the patient, not the report’

CONCLUSION

THANK YOU

Q. A 66 years old woman had cardiac arrest in casualty. Basic and advanced life support was started in casualty. While patient was being ventilated blood sample was taken:


pH 6.86

PaO2 6o mm of Hg

PaCO2 38.9 mm of Hg

ABC 6.6 mmol/L

BE - 23.9 mmol/L

Ans :Metabolic Acidaemia

Q. A 33 years old man with insulin dependent diabetes brought to emergency department not well for 3 days c/o frequency of urination, fever, nausea, sweating had not been eating hence not taking insulin:


pH 7.2

PaCO2 22 mm of Hg

ABC 5.4 mmol/L

SBC 7.5 mmol/L

BE -26 mmol/L

Ans :Metabolic Acidaemia with poor compensation

Q. A 68 years old man c/o severe chest pain and shortness of breath. O/E cold and clammy R/R 8 per mt. brought in emergency


pH 6.99

PaCO2 109.5 mm of Hg

ABC 15.5 mmol/L

SBC 15.3 mmol/L

BE -12.4 mmol/L

Ans :Combined respiratory and metabolic Acidaemia

Q. A 40 years old woman who had taken overdose of Dothiepin (a tricyclic antidepressant) 2 hrs previously. She was unconscious with a shallow breathing RR 8 / mt.


pH 6.76

PaCO2 46.5 mm of Hg

ABC 6.5 mmol/L

SBC 6.2 mmol/L

BE -29 mmol/L

Ans :Mixed Respiratory and Metabolic Acidaemia

Q. A patient admitted in surgical ward was on gastric aspiration through Ryle’s tube because of persistent vomitings:


pH 7.5

PaCO2 50 mm of Hg

ABC 35 mmol/L

SBC 32 mmol/L

BE +8 mmol/L

Ans : Metabolic Alkalaemia

Q. A 26 years old man having vomiting and diarrhoea from 4 days came in emergency:


pH 7.48

PaCO2 43 mm of Hg

ABC 31 mmol/L

SBC 30 mmol/L

BE +4.5 mmol/L

Ans :Metabolic Alkalaemia

Q. A 51 years old woman presented in emergency department c/o of shortness of breath. The symptoms were present from years. She had no other medical problems:


pH 7.48

PaCO2 17.9 mm of Hg

ABC 12.5 mmol/L

SBC 14.5 mmol/L

BE -6.6 mmol/L

Ans :Chronic Respiratory Alkalaemia

Q. A 30 years old lady attempted to commit suicide by throwing herself into a canal having taken overdose of alcohol . Passing policeman saved her & brought her to casualty. O/E she was pale, cold and confused. Her tympanic temperature was 28.40C, R/R 18/mt, PR 108/mt, BP 136/90 chest – bilateral crepts and ronchi.

Arterial blood gases on breathing room air shows:

pH 7.23

PaCO2 41.5 mm of Hg

ABC 16.7 mmol/L

SBC 16.4 mmol/L

BE -10.2 mmol/L

PaO2 86.0 mm of Hg

Ans :Combined metabolic and respiratory acidaemia

Q. A patient having fever since 3 days, having some respiratory difficulty was admitted with PR=130/ minute and BP = 100/ 60 mm Hg

pH 6.56

PaO2 60 mm of Hg

PaCO2 80 mm of Hg

ABC 25 mmol / L

Ans: IS IT COMPATIBLE?

TREAT THE PATIENTNOT THE ABG!

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