Blood Gas Analysis and it’s Clinical Interpretation

Dr R.S.Gangwar MD, PDCC, FIPM

Assistant ProfessorGeriatric ICU,DGMH

Outline1. Common Errors During ABG Sampling2. Components of ABG3. Discuss simple steps in analyzing ABGs4. Calculate the anion gap 5. Calculate the delta gap6. Differentials for specific acid-base disorders

Delayed Analysis

Consumptiom of O2 & Production of CO2 continues after blood drawn

Iced Sample maintains values for 1-2 hours Uniced sample quickly becomes invalid within 15-20 minutes

PaCO2 3-10 mmHg/hour PaO2 pH d/t lactic acidosis generated by glycolysis in R.B.C.

Parameter 37 C (Change every 10 min)

4 C (Change every 10 min)

pH 0.01 0.001

PCO2 1 mm Hg 0.1 mm Hg

PO2 0.1 vol % 0.01 vol %

Temp Effect On Change of ABG Values

FEVER OR HYPOTHERMIA

1. Most ABG analyzers report data at N body temp2. If severe hyper/hypothermia, values of pH &

PCO2 at 37 C can be significantly diff from pt’s actual values

3. Changes in PO2 values with temp also predictable

Hansen JE, Clinics in Chest Med 10(2), 1989 227-237

If Pt.’s temp < 37C Substract 5 mmHg Po2, 2 mmHg Pco2 and

Add 0.012 pH per 1C decrease of temperature

AIR BUBBLES: 1. PO2 150 mmHg & PCO2 0 mm Hg in air bubble(R.A.)

2. Mixing with sample, lead to PaO2 & PaCO2

To avoid air bubble, sample drawn very slowly and preferabily in glass syringe

Steady State:

Sampling should done during steady state after change in oxygen therepy or ventilator parameterSteady state is achieved usually within 3-10 minutes

Leucocytosis : pH and Po2 ; and Pco2 0.1 ml of O2 consumed/dL of blood in

10 min in pts with N TLC Marked increase in pts with very high

TLC/plt counts – hence imm chilling/analysis essential

EXCESSIVE HEPARIN Dilutional effect on results HCO3

- & PaCO2 Only .05 ml heperin required for 1 ml blood.

So syringe be emptied of heparin after flushing or only dead space volume is sufficient or dry heperin should be used

TYPE OF SYRINGE1. pH & PCO2 values unaffected2. PO2 values drop more rapidly in plastic syringes (ONLY

if PO2 > 400 mm Hg) Differences usually not of clinical significance so

plastic syringes can be and continue to be used Risk of alteration of results with:

1. size of syringe/needle2. vol of sample

HYPERVENTILATION OR BREATH HOLDING

May lead to erroneous lab results

COMPONENTS OF THE ABG pH: Measurement of acidity or alkalinity, based on the

hydrogen (H+). 7.35 – 7.45

Pao2 :The partial pressure oxygen that is dissolved in arterial blood. 80-100 mm Hg.

PCO2: The amount of carbon dioxide dissolved in arterial blood. 35– 45 mmHg

HCO3 : The calculated value of the amount of bicarbonate in the blood. 22 – 26 mmol/L

SaO2:The arterial oxygen saturation.

>95% pH,PaO2 ,PaCO2 , Lactate and electrolytes are measured

Variables HCO3 (Measured or calculated)

Contd… Buffer Base:

It is total quantity of buffers in blood including both volatile(Hco3) and nonvolatile (as Hgb,albumin,Po4)

Base Excess/Base Deficit: Amount of strong acid or base needed to restore

plasma pH to 7.40 at a PaCO2 of 40 mm Hg,at 37*C. Calculated from pH, PaCO2 and HCT Negative BE also referred to as Base Deficit True reflection of non respiratory (metabolic) acid

base status Normal value: -2 to +2mEq/L

CENTRAL EQUATION OF ACID-BASE PHYSIOLOGY

Henderson Hasselbach Equation:

where [ H+] is related to pH by

To maintain a constant pH, PCO2/HCO3- ratio should be constant

When one component of the PCO2/[HCO3- ]ratio is altered, the compensatory response alters the other component in the same direction to keep the PCO2/[HCO3- ] ratio constant

[H+] in nEq/L = 24 x (PCO2 / [HCO3 -] )

[ H+] in nEq/L = 10 (9-pH)

Compensatory response or regulation of pH By 3 mechanisms: Chemical buffers:

React instantly to compensate for the addition or subtraction of H+ ions

CO2 elimination: Controlled by the respiratory system Change in pH result in change in PCO2 within minutes

HCO3- elimination: Controlled by the kidneys Change in pH result in change in HCO3- It takes hours to days and full compensation occurs in 2-

5 days

Normal ValuesVariable Normal Normal

Range(2SD)

pH 7.40 7.35 - 7.45

pCO2 40 35-45

Bicarbonate 24 22-26

Anion gap 12 10-14

Albumin 4 4

Steps for ABG analysis1. What is the pH? Acidemia or Alkalemia?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 check the delta gap?7. What is the differential for the clinical

processes?

Step 1: Look at the pH: is the blood acidemic or

alkalemic?

EXAMPLE : 65yo M with CKD presenting with nausea, diarrhea

and acute respiratory distress ABG :ABG 7.23/17/235 on 50% VM BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr

5.1 ACIDMEIA OR ALKALEMIA ????

EXAMPLE ONE ABG 7.23/17/235 on 50% VM BMP Na 123/ Cl 97/ HCO3 7/BUN

119/ Cr 5.1 Answer PH = 7.23 , HCO3 7 Acidemia

Step 2: What is the primary disorder?

What disorder is present?

pH pCO2 HCO3

Respiratory Acidosis

pH low high high

Metabolic Acidosis pH low low low

Respiratory Alkalosis

pH high low low

Metabolic Alkalosis

pH high high high

Contd….Metabolic Conditions are suggested if pH changes in the same direction as pCO2 or pH is abnormal but pCO2 remains unchanged

Respiratory Conditions are suggested if:pH changes in the opp direction as pCO2 or pH is abnormal but HCO3- remains unchanged

EXAMPLE

ABG 7.23/17/235 on 50% VM BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.

PH is low , CO2 is Low PH and PCO2 are going in same directions then its

most likely primary metabolic

EXPECTED CHANGES IN ACID-BASE DISORDERS

Primary Disorder Expected Changes

Metabolic acidosis PCO2 = 1.5 × HCO3 + (8 ± 2) Metabolic alkalosis PCO2 = 0.7 × HCO3 + (21 ± 2) Acute respiratory acidosis delta pH = 0.008 × (PCO2 - 40) Chronic respiratory acidosis delta pH = 0.003 × (PCO2 - 40) Acute respiratory alkalosis delta pH = 0.008 × (40 - PCO2) Chronic respiratory alkalosis delta pH = 0.003 × (40 - PCO2)

From: THE ICU BOOK - 2nd Ed. (1998) [Corrected]

Step 3-4: Is there appropriate compensation? Is it chronic or acute? Respiratory Acidosis

Acute (Uncompensated): for every 10 increase in pCO2 -> HCO3 increases by 1 and there is a decrease of 0.08 in pH

Chronic (Compensated): for every 10 increase in pCO2 -> HCO3 increases by 4 and there is a decrease of 0.03 in pH

Respiratory Alkalosis Acute (Uncompensated): for every 10 decrease in pCO2 ->

HCO3 decreases by 2 and there is a increase of 0.08 in PH Chronic (Compensated): for every 10 decrease in pCO2 -> HCO3

decreases by 5 and there is a increase of 0.03 in PH

Partial Compensated: Change in pH will be between 0.03 to 0.08 for every 10 mmHg change in PCO2

Step 3-4: Is there appropriate compensation? Metabolic Acidosis

Winter’s formula: Expected pCO2 = 1.5[HCO3] + 8 ± 2 OR pCO2 = 1.2 ( HCO3) If serum pCO2 > expected pCO2 -> additional respiratory

acidosis and vice versa Metabolic Alkalosis

Expected PCO2 = 0.7 × HCO3 + (21 ± 2) OR pCO2 = 0.7 ( HCO3) If serum pCO2 < expected pCO2 - additional respiratory

alkalosis and vice versa

EXAMPLE ABG 7.23/17/235 on 50% VM BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5.

Winter’s formula : 17= 1.5 (7) +8 ±2 = 18.5(16.5 – 20.5)

So correct compensation so there is only one disorder Primary metabolic

Step 5: Calculate the anion gap AG used to assess acid-base status esp in D/D of

met acidosis AG & HCO3

- used to assess mixed acid-base disorders

AG based on principle of electroneutrality: Total Serum Cations = Total Serum Anions Na + (K + Ca + Mg) = HCO3 + Cl + (PO4 + SO4

+ Protein + Organic Acids) Na + UC = HCO3 + Cl + UA Na – (HCO3 + Cl) = UA – UC Na – (HCO3 + Cl) = AG Normal =12 ± 2

Contd… AG corrected = AG + 2.5[4 – albumin] If there is an anion Gap then calculate the

Delta/delta gap (step 6) to determine additional hidden nongap metabolic acidosis or metabolic alkalosis

If there is no anion gap then start analyzing for non-anion gap acidosis

EXAMPLE Calculate Anion gap

ABG 7.23/17/235 on 50% VM BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5/

Albumin 2.

AG = Na – Cl – HCO3 (normal 12 ± 2) 123 – 97 – 7 = 19

AG corrected = AG + 2.5[4 – albumin] = 19 + 2.5 [4 – 2] = 19 + 5 = 24

Step 6: Calculate Delta Gap Delta gap = (actual AG – 12) + HCO3 Adjusted HCO3 should be 24 (+_ 6) {18-30} If delta gap > 30 -> additional metabolic alkalosis If delta gap < 18 -> additional non-gap metabolic

acidosis If delta gap 18 – 30 -> no additional metabolic

disorders

EXAMPLE : Delta Gap ABG 7.23/17/235 on 50% VM BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr 5/

Albumin 4.

Delta gap = (actual AG – 12) + HCO3

(19-12) +7 = 14 Delta gap < 18 -> additional non-gap

metabolic acidosisSo Metabolic acidosis anion and non

anion gap

Metobolic acidosis: Anion gap acidosis

EXAMPLE: WHY ANION GAP? 65yo M with CKD presenting with nausea, diarrhea

and acute respiratory distress ABG :ABG 7.23/17/235 on 50% VM BMP Na 123/ Cl 97/ HCO3 7/BUN 119/ Cr

5.1 So for our patient for anion gap portion its due

to BUN of 119 UREMIA But would still check lactic acid

Nongap metabolic acidosis For non-gap metabolic acidosis, calculate the urine anion

gap URINARY AG

Total Urine Cations = Total Urine AnionsNa + K + (NH4 and other UC) = Cl + UA(Na + K) + UC = Cl + UA(Na + K) – Cl = UA – UC (Na + K) – Cl = AG

Distinguish GI from renal causes of loss of HCO3 by estimating Urinary NH4+ .

Hence a -ve UAG (av -20 meq/L) seen in GI, while +ve value (av +23 meq/L) seen in renal problem.

UAG = UNA + UK – UCL

Kaehny WD. Manual of Nephrology 2000; 48-62

EXAMPLE : NON ANION GAP ACIDOSIS 65yo M with CKD presenting with nausea, diarrhea

and acute respiratory distress ABG :ABG 7.23/17/235 on 50% VM BMP Na 123/ Cl 97/ HCO3 14 AG = 123 – 97-14 = 12

Most likely due to the diarrhea

Causes of nongap metabolic acidosis - DURHAM

Diarrhea, ileostomy, colostomy, enteric fistulas

Ureteral diversions or pancreatic fistulas

RTA type I or IV, early renal failure

Hyperailmentation, hydrochloric acid administration

Acetazolamide, Addison’s

Miscellaneous – post-hypocapnia, toulene, sevelamer, cholestyramine ingestion

Metabolic alkalosis Calculate the urinary chloride to differentiate saline

responsive vs saline resistant Must be off diuretics in order to interpret urine chloride

Saline responsive UCL<25

Saline-resistant UCL >25

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

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

Over-diuresis Exogenous corticosteroid administration

Post-hypercapnia

Respiratory Alkalosis Causes of Respiratory Alkalosis

Anxiety, pain, fever

Hypoxia, CHF

Lung disease with or without hypoxia – pulmonary embolus, reactive airway, pneumonia

CNS diseases

Drug use – salicylates, catecholamines, progesterone

Pregnancy

Sepsis, hypotension

Hepatic encephalopathy, liver failure

Mechanical ventilation

Hypothyroidism

High altitude

Case1. 7.27/58/60 on 5L, HCO3

- 26, anion gap is 12, albumin is 4.0 1. pH= Acidemia (pH < 7.4) 2.CO2= Acid (CO2>40) Opposite direction so Primary disturbance =

Respiratory Acidosis 3 &4: Compensation : Acute or chronic? ACUTE

CO2 has increased by (58-40)=18 If chronic the pH will decrease 0.05 (0.003 x 18 =

0.054) pH would be 7.35 If acute the pH will decrease 0.14 (0.008 x 18 = 0.144)

pH would be 7.26.

Contd. 5: Anion gap –N/A 6: There is an acute respiratory acidosis, is there

a metabolic problem too? ΔHCO3

- = 1 mEq/L↑/10mmHg↑pCO2

The pCO2 is up by 18 so it is expected that the HCO3-

will go up by 1.8. Expected HCO3- is 25.8, compared to

the actual HCO3- of 26, so there is no additional

metabolic disturbance.

Dx-ACUTE RESPIRATORY ACIDOSIS

Case.2 7.54/24/99 on room air, HCO3

- 20, anion gap is 12, albumin is 4.0. 1: pH= Alkalemia (pH > 7.4) 2.CO2= Base (CO2<40) pH & pCO2 change in opposite Direction So

Primary disturbance = Respiratory Alkalosis 3 &4: Compensation ? acute or chronic? ACUTE

ΔCO2 =40-24=16 If chronic the pH will increase 0.05 (0.003 x 16 = 0.048)

pH would be 7.45 If acute the pH will increase 0.13(0.008 x 16 = 0.128)

pH would be 7.53

Contd… 5:Anion gap – N/A 6: There is an acute respiratory alkalosis, is

there a metabolic problem too? ΔHCO3

- = 2 mEq/L↓/10mmHg↓pCO2

The pCO2 is down by 16 so it is expected that the HCO3

- will go down by 3.2. Expected HCO3- is 20.8,

compared to the actual HCO3- of 20, so there is no

additional metabolic disturbance. Dx-ACUTE RESPIRATORY ALKALOSIS

Case-3 7.58/55/80 on room air, HCO3

- 46, anion gap is 12, albumin is 4.0. Ucl -20 1: pH= Alkalemia(pH > 7.4)

2:CO2= Acid (CO2>40) Same direction so Primary disturbance = Metabolic

Alkalosis 3&4: Compensation:

∆ pCO2=0.7 x ∆ HCO3-

The HCO3- is up by 22.CO2 will increase by 0.7x22 = 15.4.

Expected CO2 is 55.4, compared to the actual CO2 of 55, therefore there is no additional respiratory disturbance.

contd 5: No anion gap is present; and no adjustment

needs to be made for albumin. Metabolic Alkalosis

Urinary chloride is 20 meq/l (< 25 meq/l)so chloride responsive, have to treat with Normal saline.

Dx-METABOLIC ALKALOSIS

Case-4 7.46/20/80 on room air, HCO3

- 16, anion gap = 12, albumin = 4.0 1: pH = Alkalemia (pH > 7.4) 2:CO2 = Base (CO2<40) So Primary disturbance = Respiratory

Alkalosis 3 &4: Compensation? acute or chronic? Chronic

ΔCO2 =40-20= 20. If chronic the pH will increase 0.06 (0.003 x 20 = 0.06)

pH would be 7.46. If acute the pH will increase 0.16 (0.008 x 20 = 0.16)

pH would be 7.56.

Contd…. 5: Anion gap – N/A 6: There is a chronic respiratory alkalosis, is

there a metabolic problem also? Chronic: ΔHCO3

- = 4 mEq/L↓/10mmHg↓pCO2

The pCO2 is down by 20 so it is expected that the HCO3

- will go down by 8. Expected HCO3- is 16,

therefore there is no additional metabolic disorder.

Dx-CHRONIC RESPIRATORY ALKALOSIS

Case-5 7.19/35/60 on 7L, HCO3

- 9, anion gap = 18, albumin = 4.0 1: pH = Acidemia (pH < 7.4) 2:CO2= Base (CO2<40) So Primary disturbance: Metabolic Acidosis 3&4: Compensation ? ∆ pCO2=1.2 x ∆ HCO3

-

CO2 will decrease by 1.2 (∆HCO3-) 1.2 (24-9) 18. 40 – 18=

22 Actual CO2 is higher than expected Respiratory Acidosis

5: Anion Gap = 18 (alb normal so no correction necessary)

Contd…..6: Delta Gap: Delta gap = (actual AG – 12) + HCO3 = (18-12) + 9 = 6 + 9 = 15 which is<18 Non-AG Met Acidosis

Dx-ANION GAP METABOLIC ACIDOSIS with NON-ANION GAP METABOLIC ACIDOSIS with RESPIRATORY ACIDOSIS

Case-6 7.54/80/65 on 2L, HCO3

- 54, anion gap 12,albumin = 4.0 , Ucl 40 meq/l 1: pH = Alkalemia (pH > 7.4) 2:CO2= Acid (CO2>40) So Primary disturbance: Metabolic Alkalosis 3&4: Compensation?

∆ pCO2=0.7 x ∆ HCO3-

CO2 will increase by 0.7 (∆HCO3-) 0.7 (54-24)

2140 + 21 = 61 Actual CO2 is higher than expected Respiratory Acidosis

Contd…. 5: Anion Gap = 12 (alb normal so no correction

necessary) Urinary chloride is 40 meq/l (> 25 meq/l)so

chloride resistant. So treatment would be disease specific and repletion of potassium

Dx-METABOLIC ALKALOSIS with RESPIRATORY ACIDOSIS

Case-7 7.6/30/83 on room air, HCO3

- 28, anion gap = 12, albumin = 4.0 1: pH = Alkalemia (pH > 7.4) 2:CO2= Base (CO2<40) SoPrimary Disturbance: Metabolic Alkalosis 3&4: Compensation ?

∆ pCO2=0.7 x ∆ HCO3-

CO2 will increase by 0.7 (∆HCO3-) 0.7 (28-24) 2.8 40 + 2.8 =

42.8 Actual CO2 is lower than expected Respiratory Alkalosis

Anion Gap = 12 (alb normal so no correction necessary) See urinary chloride for further Dx.

Dx-METABOLIC ALKALOSIS with RESPIRATORY ALKALOSIS

Case-8 A 50 yo male present with sudden onset of SOB with

following ABG 7.25/46/78 on 2L, HCO3- 20, anion gap = 10,

albumin = 4.0 1: pH = Acidemia (pH < 7.4) 2:CO2= Acid (CO2>40) So Primary disturbance: Respiratory Acidosis 3 &4: If respiratory disturbance is it acute or chronic?

ACUTE ∆ CO2 = 46-40= 6 If chronic the pH will decrease 0.02 (0.003 x 6 = 0.018)

pH would be 7.38 If acute the pH will decrease 0.05 (0.008 x 6 = 0.048)

pH would be 7.35.

Contd… Anion Gap = 10 (alb normal so no correction necessary) 6: There is an acute respiratory acidosis, is there a metabolic

problem too? ∆ HCO3

- = 1 mEq/L↑/10mmHg↑pCO2

The HCO3- will go up 1mEq/L for every 10mmHg the pCO2goes up

above 40 The pCO2 is up by 6 so it is expected that the HCO3

- will go up by 0.6. Expected HCO3

- is 24.6, compared to the actual HCO3- of 20.

Since the HCO3- is lower than expected Non-Anion Gap Metabolic

Acidosis (which we suspected).

Dx-RESPIRATORY ACIDOSIS with NON-ANION GAP METABOLIC ACIDOSIS

 

Case-9 7.15/22/75 on room air, HCO3

- 9, anion gap = 10, albumin = 2.0 1: pH = Acidemia (pH < 7.4) 2:CO2= Base (CO2<40) So Primary disturbance: Metabolic Acidosis 3&4:∆ Compensation ?

pCO2=1.2 x ∆ HCO3-

Expected pCO2 = 1.2 x ∆ HCO3- 1.2 (24 -9) 1.2

(15) 18. The expected pCO2is 22mmHg. The actual pCO2 is 22, which is expected, so there is no concomitant disorder.

Contd…. 5: Anion Gap = 10

AGc = 10 + 2.5(4-2) = 15 Anion Gap Metabolic Acidosis

6: Delta Gap: Delta gap = (actual AG – 12) + HCO3

= (15-12) + 9 = 3+ 9 = 12 which is<18 Non-AG

Met Acidosis

Dx-ANION GAP METABOLIC ACIDOSIS with NON-ANION GAP METABOLIC ACIDOSIS