arterial blood gases made easy 1204798448914025 4

Dr. Vishram BucheOm Child Trust Hospital NAGPUR

The Goal :

In detail

…A respiratory component …A respiratory acid …Moves opposite to the direction of pH.

…A metabolic component …It is a base (Metabolic) …Moves in the same direction of pH.

…Moves in same direction ...Primary disorder …Moves in opposite direction …Mixed Disorder

COCO22

HCOHCO33

COCO22

HCOHCO33

H 1pH

H+ nmoles /L. pH

20 7.60

30 7.50

40 7.40

50 7.30

60 7.200

14

H+ = 80- last two digits of pH

OH ion

H+ ion

Alkaline

Acidic

CO 2 CHANGES pH in opposite direction

Primary lesion

compensation

pH

HCO3

CO2

METABOLIC ACIDOSIS

HYPER VENTILATION

BICARB CHANGES pH in same direction

LOW HCO3

LOW pH

LOW pCO2

LowAlkali

CO 2 CHANGES pH in opposite direction

Primary lesion

compensation

pH

HCO3

CO2

METABOLIC ALKALOSIS

HYPO VENTILATION

BICARB CHANGES pH in same direction

HIGH HCO3

HIGH pH

HIGH CO2

High Alkali

CO 2 CHANGES pH in opposite direction

Primary lesion

compensation

pH

CO 2

BICARB

Respiratory acidosis

HIGH pCO2

LOW pH

HIGH HCO3

High CO2

CO 2 CHANGES pH in opposite direction

Primary lesion

compensation

pH

CO 2

BICARB

Respiratory alkalosis

LOW pCO2

HIGH pH

LOW HCO3

Low CO2

Body’s physiologic response to Primary disorder in order to bring pH towards NORMAL limit

Full compensationPartial compensationNo compensation…. (uncompensated)

BUT never overshoots, If overcompensation is there, Take it granted it is a MIXED disorder

How to identify the type of compensation…..?

pH HCO3 CO2

7.20 15 40

7.20 15 30

7.37 15 20

Un Compensated

Partially Compensated

Fully Compensated

PaO2

SaO2OXY (Sat) 98%

HAEMOGLOBIN

2 % DissolvedOxygen

O.

D.

C.

PAO2

A.C.I.

CaO2Content of oxygenMl/100 of blood

DeliveryOf

OxygenTo

TissuesDaO2

O2

Cardiac output A.C.I.( Alveolar capillary interface)

FiO2….21%....150 mm of Hg

Now that I have this data,

what does it mean?

----- XXXX Diagnostics ------

Blood Gas Report248 05:36 Jul 22 2000Pt ID 2570 / 00

Measured 37.0o CpH 7.463pCO2 44.4 mm HgpO2 113.2 mm Hg

Corrected 38.6o CpH 7.439pCO2 47.6 mm HgpO2 123.5 mm Hg

Calculated DataHCO3 act 31.1 mmol / LHCO3 std 30.5 mmol / LBE 6.6 mmol / LO2 CT 14.7 mL / dlO2 Sat 98.3 %ct CO2 32.4 mmol / LpO2 (A - a) 32.2 mm HgpO2 (a / A) 0.79

Entered DataTemp 38.6 oCct Hb 10.5 g/dlFiO2 30.0 %

output

-----XXXX Diagnostics-----

Blood Gas Report328 03:44 Feb 5 2006Pt ID 3245 / 00

Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg

Corrected 38.6 0CpH 7.436pCO2 47.6 mm HgpO2 122.4 mm Hg

Calculated Data

HCO3 act 31.2 mmol / LHCO3 std 30.5 mmol / LB E 6.6 mmol / LO2 ct 15.8 mL / dlO2 Sat 98.4 %ct CO2 32.5 mmol / LpO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78

Entered DataTemp 38.6 0CFiO2 30.0 %ct Hb 10.5 gm/dl

Measured values…most important

Temperature Correction :Is there any value to it ?

Calculated Data :Which are useful one?

Entered Data :Important

Uncorrected pH & pCO2 are reliable reflections of in-vivo acid base status

Temperature correction of pH & pCO2 do not affect calculated bicarbonate“ There is no scientific basis ... for applying temperature corrections to blood gas measurements…” Shapiro BA, OTCC, 1999.

pCO2 reference points at 37o C are well established as a reliable reflectors of alveolar ventilation

Reliable data on DO2 and oxygen demand are

unavailable at temperatures other than 37o C

Bicarbonate is calculated on the basis of the Henderson equation:

[H+] = 24 pCO2 / [HCO3-]

or for the

Mathematically inclined…

-----XXXX Diagnostics-----

Blood Gas Report328 03:44 Feb 5 2006Pt ID 3245 / 00

Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg

Corrected 38.6 0CpH 7.436pCO2 47.6 mm HgpO2 122.4 mm Hg

Calculated Data

HCO3 act 31.2 mmol / LHCO3 act 31.2 mmol / LHCO3 std 30.5 mmol / LB E 6.6 mmol / LO2 ct 15.8 mL / dlO2 Sat 98.4 %ct CO2 32.5 mmol / LpO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78

Entered DataTemp 38.6 0CFiO2 30.0 %ct Hb 10.5 gm/dl

-----XXXX Diagnostics-----

Blood Gas Report328 03:44 Feb 5 2006Pt ID 3245 / 00

Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg

Corrected 38.6 0CpH 7.436pCO2 47.6 mm HgpO2 122.4 mm Hg

Calculated Data

HCO3 act 31.2 mmol / LHCO3 std 30.5 mmol / LHCO3 std 30.5 mmol / LB E 6.6 mmol / LB E 6.6 mmol / LO2 ct 15.8 mL / dlO2 Sat 98.4 %ct CO2 32.5 mmol / LpO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78

Entered DataTemp 38.6 0CFiO2 30.0 %ct Hb 10.5 gm/dl

Standard Bicarbonate:Plasma HCO3 after equilibrationto a PCO2 of 40 mm Hg

: reflects non-respiratory acid base change: does not quantify the extent of the buffer base abnormality : does not consider actual buffering capacity of blood

Base Excess: D base to normalise HCO3 (to 24) with PCO2 at 40 mm Hg(Sigaard-Andersen)

: reflects metabolic part of acid base D: no info. over that derived from pH, pCO2 and HCO3: Misinterpreted in chronic or mixed disorders

-----XXXX Diagnostics-----

Blood Gas Report328 03:44 Feb 5 2006Pt ID 3245 / 00

Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg

Corrected 38.6 0CpH 7.436pCO2 47.6 mm HgpO2 122.4 mm Hg

Calculated Data

HCO3 act 31.2 mmol / LHCO3 std 30.5 mmol / LB E 6.6 mmol / LO2 ct 15.8 mL / dlO2 ct 15.8 mL / dlO2 Sat 98.4 %O2 Sat 98.4 %ct CO2 32.5 mmol / LpO2 (A -a) 30.2 mm Hg pO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78pO2 (a/A) 0.78

Entered DataTemp 38.6 0CFiO2 30.0 %ct Hb 10.5 gm/dl

Alveolar-arterial O2 Difference

* When FiO2 = 21 % :PiO2 = (760-45) x .21= 150 mmHg

OO22

COCO22

PAO2 = 150 – 1.2 (PCO2)

= 150 – 1.2 40

= 150 – 50 = 100 mm Hg

PaO2 = 90 mmHg

………..PAO2 – PaO2 = ? PAO2 = PiO2* -(PCO2/0.8)

PAO2 – PaO2 = 10 mmHg PaO2

PAO2

Alveolar-arterial Difference

OO22

COCO22

Alveolar – arterial G.100 - 45 = 55 ……………….Wide A-a

Oxygenation Failure Wide Gap

PCO2 = 40PaO2 = 45PAO2 = 150 – 1.2 (40) = 150 - 50 = 100

Ventilation FailureNormal Gap

PCO2 = 80PaO2 = 45PAO2 = 150-1.2(80) = 150-100 = 50 Alveolar arterial G.50 – 45 = 5…………….Normal A-a

20 × 5 = 100

Expected PaO2 =

FiO2 × 5 = PaO2

Normal

Always mention and see… FiO2 ct Hb

-----XXXX Diagnostics----Blood Gas Report

Measured 37.0 0CpH 7.452 pCO2 45.1 mm HgpO2 112.3 mm Hg

Calculated Data

HCO3 act 31.2 mmol / LO2 Sat 98.4 %O2 ct 15.8pO2 (A -a) 30.2 mm Hg pO2 (a/A) 0.78

Entered Data

FiO2 %Ct Hb gm/dl

Technical Errors

Glass vs. plastic syringe: Changes in pO2 are not clinically importantNo effect on pH or pCO2

Heparin (1000 u / ml):Need <0.1 ml / ml of bloodpH of heparin is 7.0; pCO2 trends downAvoided by heparin flushing & drawing 2-4 cc blood

Delay in measurement:Rate of changes in pH, pCO2 and pO2 can be reduced to 1/10 by cooling in ice slush(4o C) No major drifts up to 1 hour

1.

2. Look at pH?

3. Who is the culprit ?...Metabolic / Respiratory

4. If respiratory…… acute and /or chronic

5. If metabolic acidosis,

Anion gap ed and/or normal or both?

6. Is more than one disorder present?

7. Correlate clinically

Consider the clinical settings! Anticipate the disorder

7 st

eps t

o an

alyz

e AB

G

Step 2

Look at the pHIs the patient acidemic pH < 7.35or alkalemic pH > 7.45

If pH = 7.4 …… Normal Mixed or Fully compensated

Step 3 ……. CULPRIT?

HCO3…… METABOLIC

> 26 ….. Met. Alkalosis

< 22 ……Met. Acidosis

PCO2 ……RESPIRATORY

> 45 …… Resp. Acidosis

< 35 …… Resp. Alkalosis

HCO3 = BaseNormal…22-26

CO2 = ACIDNormal…35-45

Step 4

If there is a primary Respiratory disturbance, is it acute ?

.08 change in pH ( Acute )

.03 change in pH (Chronic)

10 mm Change PaCO2

=

Remember………… relation of CO2 and pH

pH

Step 5If it is a primary Metabolic disturbance,whether respiratory compensation appropriate?For metabolic acidosis:Expected PCO2 = (1.5 x [HCO3]) + 8 + 2

(Winter’s equation)

For metabolic alkalosis:Expected PCO2 = 6 mm… for 10 mEq. rise in Bicarb.………UNCERTAIN COMPENSATION

CO2 is equal to Last two digits

of pH

Remember If : Suspect .............

actual PaCO2 is more than expected : additional ...respiratory acidosis

actual PaCO2 is less than expected : additional...respiratory alkalosis

Step 5 cont.If metabolic acidosis is thereHow is anion gap ? Is it wide ...

Na - (Cl-+ HCO3-) = Anion Gap usually <12

If >12, Anion Gap Acidosis : M ethanolU remiaD iabetic KetoacidosisP araldehydeI nfection (lactic acid)E thylene GlycolS alicylate

Common pediatric causes

Lactic acidosisMetabolic disordersRenal failure

Step 6…

-- Clinical history

-- pH normal, abnormal PCO2 n HCO3

-- PCO2 n HCO3 moving opposite directions

-- Degree of compensation for primary

disorder is inappropriate

-- Rise of anion gap and Fall of HCO3….. ……..R/F equation

Validity of ABG report… a lab error

H= 24 xPCO2

HCO3

e.g. pH = 7.30, PCO2 = 38.1, HCO3 = 30

By Henderson-Hasselbach H+ = 24 x pCO2/HCO3

= 24 x (38/30) = 30 80 - last two digit pH = H+

80 - H+ = last two digit pH (after 7) pH should be 7.50

Ready Chart………

Limitations…..

SIMPLE DISORDERS LOOKS LIKE MIXED1. Not enough time lapsed for compensation 2. 5% out of confidence Bandse.g. pH = 7.20, HCO3 = 18, PCO2 = 33

MIXED DISORDERS LOOKS LIKE SIMPLEe.g. pH =7.24, PCO2 = 65, HCO3 = 26Chronic Resp acidosis + Metabolic AcidosisChronic case …. History helps

Partially compensated Metabolic Acidosis

pH = 7.4PaCO2 = 40 HCO3 = 24

1

Uncompensated Metabolic Acidosis

pH = 7.4PaCO2 = 40 HCO3 = 24

2

Partially compensated Metabolic Alkalosis

pH = 7.4PaCO2 = 40 HCO3 = 24

3

Fully compensated Respiratory Alkalosis

pH = 7.4PaCO2 = 40 HCO3 = 24

4

Partially compensated Respiratory Acidosis

pH = 7.4PaCO2 = 40 HCO3 = 24

5

Uncompensated Uncompensated Metabolic AlkalosisMetabolic Alkalosis

pH = 7.4PaCO2 = 40 HCO3 = 24

6

Normal A.B.G.

pH = 7.4PaCO2 = 40 HCO3 = 24

7

Uncompensated Respiratory Acidosis

pH = 7.4PaCO2 = 40 HCO3 = 24

8

Uncompensated Respiratory Alkalosis

pH = 7.4PaCO2 = 40 HCO3 = 24

9

Fully compensated Respiratory Acidosis

pH = 7.4PaCO2 = 40 HCO3 = 24

10

Combined Alkalosis

pH = 7.4PaCO2 = 40 HCO3 = 24

11

Combined Acidosis

pH = 7.4PaCO2 = 40 HCO3 = 24

12

-----XXXX Diagnostics----Blood Gas Report

Measured 37.0 0CpH 7.301 pCO2 75.1 mm HgpO2 45.3 mm Hg

Calculated Data

HCO3 act 35.2 mmol / LO2 Sat 78.4 %O2 ct 15.8pO2 (A -a) 9.5 mm Hg pO2 (a/A) 0.83

Entered Data

FiO2 21 %Ct Hb 12 gm/dl

CO2 =75-40=35Expected pH ( Acute ) = 7.11Expected pH ( Chronic ) = 7.30

Chronic resp. acidosis

pH <7.30 …Acidosis

Respiratory Acidosis

Normal A-a gradientHypoxia due toDue to hypoventilation

Hypoxia….???

Case 1 6 year old male with progressive

respiratory distress due to

Muscular dystrophy .

Case 28-year-old male asthmatic;3 days of cough, dyspneaand orthopnea notresponding to usualbronchodilators.

O/E: Respiratory distress;suprasternal and intercostal retraction;tired looking; on 4 L NC.

----- XXXX Diagnostics ------

Blood Gas Report

Measured 37.0o C

pH 7. 24pCO2 49.1 mm HgpO2 66.3 mm Hg

Calculated DataHCO3 act 18.0 mmol / L

O2 Sat 92 %pO2 (A - a) mm Hg pO2 (a / A)

Entered DataFiO2 30 %

153-66= 87

pH <7.35 , acidosis

pCO2 >45; respiratory acidosis

Wide A / a gradient

Hypoxia

WITH INCREASE IN CO2 BICARB MUST RISE ? Metabolic acidosis + respiratory acidosis

CO2 = 49 - 40 = 9Expected pH ( Acute ) = 9/10 x 0.08 = 0.072Expected pH ( Acute ) = 7.40 - 0.072 = 7.328Acute resp. acidosis

30 × 5 = 150

Case 3

8 year old diabetic

with respi. distress

fatigue and loss of

appetite.

----- XXXX Diagnostics ------

Blood Gas Report

Measured 37.0o C

pH 7.23pCO2 23 mm HgpO2 110.5 mm Hg

Calculated DataHCO3 act 14 mmol / L

O2 Sat %pO2 (A - a) mm Hg pO2 (a / A)

Entered DataFiO2 21.0 %

pH <7.35 , Acidosis

HCO3 <22; metabolic acidemia

Last two digits of pHCorrespond with co2

If Na = 130, Cl = 90Anion Gap = 130 - (90 + 14)

= 130 – 104 = 26

----- XXXX Diagnostics ------

Blood Gas Report

Measured 37.0o

CpH 7.34pCO2 38.1 mm HgpO2 90.3 mm Hg

Calculated DataHCO3 act 30 mmol / L

O2 Sat 98.3 %pO2 (A - a) 10 mm Hg pO2 (a / A) 0.93

Entered DataFiO2 21.0 %

Case 4

16 year old female withsudden onset of dyspnea.

No Cough or Chest Pain

Vitals normal but RR 56,anxious.

Acidosis

Low CO2..???

High HCO3…???LAB ERROR!

By Henderson-Hasselbach H+ = 24 x pCO2/HCO3

= 24 x (38/30) = 30 80 - last two digit pH = H+

80 - H+ = last two digit pH (after 7) pH should be 7.50

3

224HCOPaCO

H

----- XXXX Diagnostics ------

Blood Gas Report

Measured 37.0o C

pH 7.46pCO2 28.1 mm HgpO2 55.3 mm Hg

Calculated DataHCO3 act 19.2 mmol / L

O2 Sat %pO2 (A - a) mm Hg pO2 (a / A)

Entered DataFiO2 24.0 %

Case 5 :

10 year old child with encephalitis

pH almost within normal rangeMild alkalosis

Co2 is low , respiratoryCo2 low by around 10 ( Acute ) by .08 (Chronic ) by .03

Bicarb looks low ?Is it expected ?

Case 6……

6 yrs old girl having type 1 Diabetic with H/O persistant vomiting

Lab: pH 7.37, pCO2 35 mm Hg, HCO3 22 Na 140, Cl 90, Blood sugar : 300

Mild Metabolic acidosis ? Should we send her Home?

NO !!Anion gap = (140 - 112) = 28Correlate Rise of Anion Gap with Fall of HCO3

Anion Gap ↑ed by 18, HCO3 should ↓ed by 18, but ↓ed by 2 onlyHCO3 retention (production) due to vomiting

Metabolic acidosis, metabolic alkalosis

1 month baby having malrotation, (having intermitent vomiting) posted for surgery .

His pre-op ABG shows on Room air pH ………. 7.39pCO2 ……..l5paO2 ……...90 HCO3 ……..8

b) Metabolic acidosis with compensatory Hypocapnia c) Acute respiratory alkalosis fully compensated. d) Chronic respiratory alkalosis fully compensated.

Case 3……….

a) Primary metabolic acidosis with respiratory alkalosis

What is the probable cause for the above findings ? Are they OK ?As far as oxygenation is concerned ?

Patient was hypo- volumic , received Normal Saline bolus... Corrected acidosisHe was operated …but post-op… drowsy His ABG……..FiO2….30%

pH ……..7.39PaCO2 …38PaO2 ……60

1) Why hypoxemia ?2) Were the lungs bad to begin with ? ( Pre OP PaO2 … 90 mmHg )3) Micro atelectesis during surgery ? Anesthetist goofed up the case 4) Pure and simple hypoventilation …..Sedation ?

Why hypoxemia ?Lungs were bad to begin with ?Micro atelectesis during surgeryPure and simple hypoventilation ? sedation

PRE OP ….ABG on room airpH 7.39 PaCO2 l5mmHg PaO2 90 mmHgHCO3 8mmol/L

Pre OP .....A/a gradient PAO2 = PiO2 – 1.2 (PaCO2 )

= 150 – 1.2 x 90= 150 – 18 = 132 mm Hg

132 – 90= 42 WIDE A / a gradient

Oxygenation status good …..?

One click

Apparently the lungs looked good with PaO2 of 90…But have a good look at the ABG again With wash out of CO 2 ……….The expected PaO2 should have been more than 90 .

This coupled with correction of acidosis( normalizing PaCO2 )Lowered the PaO2 …post operatively.

Conclusion …….. Lungs were not normal at the beginning

Correlate PaO2 with FiO2

But please also correlate with PaCO2

Learning point

No click

1. I shall use only minimal amount of heparin to rinse the syringe.

(Excess heparin causes pCO2 & shift pO2 to near 150. pH remains unchanged.)

2. I shall do ALLEN’S test for collateral circulation and, ALSO confirm that the sample sent is arterial and not venous

3. I shall ensure there are no air bubbles in the blood .

4. I shall send the sample in ice and analyze it quickly, … and keep the TLC in mind, esp. when there is a delay.

5. I shall always take FiO2 into consideration when interpreting pO2 values. I shall also look at the pCO2 values carefully.

6. I shall take the history into consideration before instituting therapy for Chronic respiratory failure.7. I shall always remember the acronym “ DOPE “ in situations of sudden deterioration of ABG values

D- DisplacementO- ObstructionP- PneumothoraxE- Equipment failure

8. I shall practice gentle mechanical ventilation and not try to bring ABG to perfect normal.

9. I shall treat the patient not the ABG report

10. I shall always correlate ABG report clinically.

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please copy for educational purposes….

THANKS For any queries: vbuche@rediffmail.com