10. abg analysis

Arterial Blood Gas Analysis

Ram E. Rajagopalan, MBBS, AB (Int. Med & Crit. Care)

Consultant & Head, Dept. of Critical Care Medicine

SUNDARAM MEDICAL FOUNDATION, Chennai

Goals of this talk:

To provide one approach to interpretation of blood gases (Oxygenation & Acid-base disorders)

Not:To teach physiology of respiratory gas exchange

To discuss theories on acid-base regulation

To debate alternative approaches to interpretation

As required for ECG interpretation,

a systematic approach to ABGs enhances accuracy.

There are NO short-cuts!

A Systematic Approach

The Anatomy of a Blood Gas Report

----- 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


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 %

Measured Values

Temperature Correction:Is there any value to it?

Calculated Data:Which are the useful ones?

Entered Data:Derived from other sources

Traditional Measurements

pH electrode

pCO2 electrode (Severinghaus)

pO2 electrode (Clark)

Additional options include:Co-oximeter; measures O2 saturation

Na+, K+, Ca2+, Cl-

Haematocrit

Temperature Correction:A spin-off of microprocessor capability?

“ There is no scientific basis ... for applying temperature corrections to blood gas measurements…” Shapiro BA, OTCC, 1999.

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

Temperature correction of pH & pCO2 do not affect calculated bicarbonate

pCO2 reference points at 37o C are well

established as reliable reflectors of alveolar ventilation

Reliable data on DO2 and oxygen demand are

unavailable at temperatures other than 37o C


Blood Gas Report

Measured 37.0o


Corrected 38.6o


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



Blood Gas Report

Measured 37.0o


Corrected 38.6o

C



Bicarbonate is calculated on the basis of the Henderson equation:

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

or for the mathematically inclined…

Bicarbonate:

Henderson-Hasselbach equation:

pH = pKc + Log [HCO3-]

pCO2


Blood Gas Report

Measured 37.0o


Corrected 38.6o

C



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

: reflects metabolic part of acid base : no info. over that derived from pH, pCO2 and HCO3

: Misinterpreted in chronic or mixed disorders


Blood Gas Report

Measured 37.0o


Corrected 38.6o

C



Oxygenation Parameters:

O2 Content of blood:Hb x O2 Sat x Const. + Dissolved O2

Oxygen Saturation:

Alveolar / arterial gradient:

Arterial / alveolar ratio:

Oxygen Saturation

pO2

Sat

ura

tion

0 60 120

100% Most blood gasmachines estimate saturation from an idealized dissociation curve

Gold standard is co-oximetry

Errors may occur with abnormal haemoglobins.

Oxygen content is calculated from this.

Alveolar-arterial DifferenceInspired O2 = 21%= piO2 = (760-45) x .21=150 mmHg

O2

CO2

palvO2 = piO2 - pCO2 / RQ= 150 - 40/0.8= 150 – 50 = 100 mm Hg

partO2 = 90 mmHg

palvO2- partO2 = 10 mmHg

Alveolar-arterial Difference

O2

CO2

Oxygenation FailurepiO2 = 150

pCO2 = 40

palvO2= 150 – 40/.8=150-50 =100

pO2 = 45

= 100-45 = 55

Ventilation FailurepiO2 = 150

pCO2 = 80

palvO2= 150-80/.8 =150-100

= 50

pO2 = 45

= 50-45 = 5

Arterial-alveolar Difference

% Inspired Oxygen

A-a

O2

Severe

ModerateMild

Normal

A

B

C

D


Blood Gas Report

Measured 37.0o


Corrected 38.6o

C

Calculated DataHCO3 act 31.1 mmol / L

O2 CT 14.7 mL / dlO2 Sat 98.3 %t CO2 32.4 mmol / LpO2 (A - a) 32.2 mm HgpO2 (a / A) 0.79


Oxygenation: Limitations of parameters:

O2 Content of blood:Useful in oxygen transport calculationsDerived from calculated saturation

Oxygen Saturation:Ideally measured by co-oximetryCalculated values may be error-prone

Alveolar / arterial gradient:Reflects O2 exchange with fixed FiO2 ImpracticalDifferentiates hypoventilation as cause

Arterial / alveolar ratio:Proposed to be less variableSame limitations as A-a gradient


Blood Gas Report

Measured 37.0o



O2 Sat 98.3 %pO2 (A - a) 32.2 mm Hg

Entered DataFiO2 30.0 %

The Blood Gas Report:The essentials

pH 7.40 + 0.05PCO2 40 + 5 mm HgPO2 80 - 100 mm Hg

HCO3 24 + 4 mmol/L

O2 Sat >95A-a 2.5+(0.21 x Age) mm Hg

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

7The

Habits ofHighly

SuccessfulBlood Gas

Analysts

Dr. Ram “Covey” Rajagopalan(With apologies to Narins, Emmett & Morganroth)

Step 1Look at the pH

Is the patient acidemic pH < 7.35or alkalemic pH > 7.45

Step 2Is it a metabolic or respiratory disturbance ?

Acidemia: With HCO3 < 20 mmol/L = metabolicWith PCO2 >45 mm hg = respiratory

Alkalemia: With HCO3 >28 mmol/L = metabolicWith PCO2 <35 mm Hg = respiratory

Step 3If there is a primary respiratory disturbance, is it acute?

Expect pH = 0.08 x PCO2 / 10

Step 4For a respiratory disorder is renal compensation OK?

Respiratory acidosis: <24 hrs: [HCO3] = 1/10 PCO2

>24 hrs: [HCO3] = 3/10 PCO2

Respiratory alkalosis: 1- 2 hrs: [HCO3] = 2/10 PCO2

>2 days: [HCO3] = 6/10 PCO2

Step 5If the disturbance is metabolic is the respiratorycompensation appropriate?

For metabolic acidosis:Expect PCO2 = (1.5 x [HCO3]) + 8 + 2(Winter’s equation)

For metabolic alkalosis:Expect PCO2 = (0.7 x [HCO3]) + 21 + 1.5

If not: actual PCO2 > expected : hidden respiratory acidosisactual PCO2 < expected : hidden respiratory alkalosis

Step 6If there is metabolic acidosis, is there an anion gap?

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

If >12, Anion Gap Acidosis : MethanolUremiaDiabetic KetoacidosisParaldehydeInfection (lactic acid)Ethylene GlycolSalicylate

Step 7Does the anion gap explain the change in bicarbonate?

anion gap (Anion gap -12) ~ [HCO3]

If anion gap is greater; consider additional metabolic alkalosis

If anion gap is less; consider a nonanion gap metabolic acidosis


Blood Gas Report

Measured 37.0o



O2 Sat 78 %pO2 (A - a) 9.5 mm Hg pO2 (a / A) 0.83

Entered DataFiO2 21 %


Blood Gas Report

Measured 37.0o



O2 Sat 78 %pO2 (A - a) 9.5 mm Hg pO2 (a / A) 0.83


Case 2

60 year old male smokerwith progressiverespiratory distressand somnolence.

CO2 =76-40=36Expected pH = 36/10 x0.08=0.29Expected pH = 7.40-0.29=7.11Chronic resp. acidosis

pH <7.35 ; acidemia

pCO2 >45; respiratory acidemia

Limits:HCO3 = 3/10 of pCO2=3/10x36=10.8Limits of HCO3 = 24+11=35Pure Resp Acidosis

HypoxiaNormal A-a gradientDue to hypoventilation


Blood Gas Report

Measured 37.0o

CpH 7. 24pCO2 49.1 mm HgpO2 66.3 mm Hg


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



Blood Gas Report

Measured 37.0o

CpH 7. 24pCO2 49.1 mm HgpO2 66.3 mm Hg


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


Case 418-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.

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

pH <7.35 ; acidemia

pCO2 >45; respiratory acidemia

Limits:HCO3 = 1/10 of pCO2=1/10x9=0.9Limits of HCO3 = 24+1=25Resp Acidosis + Metabolic Acidosis

Hypoxia

piO2 = 715x.3=214.5 / palvO2 = 214-49/.8=153

153-66= 87

Normal AG= 12; Gap = 16 - 12 = 4HCO3 = 24 - 14 = 10, 2.5 accounted for by resp. alkalosis; / 3.5 indicates additional non-gap acidosis


Blood Gas Report

Measured 37.0o

CpH 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)



Blood Gas Report

Measured 37.0o

CpH 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)


Case 5

28 year old diabetic withrespiratory distressfatigue andloss of appetite.

pH <7.35 ; acidemia

HCO3 <22; metabolic acidemia

Limits:Expected pCO2 = (1.5 x HCO3)+8 + 2

= (1.5 x 14)+ 8 + 2 = 29 + 2 = 27 to 31

Met. Acidosis + Resp. alkalosis

If Na = 130, Cl = 100Anion Gap = 130 - (100 + 14)

= 130 - 114= 16

There is no copyright on this material….please copy for educational purposes….

… and thanks for not becoming comatose!

Technology

10. abg analysis