Clinical Aspect of Interpretation of Blood Gas Analysis

For medical students of PUMC

Class 1999, Sept. 15, 2003

What Does Arterial Blood Gas (ABG) Measure?

Pulmonary function tests are concern with ventilation: the movement of air into and out of the lung

MIXED VENOUS BLOOD

pH 7.36PCO2 46 mmHgPO2 40 mmHgSO2 75%

pH 7.40PCO2 40 mmHgPO2 95 mmHgSO2 95%

ARTERIAL BLOOD

External Respiration

Internal Respiration

What Information Does Arterial Blood Gas provide?

• Arterial oxygenation

• Alveolar ventilation

• Respiratory/metabolic acid-base balance

• Carboxyhemoglobin levels

Alveolar Ventilation EquationInverse relationship between VA and PaCO2

KV

VPCO

A

CO .

.

22

Arterial Blood Gas AnalysisIndications

• Evaluate adequacy of lung function– Ventilation, acid-base status– Oxygenation

• Determine need for supplemental O2

• Monitor ventilatory support• Document severity or progression of known

pulmonary disease• Diagnose the toxicity of CO

Henderson-Hasselbalch Equation

The relationship between pH, PaCO2 , HCO3-

1

20,,

][log

32

3 orLungs

Kidneysor

COH

HCOpKpH

Why the assessment of a single buffer system is adequate despite multiple

buffer systems?

• Bicarbonate buffer system: of primary importance, open system in communication with external environment via kidneys

• Hemoglobin buffer: of second importance

• Phosphate buffer system

• Plasma protein buffer system

• All buffer systems are linked together through H+

Why the assessment of carbonic acid is adequate?

• Each day our body produces large amount of acid from metabolism. 99% of the total acid is in the form of CO2. Only 1% is fixed acid

Henderson-Hasselbalch Equation

The relationship between pH, PaCO2 , HCO3-

1

20,,

][log

32

3 orLungs

Kidneysor

COH

HCOpKpH

Case 1: normal

0301.0*

][log

2

3

COP

HCOpKpH

a

Case 1: normal

0301.0*40

/24log1.6

LmEqpH

Case 1: normal

LmEq

LmEqpH

/2.1

/24log1.6

LmEq

LmEqpH

/2.1

/24log1.6

Case 1: normal

1

20log1.6 pH

Case 1: normal

4.73.11.6 pH

Case 2Uncompensated Respiratory Acidosis

0301.0*3.68

/3.25log1.6

LmEqpH

Case 2Uncompensated Respiratory Acidosis

LmEq

LmEqpH

/2.1

/24log1.6

LmEq

LmEqpH

/06.2

/3.25log1.6

Case2 Uncompensated Respiratory Acidosis

1

12log1.6 pH

Case 2Uncompensated Respiratory Acidosis

18.708.11.6 pH

Case 3Compensated Respiratory Acidosis

0301.0*3.68

/4.36log1.6

LmEqpH

Case3 Compensated Respiratory Acidosis

1

7.17log1.6 pH

Case 3Compensated Respiratory Acidosis

35.725.11.6 pH

PEARL: The compensations of either the renal system or the respiratory system can never be complete.

Clinically Relevant Parameters (1)

Through the years, opinions have changed regarding what are the most clinically relevant parameters. Today, for a nearly complete description of the oxygenation, ventilation, and acid-base status, pH, PaCO2, PaO2 and actual HCO3

- are generally sufficient.

Clinically Relevant Parameters (2)

Indeed, the literature or text book contains literally several parameters, i.e. standard HCO3

-, buffer base (BB), base excess (BE) from in vitro measures. Because intro and in vivo changes in response to hypercapnia are different, their actual clinical benefit is limited.

Burton GG, Hodgkin JE, Ward JJ. Respiratory care: A guide to clinical practice. 1997, 260-265.

Primary Respiratory Acidosis

• Initiating event: hypoventilation

• Resultant effects: CO2 retention

• Compensation: HCO3- retention via

renal system

Primary Respiratory Alkalosis

• Initiating event: hyperventilation

• Resultant effects: CO2 elimination

• Compensation: HCO3- elimination

via renal system

Primary Metabolic Acidosis

• Initiating event: renal, extrarenal

• Resultant effects: HCO3- deficit

• Compensation: CO2 elimination via respiratory system

Primary Metabolic Alkalosis

• Initiating event: renal, extrarenal

• Resultant effects: HCO3- increase

• Compensation: CO2 retention via respiratory system

Normal Range of Arterial Blood Gases

Normal Range Clinical Indication

pH

PCO2

HCO3-

7.35-7.45

35-45

22-27

Acid-base parameter

Respiratory parameter

Metabolic parameter

Interpretation of Arterial Blood Gases

Interpretation StrategiesStep 1

Was the blood gas specimen obtained

acceptably? Free of air bubbles and clots?

Analyzed promptly and/or iced properly?

Air Contamination of Sample

In vivo values Air contamination

PH

PCO2

PO2

7.40

40

95

7.45

30

110

Step 2

Did the blood gas analyzer function

properly? Was there a recent acceptable

calibration of all electrodes? Was analyzer

function validated by appropriate quality

control?

Data Quality in Blood GasesAcceptability criteria of AARC

• Blood collected anaerobically

• The specimen adequately anticoagulated

• A 2-4 ml sample recommended

• The specimen analyzed in a few minutes, otherwise stored in ice within 1 hour

• Equipment calibration and quality control

• The specimen adequately identified

Step3 Determine acid-base imbalance

The normal limits of pH is 7.35 - 7.45.

If below 7.35, acidosis is present; If above

7.45, alkalosis is present. Otherwise look for

compensation.

Is pH within normal limits?

Step 4the cause of acid-base imbalance?

Respiratory?

• If PCO2 >45 and pH <7.35, respiratory acidosis.

• If PCO2 >45 and pH 7.35-7.45, then compensated respiratory acidosis

• If PCO2 <35 and pH >7.45, respiratory alkalosis

• If PCO2 <35 and pH 7.35-7.45, then compensated respiratory alkalosis

Step 4the cause of acid-base imbalance?

Metabolic?

• If HCO3- <22 and pH <7.35, metabolic acidosis.

• If HCO3- <22 and pH 7.35-7.45, then compensated

metabolic acidosis

• If HCO3- >27 and pH >7.45, metabolic alkalosis

• If HCO3- >27 and pH 7.35-7.45, then compensated

metabolic alkalosis

Step 5 Oxygenation?

Is PaO2 within normal limits of 80 to 100

mm Hg? If PaO2 < 50 mm Hg, severe hy

poxemia is present.

The Hypoxemic State

Hypoxemia is defined as PaO2 < 80 mm Hg

while breathing room air. When patients

are already on oxygen it is not necessary

and may be dangerous to interrupt the o

xygen therapy to assess hypoxemia.

Step 6Correlated with clinical picture?

Are blood gas results consistent with p

atient's clinical status?

Case 1

• pH 7.35

• PCO2 30 mm Hg

• HCO3- 16 mEq/L

What is your interpretation?

Case 2

• pH 7.45

• PCO2 30 mm Hg

• HCO3- 20 mEq/L

What is your interpretation?

Case 3

• pH 7.55

• PCO2 27 mm Hg

• HCO3- 23 mEq/L

• PO2 104 mm Hg

• Get the plastic bag out!!!

What is your interpretation?

Case 4

• pH 7.30

• PCO2 34 mm Hg

• HCO3- 24 mEq/L

• Get the technician out!!!

What is your interpretation?

Case 5

A patient referred to PFT Lab. for shortness of breath

Case 5

pH 7.28 HCO3- 25.8 mEq/L

PCO2 51 mm Hg

PO2 55 mm Hg

What is your interpretation?

Case 6

A 17 y/o diabetic, entered Emergencywith Kussmaul breathing

Case 6Interpretation?

pH 7.05 HCO3- 5 mEq/L

PCO2 12 mm Hg

PO2 108 mm Hg

Case 7

34 y/o female, entered Emergencyin coma, drug overdose suspected

Case 7

pH 7.15 HCO3- 28 mEq/L

PCO2 80 mm Hg

PO2 42 mm Hg

What is your interpretation?

Case 8

A 63 y/o male, admitted for elective knee surgery

Case 8

pH 7.36 BP 122/84

PCO2 46 mm Hg P 80, regular

PO2 41 mm Hg RR 15/min

Preoperative blood gas

Suggested panic values of ABG• pH < 7.20

• pH > 7.60

• PaCO2 > 65mmHg (check pH and HCO3- to

see compensation)

• PaO2 < 50mmHg (exception: congenital cardiac malformations)

• COHb > 20%

• MetHb > 10%

Summary

Since arterial blood gas analysis is the reflection of efficiency or inefficiency of several organ systems, proper interpretation is essential in the care of critically ill patients.