"New Perspectives in Blood Gas Measurement, Assessment and Interpretation"

William J Malley, M.S., RRT, CPFT

“Blood gas and pH analysis has more immediacy and potential impact on patient care than any other laboratory determination.”

National Committee for Clinical Laboratory Standards

(NCCLS)

OUTLINE

1) Review pre-analytical ABG issues and consequences of improper technique.

2) Discuss the value of temperature correction of ABG’s.

3) Use case studies to highlight some key clinical concepts.

4) Explore appropriate clinical ABG targets in different clinical scenarios.

Interactive

Sample Handling

Icing blood gas samples

Routine arterial blood gases in plastic syringes should be iced if they will be run within 30 minutes.

Yes/No

Rationale for Icing

To minimize the effect of metabolism of living tissue

.

No

NCCLS. Blood gas and pH analysis and related measurements. C46-A, 2001

Icing blood gas samples

PaO2 decreased in a linear fashion when measured up to 30 minutes after the blood sample in a plastic syringe had been placed in an ice-water bath.

YES/NO

Factors which could theoretically impact results:

1) Metabolism 2) Diffusion of gases between the

sample and the ambient gas 3) The change in gas solubility due to

the temperature decrease

No…PaO2 actually increased

Liss, HP, Payne, CB. Stability of blood gases in ice and at room temperature. Chest 103:1120, 1993.

Icing blood gas samples when also measuring electrolytes

Potassium will tend to be higher in iced samples or samples sent through pneumatic tubes.

Yes/No

Factors which may falsely elevate plasma potassium levels

Hemolysis Physical Stress on sample

Icing Pneumatic Tubes

Yes

Learning Styles

“I learn best least from facts and figures and most from pictures and stories!”

A 38 y.o. female patient is admitted to the emergency department with severe pneumonia and a temperature of 41oC.

ARTERIAL BLOOD GASES

SaO2 85% pH 7.30

PaCO2 41 mm Hg

PaO2 62 mm Hg [HCO3] 25 mEq/L

(data has been temp corrected to 410C)

Oxygenation status of this patient is satisfactory.

Yes/No

NO!

ARTERIAL BLOOD GASES SaO286% 69% pH 7.24 7.30

PaCO2 41 mm Hg 32 mm Hg

PaO2 62 mm Hg 40 mm Hg

[HCO3] 17 mEq/L 15 mEq/L 41oC 37oC

A 51 y.o. male with severe leukemia has a blood gas drawn. He is also on a pulse oximeter which is reading 92%.

ARTERIAL BLOOD GASES

SaO2 85%

pH 7.36

PaCO2 37 mm Hg

PaO2 50 mm Hg

[HCO3] 25 mEq/L

The patient must have carboxyhemoglobinemia which is not being picked up by the pulse oximeter.

Yes/No

KEY OBSERVATIONS

Pulse oximetry reading is satisfactory. There is a discrepancy between the

pulse oximeter reading and the PaO2

The patient has leukemia

NO

Haynes JM, A case of disparity between pulse oximetry

measurements and blood gas analysis values, Resp Care 49,1059-

60, 2004

A 55 year old male patient in the burn unit suffers a cardiac arrest. Blood gases drawn during the arrest are called to the floor. Due to the negative base excess reported the patient is given two

ampoules of sodium bicarbonate.

ARTERIAL BLOOD GASES SaO2 91%

pH 7.52 PaCO2 47 mm Hg

PaO2 63 mm Hg

[BE] -11 mEq/L

BICARBONATE THERAPY

Bicarbonate therapy for metabolic acidosis should be directed primarily at the negative base excess.

YES/NO

BICARBONATE THERAPY

May be indicated is metabolic acidosis but is controversial

NO

A 55 year old male patient in the burn unit suffers a cardiac arrest. Blood gases drawn during the arrest are called to the floor. Due to the negative base excess reported the patient is given two

ampoules of sodium bicarbonate.

ARTERIAL BLOOD GASES SaO2 91%

pH 7.52 PaCO2 47 mm Hg

PaO2 63 mm Hg

[BE] -11 mEq/L

Blood Gas Interpretation

The blood gas reported is impossible.

Yes/NO

If you know two of the three acid-base variables

(pH- HCO3- PaCO2)you know the third!

Arterial Blood Gas

pH 7.55 PaCO2 40 mm Hg [HCO3] ?

Blood Gas Interpretation

The blood gas reported is impossible.

Yes/NO

Yes

ARTERIAL BLOOD GASES SaO2 91%

pH 7.52 PaCO2 47 mm Hg

PaO2 63 mm Hg

[BE] +11 mEq/L

A 25 year old female patient arrives in the emergency room in a coma.

ARTERIAL BLOOD GASES

SaO2 85% pH 7.16 PaCO2 80 mm Hg

PaO2 52 mm Hg

[BE] blood - 4 mEq/L [HCO3] 28 mEq/L

METABOLIC INDICES

It is impossible to have an elevated plasma bicarbonate with a decreased base excess of the blood.

YES/NO

METABOLIC INDICES

HYPERCAPNIA CAUSES Increased bicarbonate via the hydrolysis

reaction Decreased blood base excess because of

in vivo – in vitro discrepancies in calculation

NO

Arterial Blood Gases

pH 7.38 PaCO2 73 mm Hg [HCO3] 42 mEq/L [K] 2.8 mEq/L

COPD

This blood gas represents a compensated respiratory acidosis.

YES/NO

NO

HYPOKALEMIA

Hypokalemia may hamper weaning from mechanical ventilation

YES/NO

CONSEQUENCES OF HYPOKALEMIA

ARRYTHMIA METABOLIC ALKALOSIS MUSCLE WEAKNESS

YES

Clinical Blood Gas Targets

PaCO2 ?

PaO2

pH

Permissive Hypercapnia

PaCO2 as high as 80 – 100 mm Hg

Avoidance of Volume / Pressure Induced Lung Injury

Avoid hypercapnia patients vulnerable to increased intracranial pressure

METABOLIC ACIDOSIS

Some evidence suggests acidosis may actually be protective to cells, but it should be treated in the presence of hyperkalemia.

YES/NO

TREATMENT OF METABOLIC ACIDOSIS

Treatment of metabolic acidosis has been becoming progressively conservative

Metabolic acidosis exacerbates hyperkalemia

YES

Permissive Acidemia

pH down to 7.10 surprisingly well tolerated

Some evidence that low pH is actually protective to the organism

Must treat when acidosis is associated with significant hyperkalemia

PERMISSIVE HYPOXEMIA ?

In paraquot poisoning, it is probably best to keep the patient in mild hypoxemia.

YES/NO

PERMISSIVE HYPOXEMIA

Formation of oxygen free radicals is accelerated in the presence of Paraquot Bleomycin Rx

YES

Permissive Hypoxemia

According to Downs, hypoxia may not occur until PaO2 is less than 30 mm HG

Downs, JB. Has oxygen administration delayed appropriate respiratory care? Resp Care48:611, 2003.