New Perspectives In Blood Gas Measurement
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"New Perspectives in Blood Gas Measurement, Assessment and Interpretation"
William J Malley, M.S., RRT, CPFT
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“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)
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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.
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Interactive
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Sample Handling
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Icing blood gas samples
Routine arterial blood gases in plastic syringes should be iced if they will be run within 30 minutes.
Yes/No
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Rationale for Icing
To minimize the effect of metabolism of living tissue
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.
No
NCCLS. Blood gas and pH analysis and related measurements. C46-A, 2001
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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
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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
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No…PaO2 actually increased
Liss, HP, Payne, CB. Stability of blood gases in ice and at room temperature. Chest 103:1120, 1993.
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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
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Factors which may falsely elevate plasma potassium levels
Hemolysis Physical Stress on sample
Icing Pneumatic Tubes
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Yes
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Learning Styles
“I learn best least from facts and figures and most from pictures and stories!”
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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)
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Oxygenation status of this patient is satisfactory.
Yes/No
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NO!
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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
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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
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The patient must have carboxyhemoglobinemia which is not being picked up by the pulse oximeter.
Yes/No
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KEY OBSERVATIONS
Pulse oximetry reading is satisfactory. There is a discrepancy between the
pulse oximeter reading and the PaO2
The patient has leukemia
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NO
Haynes JM, A case of disparity between pulse oximetry
measurements and blood gas analysis values, Resp Care 49,1059-
60, 2004
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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
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BICARBONATE THERAPY
Bicarbonate therapy for metabolic acidosis should be directed primarily at the negative base excess.
YES/NO
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BICARBONATE THERAPY
May be indicated is metabolic acidosis but is controversial
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NO
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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
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Blood Gas Interpretation
The blood gas reported is impossible.
Yes/NO
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If you know two of the three acid-base variables
(pH- HCO3- PaCO2)you know the third!
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Arterial Blood Gas
pH 7.55 PaCO2 40 mm Hg [HCO3] ?
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Blood Gas Interpretation
The blood gas reported is impossible.
Yes/NO
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Yes
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ARTERIAL BLOOD GASES SaO2 91%
pH 7.52 PaCO2 47 mm Hg
PaO2 63 mm Hg
[BE] +11 mEq/L
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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
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METABOLIC INDICES
It is impossible to have an elevated plasma bicarbonate with a decreased base excess of the blood.
YES/NO
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METABOLIC INDICES
HYPERCAPNIA CAUSES Increased bicarbonate via the hydrolysis
reaction Decreased blood base excess because of
in vivo – in vitro discrepancies in calculation
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NO
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Arterial Blood Gases
pH 7.38 PaCO2 73 mm Hg [HCO3] 42 mEq/L [K] 2.8 mEq/L
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COPD
This blood gas represents a compensated respiratory acidosis.
YES/NO
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NO
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HYPOKALEMIA
Hypokalemia may hamper weaning from mechanical ventilation
YES/NO
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CONSEQUENCES OF HYPOKALEMIA
ARRYTHMIA METABOLIC ALKALOSIS MUSCLE WEAKNESS
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YES
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Clinical Blood Gas Targets
PaCO2 ?
PaO2
pH
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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
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METABOLIC ACIDOSIS
Some evidence suggests acidosis may actually be protective to cells, but it should be treated in the presence of hyperkalemia.
YES/NO
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TREATMENT OF METABOLIC ACIDOSIS
Treatment of metabolic acidosis has been becoming progressively conservative
Metabolic acidosis exacerbates hyperkalemia
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YES
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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
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PERMISSIVE HYPOXEMIA ?
In paraquot poisoning, it is probably best to keep the patient in mild hypoxemia.
YES/NO
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PERMISSIVE HYPOXEMIA
Formation of oxygen free radicals is accelerated in the presence of Paraquot Bleomycin Rx
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YES
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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.
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