Arterial versus Venous Blood Gas Analysis

Rama B Rao, MDBellevue Hospital

Center/NYUMC2005

Case 1 A 78 year old woman with a history of HTN, A fib, DM, and COPD presents with severe

abdominal pain. On examination she has diffuse severe tenderness throughout the abdomen, and mild wheezes with the following vital signs:

HR 110 bpm, RR 22/min, T37°C, BP 105/70 mmHg Oxygen Saturation of 93% on RA

A blood gas is obtained with a lactate VBG 7.20/29/33 HCO3 12 Lactate 9

Case 2 A 30 year old male with a CD4 of 8

presents with dyspnea on exertion. Oxygen saturation is 88% and rises to 95% on 100% NRB.

An ABG is attempted, but the sample obtained is not pulsating and is likely to be venous.

VBG on room air results are 7.38/35/40 HCO3 23

Arterial Blood Gas Sampling A-a gradient Ventilation Acid-base status Lactate Electrolytes Co-oximetry

A-a Gradient Difference between what is measured in

the artery on an ABG, and what exists in the alveoli

Alveolar gas =Ambient gas minus what displaces it from the internal environment pAO2= Inspired O2 - (CO2/0.8)

A-a gradient is calculated pAO2 - measured paO2

A-a Gradient and paO2

When is it useful to calculate a gradient?

When will it affect your interventions in the emergency department?

A-a Gradient Indications• Assessment of PaO2 for subsequent

interventions• A-a gradient > 35 mmHg or paO2 <

70 mmHg Anonymous. Consensus statement on the use of corticosteroids

as adjunctive therapy for pneumocystis pneumonia in the acquired immunodeficiency syndrome. The National Institutes of Health-University of California Expert Panel for Corticosteroids as Adjunctive Therapy for Pneumocystis Pneumonia New England Journal of Medicine. 323(21):1500-4, 1990 Nov 22.

Venous sampling inadequate

Co-oximetry Oxyhemoglobin De-oxyhemoglobin Methemoglobin Carboxyhemoglobin

Venous co-oximetry is acceptable for MetHgb and COHgb

Touger M et al. Ann Emerg Med 1995;25:481-3

Lactate Indications Unidentified anion gap metabolic

acidosis

Management/Prognosticator Early goal directed therapy in sepsis1:

SIRS hypotension despite fluid resuscitation or lactate ≥ 4 mmol/L

Blunt trauma2

1. Rivers E, et al. New Engl J Med 2001;345:368-377; 2. Lavery RF. J Am Coll Surg 2000;190:656-664

Lactate: ABG vs VBG Not affected by tourniquet1

Venous lactate closely approximates arterial lactate, esp in blunt trauma2

Elevated venous lactate 100% sensitive for arterial lactic acidemia3

Venous lactate adequate

1.Tortella BJ Acad Emerg Med 1996;3:415, 2.Lavery RF. J Am Coll Surg 2000;190:656-664 3. Younger JG. Acad Emerg Med 1996;3:730-734

Acid-base Status Attempt to correlate arterial and

venous gases

Specific vs Nonspecific conditions

Attempt at generating an equation

Diabetic Ketoacidosis Prospective convenience sample Prior to treatment Mean difference between arterial

and venous pH 0.03 (0-0.11) Not validated for mixed acid-base

disorders, hypotensive pts, or ventilatory insufficency

VBG good correlation, useful to follow

Brandenburg MA, Ann Emerg Med 1998;31:459-465

Acute Respiratory Failure Excluded unstable hemodynamics

or pressor requiring pts 46 intubated patients in ICU Compared ABG vs VBG Created equation Validated? predictions

Chu Y. J Formosan Med Assoc 2003;102:539-43

Acute Respiratory Failure % Change pH 0.5 0.45 % Change pCO2 17.09 9.60 % Change HCO3 9.72 7.73

Authors conclude VBG predictive of ABG in stable ventilated patients

Limited applicability in ED patientsChu Y. J Formosan Med Assoc 2003;102:539-43

ED Patients Prospective 171 non-arrest, and 12 arrest pts Unable to predict arterial from

venous samples Change in pH 0.056 (SD) Change in pCO2 7.51 (SD)

Gennis PR Ann Emerg Med 1985;14:845-9

ED Patients Venous pH 7.25 98% predictive

of an arterial pH 7.20

Venous pH 7.00 98% predictive of an arterial pH 7.05

Venous pCO2 40 98% predictive of an arterial pCO2 48

Gennis PR Ann Emerg Med 1985;14:845-9

ED Patients Prospective, observational Physician questionairre Mean change in pH 0.036 ; in pCO2 6 Differences too large by questionairre 40% eligible patients captured Not many acidemic patients (pH 7.39) Limited utility, but good correlation

Rang LCF Can J Emerg Med 2002;4:7-15

Pediatric Patients ICU patients Good correlation VBG, ABG, CBG

for all parameters except for paO2 in hypotension

Change in pH difficult to assess from data

Potential utility in this subgroup

Yldzdas D. Arch Dis Childhood 2004;89;176-180

Pediatric Patients PICU patients: ABG, VBG, CBG pCO2 correlates best with capillary

sampling Venous sampling limited utility Capillary BG, and Pulse oximetry

useful Mean change pH 0.04 Potentially useful in this subgroup

Kirubakaran C. Indian J Pediatr 2003;70:781-5

COPD* Patients recovering from acute

exacerbation Compared pCO2 in venous and

arterial samples N= 48 pCO2 similar in each sample Limited utility

Elborn JS. Ulster Med J 1991;60:164-7 in Hinder K. Center for Clinical Effectiveness. www.med.monsh.edu/au/publichealthcare/cce

mean pH Gennis 0.056 Kirubakaran 0.04 Yldzdas 0.0397? Rang 0.036 Chu 0.037 (0.5%) Brandenburg 0.03

mean pCO2

Gennis 7.38 Kirubakaran - Yldzdas 3.1

Rang 6 Chu 6.75 (17.09%) Brandenburg -

mean HCO3

Gennis 1.21 2.55 SD

Kirubakaran - Yldzdas 1.67? Rang 1.5 (1.3-1.7)

Chu 2.56 (9.72%) Brandenburg very close

Case 1 A 78 year old woman with a history of

HTN, A fib, DM, and COPD presents with severe abdominal pain. On examination she has diffuse severe tenderness throughout the abdomen, mild wheezes and the following vital signs:

HR 110 bpm, RR 22/min, T37°C, BP 105/70 mmHg

A blood gas is obtained with a lactate

Case 1 VBG 7.20/29/33 HCO3 12 Lactate 9

What should you do? A. Repeat the lactate as an arterial sample B. Empirically start a bicarbonate drip C. Intubate for respiratory failure D. Repeat the sample as arterial, presume

a severe lactic acidemia is present

Case 1 VBG 7.20/29/33 HCO3 12 Lactate 9

What should you do? A. Repeat the lactate as an arterial sample B. Empirically start a bicarbonate drip C. Intubate for respiratory failure D. Presume a severe lactic acidemia is

present

Case 2 A 30 year old male with a CD4 of 8

presents with dyspnea on exertion. An ABG is attempted, but the sample obtained is not pulsating and is likely to be venous.

Case 2 VBG results are 7.38/35/40 HCO3 23 What should you do?

A. Start empiric corticosteroid therapy B. Repeat the gas as an arterial sample C. Send a lactate, urine for ketones,

and a repeat chemistry D. Correct pCO2 by adding a correction

factor of 7 mmHg

Case 2 VBG results are 7.38/35/40 HCO3

23 What should you do?

A. Start empiric corticosteroid therapy B. Repeat the gas as an arterial

sample C. Send a lactate, urine for ketones,

and a repeat chemistry D. Correct pCO2 by adding a

correction factor of 7 mmHg

Case 3 A 29 year old female is struck by a car

while crossing the street. She is awake and alert with normal vital signs and oxygen saturation and a large bruise across her right flank.

An IV line is placed. Should she get a complete gas or just a lactate? If so, venous or arterial?

Case 3 A 29 year old female is struck by a car

while crossing the street. She is awake and alert with normal vital signs and oxygen saturation and a large bruise across her right flank.

An IV line is placed. Should she get a complete gas or just a lactate? If so, venous or arterial?

Case 4 A 26 year old male with a history of

insulin requiring diabetes presents with abdominal pain, vomiting once, and polydipsia. He has missed one day of medication. His glucose is 487 mg/dL

He is mildly tachycardic, RR 24, afebrile, with clear lungs and a soft abdomen

Case 4 What should you do?

A. Send an ABG and lactate as he may have a triple acid-base disorder

B. Obtain a urine for ketones, VBG with electrolytes, and repeat as ABG if necessary

C. Obtain an ABG as he is tachypneic and may have an A-a gradient

D. Correct a venous pH by 0.05 upwards to obtain arterial value

Case 4 What should you do?

A. Send an ABG and lactate as he may have a triple acid-base disorder

B. Obtain a urine for ketones, VBG with electrolytes, and repeat as VBG after care and ABG only if necessary

C. Obtain an ABG as he is tachypneic and may have an A-a gradient

D. Correct a venous pH by 0.05 upwards to obtain arterial value

Case 5 An 8 week old male presents in

respiratory distress after 2 days of cough and nasal congestion with poor feeding. His oxygen saturation is 88% on room air. His lungs sound clear.

Case 5

What should you do? A. Presume methemoglobinemia and

empirically treat B. Obtain an arterial sample for MetHgb C. Consider congenital right to left shunt,

sepsis, pneumonia, or methemoglobinemia and send capillary blood gas

D. Consider broad differential, administer oxygen, obtain cultures, venous metHgb if no response to oxygen, and ABG

Case 5 What should you do?

A. Presume methemoglobinemia and empirically treat

B. Obtain an arterial sample for MetHgb C. Consider congenital right to left shunt,

sepsis, pneumonia, or methemoglobinemia and send capillary blood gas

D. Consider broad differential, administer oxygen, obtain cultures, venous metHgb if no response to oxygen, and an ABG to assess paO2

Conclusions Venous lactate and co-oximetry

are clinically valuable alternatives to arterial samples

paO2 is inadequately assessed with venous sampling

Conclusions Extremely acidemic venous pH will

likely predict severe arterial acidemia

A normal venous pH is likely to exclude severe arterial pH abnormalities

No single equation has been validated to predict arterial from venous sampling

Conclusions All decisions must be made with

regards to the clinical context of the patient and whether management would be potentially affected.