Arterial versus Venous Blood Gas Analysis Rama B Rao, MD Bellevue Hospital Center/NYUMC 2005

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 HCO 3 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 HCO 3 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 p A O 2 = Inspired O 2 - (CO 2 /0.8) A-a gradient is calculated p A O 2 - measured p a O 2

A-a Gradient and p a O 2 When is it useful to calculate a gradient? When will it affect your interventions in the emergency department?

A-a Gradient Indications Assessment of PaO 2 for subsequent interventions A-a gradient > 35 mmHg or p a O 2 < 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 sepsis 1 : SIRS hypotension despite fluid resuscitation or lactate ≥ 4 mmol/L Blunt trauma 2 1. Rivers E, et al. New Engl J Med 2001;345: ; 2. Lavery RF. J Am Coll Surg 2000;190:

Lactate: ABG vs VBG Not affected by tourniquet 1 Venous lactate closely approximates arterial lactate, esp in blunt trauma 2 Elevated venous lactate 100% sensitive for arterial lactic acidemia 3 Venous lactate adequate 1.Tortella BJ Acad Emerg Med 1996;3:415, 2.Lavery RF. J Am Coll Surg 2000;190: Younger JG. Acad Emerg Med 1996;3:

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:

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 pCO  9.60 % Change HCO  7.73 Authors conclude VBG predictive of ABG in stable ventilated patients Limited applicability in ED patients Chu 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 (SD) Change in pCO (SD) Gennis PR Ann Emerg Med 1985;14:845-9

ED Patients Venous pH  % predictive of an arterial pH  7.20 Venous pH  % predictive of an arterial pH  7.05 Venous pCO 2  40 98% predictive of an arterial pCO 2  48 Gennis PR Ann Emerg Med 1985;14:845-9

ED Patients Prospective, observational Physician questionairre Mean change in pH ; in pCO 2 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 p a O 2 in hypotension Change in pH difficult to assess from data Potential utility in this subgroup Yldzdas D. Arch Dis Childhood 2004;89;

Pediatric Patients PICU patients: ABG, VBG, CBG pCO 2 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 pCO 2 in venous and arterial samples N= 48 pCO 2 similar in each sample Limited utility Elborn JS. Ulster Med J 1991;60:164-7 in Hinder K. Center for Clinical Effectiveness.

mean  pH Gennis0.056 Kirubakaran0.04 Yldzdas0.0397? Rang0.036 Chu (0.5%) Brandenburg0.03

mean  pCO 2 Gennis7.38 Kirubakaran- Yldzdas3.1 Rang6 Chu6.75 (17.09%) Brandenburg-

mean  HCO 3 Gennis1.21  2.55 SD Kirubakaran- Yldzdas1.67? Rang1.5( ) Chu2.56 (9.72%) Brandenburgvery 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 HCO 3 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 HCO 3 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 HCO 3 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 pCO 2 by adding a correction factor of 7 mmHg

Case 2 VBG results are 7.38/35/40 HCO 3 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 pCO 2 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 p a O 2

Conclusions Venous lactate and co-oximetry are clinically valuable alternatives to arterial samples p a O 2 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.