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Arterial versus Venous Blood Gas Analysis

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

  2. 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

  3. 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

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

  5. 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

  6. A-a Gradient and paO2 • When is it useful to calculate a gradient? • When will it affect your interventions in the emergency department?

  7. 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

  8. 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

  9. 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

  10. 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

  11. Acid-base Status • Attempt to correlate arterial and venous gases • Specific vs Nonspecific conditions • Attempt at generating an equation

  12. 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

  13. 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

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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

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

  22. mean pCO2 • Gennis 7.38 • Kirubakaran - • Yldzdas 3.1 • Rang 6 • Chu 6.75 (17.09%) • Brandenburg -

  23. 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

  24. 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

  25. 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

  26. 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

  27. 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.

  28. 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

  29. 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

  30. 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?

  31. 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?

  32. 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

  33. 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

  34. 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

  35. 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.

  36. 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

  37. 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

  38. Conclusions • Venous lactate and co-oximetry are clinically valuable alternatives to arterial samples • paO2 is inadequately assessed with venous sampling

  39. 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

  40. Conclusions • All decisions must be made with regards to the clinical context of the patient and whether management would be potentially affected.