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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,

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arterial versus venous blood gas analysis

Arterial versus Venous Blood Gas Analysis

Rama B Rao, MD

Bellevue Hospital Center/NYUMC

2005

case 1
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
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
Arterial Blood Gas Sampling
  • A-a gradient
  • Ventilation
  • Acid-base status
  • Lactate
  • Electrolytes
  • Co-oximetry
a a gradient
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 p a o 2
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
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
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
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
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
Acid-base Status
  • Attempt to correlate arterial and venous gases
  • Specific vs Nonspecific conditions
  • Attempt at generating an equation
diabetic ketoacidosis
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
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 failure14
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

ed patients
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 patients16
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 patients17
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
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 patients19
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

slide20
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
mean pH
  • Gennis 0.056
  • Kirubakaran 0.04
  • Yldzdas 0.0397?
  • Rang 0.036
  • Chu 0.037 (0.5%)
  • Brandenburg 0.03
slide22

mean pCO2

  • Gennis 7.38
  • Kirubakaran -
  • Yldzdas 3.1
  • Rang 6
  • Chu 6.75 (17.09%)
  • Brandenburg -
slide23

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 124
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 125
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 126
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 227
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 228
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
slide29

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
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?
slide31

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
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 433
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
slide34

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
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.
slide36

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 537
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
Conclusions
  • Venous lactate and co-oximetry are clinically valuable alternatives to arterial samples
  • paO2 is inadequately assessed with venous sampling
conclusions39
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
conclusions40
Conclusions
  • All decisions must be made with regards to the clinical context of the patient and whether management would be potentially affected.