Arteriovenous blood gas agreement: A research journey


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This presentation discusses the state of evidence for arteriovenous blood gas agreement for pH, pCO2, bicarbonate and base excess and how that fits into clinical decision-making. It also describes the resaerch journey of a smal clinical team to answer important clinical questions and address an issue of concern to patients.

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  • Going to report statistical agreement, but we are probably more interested in agreement within clinically acceptable limits
  • Single study; small numbers; needs further research
  • Going to report statistical agreement, but we are probably more interested in agreement within clinically acceptable limits
  • Arteriovenous blood gas agreement: A research journey

    1. 1. Anne-Maree KellyProfessor and DirectorJoseph Epstein Centre for Emergency MedicineResearch @Western Health
    2. 2.  I received financial support for travel and accommodation fromRadiometer Pty Ltd to present a similar presentation at 4thInternationalSymposium on Blood Gas and Critical Care in France in 2008. I am undertaking some research with A/Prof Rees into calculated valueswhich may be commercialised. I have no pecuniary interest in thisprogram. I have not received industry funding for any of my blood gas researchprojects.
    3. 3.  To share a research journey To describe how venous blood gas analysis mightfit into clinical care of selected patients
    4. 4.  An emergency physician, an educator and a clinicalresearcher My research journey started when a ‘rep’ came to theED with a transcutaneous CO2 monitor and I wanted toknow if it worked in an ED population. My questions come from my clinical practice – with afocus on working smarter for better patient outcomes Venous blood gases is one of my research streams
    5. 5. How we work Clinical focus: ED,prehospital care andrelated Small team◦ Myself◦ Research nurse◦ Registrars, students Collaborativeapproach
    6. 6.  Pain management in ED Acute respiratory disease: asthma, COPD andpneumothorax Estimating children’s weight in emergencies Intranasal naloxone for heroin overdose Acute cardiology and resuscitation, especiallychest pain in ED Clinical decision rules Implementation science and EBM
    7. 7.  2001: Accuracy of pulse oximeters in resp. disease 2001: Arteriovenous pH agreement 2002: Venous pH and pCO2 as screening tests forhypercarbia (derivation) 2004: Agreement for K+, bicarbonate 2005: Validation screening VBG for hypercarbia 2006: ICU studies 2011-13: Meta-analyses and systematic reviews 2012-13: Arteriovenous agreement and clinical utility inpatients undergoing NIV 2013: External validation of mathematical prediction model(in collaboration with Aalborg University, Denmark)
    8. 8. Year PatientpopulationPaper2001 General ED population Kelly AM, McAlpine R, Kyle E. Venous pH can safely replace arterial pH in the initial evaluationof patients in the emergency department. Emerg Med J 2001; 18:340-22002 Acute respiratory disease Kelly AM, Kyle E, McAlpine R. Venous pH and pCO2 can be used to screen for significanthypercarbia in emergency patients with acute respiratory disease. J Emerg Med 2002; 15-19.2004 General ED population Fu P, Douros G, Kelly AM. Does potassium concentration measured on blood gas analysisagree with serum potassium in patients with diabetic ketoacidosis? Emerg Med Austral 2004;16:280-3.2004 General ED population Kelly AM, McAlpine R, Kyle E. Agreement between bicarbonate measured on arterial andvenous blood gases. Emerg Med Australas 2004; 16:407-9.2005 COAD Kelly AM, Kerr D, Middleton P. Validation of venous pCO2 to screen for arterial hypercarbia inpatients with chronic obstructive airways disease. J Emerg Med 2005; 28;4:377-9.2006 ICU Middleton P, Kelly AM, Brown J, Robertson M. Agreement Between Arterial And VenousValues For pH, Bicarbonate, Base Excess and Lactate Emerg Med J 2006; 23:622-4.2010 COAD Lim BL, Kelly AM. A meta-analysis on the utility of peripheral venous blood gas analyses inexacerbations of chronic obstructive pulmonary disease in the emergency department. Eur JEmerg Med 2010; 17:246-8.2010 General ED population Lim BL, Kelly AM. How useful is transcutaneous carbon dioxide monitoring in the adultemergency department? Hong Kong J Emerg Med 2010; 17:82-42011 NIV Kelly AM, Klim S. Agreement between arterial and transcutaneous pCO2 in patients undergoingnon-invasive ventilation Respir Med 2011;105:226-9.2013 NIV Kelly A, Klim S. Agreement between arterial and venous pH and pCO2 in patients undergoingnon-invasive ventilation in the emergency department Emerg Med Australas [at press]2013 NIV Agreement Between Mathematically Arterialized Venous vs. Arterial Blood Gas Values inPatients Undergoing Non-invasive Ventilation [submitted]2013 NIV Can trend of pH and pCO2 be used to monitor progress in patient undergoing non-invasiveventilation? A prospective cohort study
    9. 9.  Establishing acid-base status◦ Mainly pH; but also bicarbonate Measuring respiratory function/ ventilation Mainly pCO2; but also pH ‘Quick check’ potassium, haematocrit, someelectrolytes◦ Not addressed in this presentation
    10. 10.  Less pain for patients Fewer complications, especially vascular andinfection Fewer needle-stick injuries Easier blood draw Minimal training requirement
    11. 11. JANE TRAN◦ 26 year old, insulindependent diabetic◦ 2 days of vomiting anddiarrhoea.◦ Pulse 120 bpm, BP100/60, bedside glucose‘hi’◦ 74 year old COAD◦ Acute respiratorydistress.◦ Pulse 110, BP 140/-,oxygen saturation (onair) 88%
    12. 12.  Can we◦ Exclude / diagnose◦ Monitor progress of◦ Base therapeutic decisions forMetabolic acidosis or acute respiratory failure usingvenous blood gas analysis rather than arterial?
    13. 13.  Outcome of interest is how closelyvenous and arterial values agree,not how well they correlate Weighted mean difference gives anestimate of the accuracy betweenthe methods 95% limits of agreement giveinformation about precisionArterial valueVenous value95%LoA
    14. 14.  There is limited data about the toleranceclinicians have with respect to agreementbetween arterial and venous values of blood gasparameters Depending on this tolerance, the degree ofagreement may be acceptable or unacceptable This is included in a University of MelbourneScholarly Selective project late 2013
    15. 15.  Patient cohorts in the published literature arehighly varied Patient groups of interest are those at high risk ofacidosis or hypercarbia◦ Reporting does not always report this detail◦ Data may to be dominated by patients with normal pH,pCO2 and blood pressure◦ Need for research focussed on high risk patient groups
    16. 16.  13 studies◦ Range from 44 to 346 patients; Several JECEMR Various conditions◦ DKA (3), COAD (4), trauma (1) 2009 patients Weighted mean difference of 0.033 pH units 95% limits of agreement (7 studies) generally within +/-0.1 pH units
    17. 17. DKA COAD◦ 3 studies (265 patients)◦ Weighted meandifference = 0.02 pHunits◦ 95% limits of agreement= -0.009 to 0.02 pHunits (1 study)◦ 5 studies (643 patients)◦ Weighted meandifference= 0.034 pHunits◦ 95% limits of agreementgenerally +/- 0.1 pHunits (3 studies)
    18. 18.  One ICU-based study suggests that ashypotension increases, AV pH agreementdeteriorates◦ Very small patient numbers◦ Finding not yet validated University of Melbourne scholarly selective2013◦ Arteriovenous blood gas agreement in varying levelsof shock and cardiac output
    19. 19.  We know:◦ Generally close AV agreement in both respiratory andmetabolic disease Evidence gaps:◦ AV agreement in various levels and types of shock◦ AV difference in toxicology scenarios (1 small study inTCA OD only)◦ AV difference in mixed acid-base disease
    20. 20.  8 studies 965 patients Various conditions (COAD 4) Weighted mean difference = 6.2 mmHg 95% limits of agreement: up to -17.4 to +23.9mmHg◦ 5/7 studies reporting LoA report LoA band >20mmHg
    21. 21.  4 studies 452 patients Weighted man difference = 7.26 mmHg 95% limits of agreement: up to -14 to +26mmHg◦ All 3 studies that reported LoA report LoA band>20mmHg
    22. 22. Author, year No. Screeningcut-offSens. Spec. NPV %ABGavoidedKelly, 2002 196 45 100 57 100 43Kelly, 2005 107 45 100 47 100 29Ak, 2006 132 45 100 * 100 33McCanny,201194 45 100 34 100 23POOLEDDATA52945 100(95% CI97-100)53(95%CI 57-58)100(95% CI97-100)35%(95% CI32-41)Data limited to studies in cohorts with respiratory disease
    23. 23.  Data recently submitted for publication 47 comparisons in 34 patients Average arteriovenous difference for change in pH (v-a)was 0.001pH units (LoA -0.7 to +0.7). Average arteriovenous difference between change inpCO2 (v-a) was 0.04mmHg (LoA -17.3 to +18.2). For both pH and pCO2, in the majority of cases thedirection of change was the same although the magnitudewas variable.
    24. 24.  We know: AV agreement is NOT good enough for clinical inter-changeability Wide limits of agreement Venous pCO2 has utility as a screening test for hypercarbia Excellent NPV Evidence gaps: Whether trend in venous pCO2 and pH can safely drive a care pathwayfor COAD Subject of current international research project (JECEMR is a partner)
    25. 25.  8 studies 1211 patients Various conditions (COAD 2) Weighted mean difference = -1.3mmol/l 95% limits of agreement : up to +/- 5mmol/l(3 studies)
    26. 26.  Two studies only Data not suitable for pooling
    27. 27. JANE TRAN◦ DKA◦ AV agreement isacceptable; at least innon-shocked patients◦ Can use venous pH todiagnose/ monitor◦ Acute respiratorydistress◦ pH agreement good butpCO2 has considerableimprecision◦ Can use venous pCO2 asa screening test forhypercarbia◦ ? Can monitor trend*
    28. 28.  Team from Center for Model Based Medical Decision SupportSystems, Dept of Health Science and Technology, AalborgUniversity, Denmark (A/Prof Steven Rees) Developed venous to arterial conversion method using venousblood gas variables and pulse oximetry Designed to be incorporated into blood gas analysers
    29. 29.  The method calculatesarterial values usingmathematical models Assumes:◦ Constant value of therespiratory quotient of0.82◦ Change in base excessfrom arterial to venousblood is 0 mmol/lRees SE, Toftegaard M, Andreassen S. A method for calculation of arterial acid–base and blood gas status from measurements in the peripheralvenous blood. Comp Methods Programs Biomed. 2006, Vol 81, 18-25.
    30. 30.  Respiratory patients◦ Arterial-calculated pH difference = -0.001pH units (95% LoA-0.026 to +0.026)◦ Arterial-calculated pCO2 difference = -0.68mmHg (95% LoA-4.81 to +3.45 mmHg) ICU◦ Arterial-calculated pH difference = -0.002pH units (95% LoA-0.029 to +0.025)◦ Arterial-calculated pCO2 difference = 0.3mmHg (95% LoA -3.58to +4.18 mmHg) ED◦ pH can be calculated to within 0.02 pH units (95% LoA)◦ pCO2 can be calculated to within 4mmHg (0.5kPa)
    31. 31.  82 sample-pairs (60 patients) Mean difference for arterial pH (actual-calculated) was 0.01 pH units (95% limitsof agreement: -0.04, 0.06). Mean difference for pCO2 (actual-calculated)was -0.45mmHg (95% limits ofagreement: -10, +9).
    32. 32.  pH and bicarbonate◦ Probably close enough agreement for clinical purposesin DKA, acute respiratory failure, isolated metabolicacidosis◦ More work needed in toxicology, shock, mixed disease◦ Scholarly selective addressing agreement in shock andwith varying levels of cardiac output
    33. 33.  pCO2◦ NOT enough agreement for clinical purposes, either as one-offor to monitor change◦ Data suggests venous pCO2 is useful as a screening test Base excess◦ Probably not enough agreement for clinical purposes
    34. 34.  Mathematical modelling approaches might be moreaccurate especially for pCO2 For broad applicability an app/ similar would be morefeasible than integration into blood gas machines More work needed to prove accuracy and precision in highrisk groups
    35. 35.  JECEMR has led research into arteriovenous bloodgas agreement Our >10 year journey has steadily added piecesto build understanding of agreement in differentdisease states The accumulated data has changed practice both‘home’ and ‘away’
    36. 36. Questions?Questions?Questions?Follow JECEMR research:@kellyam_jecSubscribe to quarterly e-newsletter:Email ‘subscribe’