Assessing the accuracy of algorithm-derived cardiorespiratory fitness in surgical patients: a prospective cohort study. Canadian Journal of Anesthesiology 2017;64:361–369 doi: 10.1007/s12630-017-0812-5

Presented by:  Dr Owen Lewis

Background 

  • Widely appreciated that assessing cardiorespiratory fitness pre operatively is useful for estimating perioperative risk
  • Cardiopulmonary exercise testing (CPEX) is seen as gold standard test of cardiorespiratory fitness; however, it’s not practical for all patients to undergo CPEX testing.
  • The authors postulate that an algorithm which calculates a patients MET value based on: height, weight, waist circumference, resting HR, smoking status, and self reported fitness level will provide similar results to MET values calculated from the same patients 6 minute walk test (6MWT) and Duke Activity Status Index (DASI) and therefore be a more accurate way of quantifying cardiorespiratory fitness than simply asking how many flights of stairs they can climb

Design & Setting

  • Prospective cohort study
  • Two Tertiary hospitals in Alberta Canada

Subjects

  • Patients undergoing major elective surgery between May and September 2015 at above 2 hospitals.
  • Exclusion criteria:
    • ASA 1 or 2
    • Age <45
    • Blood pressure >180/110
    • Unstable angina
    • MI within last month
    • Heart rate >120
    • Taking B blockers
    • Going to be admitted to ITU post operatively
    • Physical or mental disability such that could not perform 6MWT

Intervention

  • All patients meeting inclusion criteria had there MET value calculated using the trial algorithm, 6MWT and DASI questionnaire.

Outcomes

  1. Correlation between trial algorithm MET score and 6MWT MET score
  2. Correlation between trial algorithm MET score and DASI MET score
  3. Correlation between trial algorithm MET score and 6MWT distance covered

Results

  • Trial algorithm and 6MWT MET score were highly correlated (Pearson R = 0.870, P <0.001)
  • Trial algorithm and DASI MET score were loosely correlated (Pearson R = 0.252, P <0.001)
  • Trial algorithm and distance walked in 6MWT were moderately correlated (Pearson R = 0.420, P<0.001)

Conclusions

  • Strong correlation between trial algorithm and 6MWT and as such could be considered as an alternative.
  • The fact that they excluded patients on B blockers meant low prevalence of heart disease in sample, limiting genralisability of results.
  • Their data suggests that DASI may over estimate cardiorespiratory fitness in those with a low 6MWT, but the number of patients with a low 6MWT in this study was very small and previous bigger studies have shown good correlation
  • There was no attempt in this paper to correlate calculated MET values with post operative outcomes
  • Further evaluation of this algorithm is warranted

Strengths

  • Attempts to answer a useful question
  • Prospective design
  • Seems adequately powered to demonstrate their outcome measure

Weaknesses

  • Effectively a single site study
  • Exclusion criteria significantly limit generalizability particularly excluding patients on B blockers
  • Half of those that met inclusion criteria declined to be in study, is this because they didn’t want a 6MWT and were less fit?
  • Didn’t compare their algorithm to CPEX (gold standard) and also didn’t compare to “current practice” i.e. just asking about flights of stairs or similar

Implications

  • Cannot apply study result to average set of patients seen preoperatively in the UK
  • No evidence as yet that MET score calculated this way is as accurate as CPEX or that it correlates to post operative outcomes and as such aids post op planning.

Potential for impact

Very limited impact at present but could be further evaluated in studies