An evaluation of the validity of the preoperative oxygen uptake efficiency slope as an indicator of cardiorespiratory fitness in elderly patients scheduled for major colorectal surgery

Anaesthesia 2017;72:1206-1216

Presented by: Dr S. Churchill

Background

CPEX testing is widely available in preassessment and guiding decision making. Those who have higher cardiorespiratory fitness may have more resilience to metabolic demands of surgical stress. Cardiorespiratory fitness is indicated by peak oxygen uptake and oxygen uptake at anaerobic threshold.

Using CPEX measures peak oxygen uptake and ventilatory anaerobic threshold, however assessing only these measures leads to loss of data. This paper investigates whether we could use submaximal data and extract relevant clinical outcomes. In particular, the paper looks to see if oxygen uptake efficiency slope can be used. The OUES is based on a curvilinear relationship between minute ventilation and oxygen uptake throughout an incremental CPEX test. OUES is thought to be a predictive marker for major cardiac events.

 Methods

This study was conducted in patients over 60 years of age who qualified and who were due to undergo colorectal surgery. Patients had completed a veteran specific activity questionnaire and those who scored <7 METS were recruited.

Those patients who required emergency surgery or who could not complete a CPEX test were excluded. Those patients who consented to the study underwent CPEX testing on an upright bike. The test was considered maximal when patient showed signs of fatigue or when they either reached a HR of>95% predicted or a RER of >1.10.

Data was analysed independently from clinicians performing test. Values at peak exercise were calculated as an average over the last 30s before termination of the test and peak heart rate was that achieved highest during the whole test. A steeper slope represents a higher oxygen uptake efficiency slope and therefore more efficient oxygen uptake (smaller ventilation is required for a given oxygen uptake).

All data collected was used not just 80-90% of the duration of the test which is what occurred in previous studies. Data was adjusted for sex, age, body surface area and body mass and all data was analysed using SPSS.

 Results

  • The study was conducted between Feb 2013 and July 2016.
  • 72 subjects were recruited and they had their CPEX test 26.8 (mean) before their surgery. One further patient was excluded.
  • There was a subgroup analysis between the Anaerobic threshold of >11ml/kg/min and < 11ml/kg/min.
  • OUES showed a statistical difference between OUES100 and OUES80 and OUES90.
  • The data appears more accurate with more data included in analysis.
  • There was a statistically significant correlation between OUES and AT and peak oxygen uptake.

 Limitations of Study

  • This was a single centre study with a small sample size. The data was analysed retrospectively which could lead to bias. There was no sample size pre determined.
  • The study already highlights those who have low METS. Why not a cross spectrum to see if any false positives/negatives?I wonder whether there may be some user error with VSAQ. This questionnaire is  dependent on patients having accurate perception of their own fitness.
  • On CPEX the assessment of maximal testing usses subjective signs of maximal effort, only 76.1% showed the objective signs of maximal effort.
  • Further corrections needed to make some variables statistically significant (Bonferroni)
  • Given that you need all the data to get accurate readings, then maybe the extrapolations wont be so accurate if test you get is submaximal.
  • The results are dependent on the patient having no significant lung disease. This is no relatable to everyday clinical practice.
  • OUES was adapted for body mass….oxygen uptake was not mentioned as being adapted for body mass.
  • At which point do we stop looking at oxygen uptake – up to the plateau or up to point of constant levelling off.
  • Levels associated with risk pre determined in previous studies.
  • No ackowledgement of actual clinical complication in the population deemed to be high risk.

Discussion

  • There are good correlations between OUES and peak oxygen uptake and AT mean it could give us an objective way of assessing cardiorespiratory fitness even in a submaximal exercise test. Data could be extrapolated and AT and peak oxygen uptake could be predicted and could facilitate decision making in high risk patients.
  • Some studies find no difference between maximal and submaximal studies. This one shows that your figures are lower in those in which all data is collected
  • Two patients who had big differences in their OUES100,90 and 80 were excluded and then the data showed no statistical difference between submaximal and maximal tests.
  • There was a strong correlation with peak oxygen uptake and anaerobic threshold is in keeping with other studies.
  • The risk of post-operative morbidity was still correlated to data surrounding AT (<11.1 being high risk). Peak oxygen uptake is still dependent on fatigue and patient effort and therefore results may be submaximal indicator of fitness.