Physiologic Reserve

Assessment of Physical Fitness and Functional Reserve

The World Health Organization (2011) declared: “there is strong scientific evidence that regular physical activity produces major and extensive health benefits particularly in older adults… In addition, strong evidence indicates that being physically active is associated with higher levels of functional health, a lower risk of falling, and better cognitive function.”

The literature is unequivocal regarding the relationship between functional capacity, or fitness, and general outcomes (Bachmann et al. 2015). Both in medical and surgical contexts, patients with lower functional capacity have a higher incidence of morbidity or mortality. Functional reserve assessment can serve to evaluate perioperative risks and influence the choice of surgical intervention. Knowing baseline functionality can help to set postoperative recovery goals, but also to orient preoperative optimization (Glance et al. 2014).

Series of tests are available to measure the physical capacity to perform normal everyday activities. We can define functional fitness as being able to perform activities of daily living safely and independently.

The evaluation of functional reserve can be obtained with performance-based and self-reported measures (Table 5.1). In deciding how to choose a measure for the relevant outcome, one needs to describe the construct to be measured, identify the measurement purpose, define the client population, plan and conduct a pilot test, and evaluate the collection and use of outcome data.

Performance measures

Performance measures could be called viewed performance because it is measured by a third party. These measures test the individual’s actual performance on an activity in a given environment at a specific time. The use of equipment to measure the level of performance provides more accurate data than self-reported measures. However, these measures have sources of inaccuracy, due to equipment, operator, test situation, individual fatigue, effort and time of day. The outcome measures here listed have been chosen as they are relevant to surgery.

Cardiopulmonary Exercise Test

The gold standard non-invasive assessment of physiological performance is the cardiopulmonary exercise test (CPET). It provides information on the integrated cardiopulmonary and musculoskeletal function (Balady et al. 2010) during an 8- to 12-minute exercise gradually increasing in intensity. It can be performed on a treadmill or a cyclo-ergometer. CPET has been used to measure exercise capacity and response to therapy in cardiac failure patients, to evaluate patients with exercise intolerance and to determine exercise training intensity in the context of cardiac rehabilitation and preoperatively (Albouaini et al. 2007). It is also used for perioperative risk assessments. It allows for the breath-by-breath measurement of respiratory, cardiovascular and exercise capacity, as well as pulmonary and metabolic gas exchange values. CPET via an incremental exercise protocol is the tool used to determine the following measures of fitness.

The single most precise value indicative of fitness obtainable from a CPET is the maximal oxygen uptake (VO_2max). It is the point at which, in an incremental exercise, the oxygen consumption reaches a plateau, despite an increasing work load. This point is extremely difficult to achieve, especially for the elderly, as it requires a high degree of motivation and favorable patient characteristics such as a healthy musculoskeletal system (Levett and Grocott, 2015). Peak oxygen consumption (VO_2peak) has commonly been used instead of VO_2max, and is the highest oxygen uptake rate recorded during the test, regardless of whether the plateau is attained. Similarly to the VO_2max, this measure can also be affected by patient motivation, environment, time of day and exercise protocol. Near-maximal exercise tests have been studied in the surgical context and it was determined that a peak oxygen uptake below 15 ml/kg/min (equivalent of 4 metabolic equivalents (METS)) is an indicator of poor fitness and a predictor for higher perioperative morbidity and mortality risks (Beckles et al. 2003, McCullough et al. 2006).

Submaximal clinically significant measurements have been investigated as they are more objective and less variable. The oxygen uptake at the ventilatory anaerobic threshold (VAT) is one of them. It is the point at which, in the ramped up effort, supplied oxygen becomes exceeded by metabolic needs, leading to anaerobic metabolism and lactate accumulation. It is determined on the ventilation and gas exchange plots where the rate of CO₂ production (VCO₂) per O₂ consumed (VO₂) increases, noticeable by a change in slope on a VCO₂ to VO₂ graph (Figure 5.1, V-slope technique).

Figure 5.1 VAT determination using the V-slope technique. The vertical black line marks the VAT, located at the point of change in slope of VCO2 to VO2.

Image obtained with the Vmax ® software by CareFusion, San Diego, CA.

It can also be determined on a graph of ventilatory equivalent ratios for oxygen and carbon dioxide (VE/VCO₂ and VE/VO₂) at the point where they diverge due to ventilation increasing disproportionately to oxygen consumption (Figure 5.2, ventilatory equivalent ratios). Being a submaximal test, it is less dependent on patient motivation but can be affected by the protocol used (initial workload and increment) and the nutritional state (carbohydrate-fed versus fat or amino-acid oxidation). VAT remains unidentifiable in some patients and in up to 25% of patients with heart failure, and suffers from high inter-observer variability (Hollenberg and Tager 2000). Low exercise capacity, defined as an oxygen uptake at VAT below 10.1 ml/kg/min has been demonstrated to predict postoperative morbidity and longer hospital length of stay (Snowden et al. 2010).

Figure 5.2 VAT determination using the respiratory equivalent ratio. The vertical black line marks the VAT, located at the point where the ventilatory equivalent ratio for oxygen (aqua diamond) diverges from the ventilatory equivalent ratio for carbon dioxide (red squares). At this point, end-tidal CO₂ (purple triangle) decreases and end-tidal O₂ (blue inversed triangle) increases.

Image obtained with the Vmax ® software by CareFusion, San Diego, CA.

The oxygen uptake efficiency slope (OUES) is another submaximal measurement of fitness. It represents the slope of the linear increase in VO₂ in relation to the logarithmic minute ventilation (VE) increase during an incremental exercise (Baba et al. 1996). The higher the oxygen uptake for a given VE, the more efficient the system. It has been studied in healthy subjects and in patients with comorbidities aged from 6 to 96 years old. The OUES has not been studied as extensively as the other CPET measurements in the surgical context. However, the small amount of literature available on using the oxygen uptake efficiency slope for risk stratification is promising. Kasikcioglu et al. (2009) observed that the preoperative OUES was, on average, significantly lower in patients who experienced complications following lung transplant than in those who did not, a finding confirmed by VO_2peak. There are equations to predict OUES based on age, body surface area, smoking status and FEV₁.

Walk Tests

Expanding on the idea of evaluating functional walking capacity, the 2- and 6-minute walk tests, TUG and stair climbing test have also been used as outcome measures. Performance on these tests depends on the ability to move the limbs.

The 2- and 6-minute walk test (2MWT, 6MWT) assess the distance covered by the patient on a flat surface over a set amount of time at their own pace, with a walking aide if needed. Subjects are instructed to walk back and forth, in a 20m stretch of hallway, for 6 min, at a pace that would make them tired by the end of the walk; encouragement and feedback are given according to published guidelines (Butland et al. 1982). They are allowed to rest during the test if needed, although this time is included in the 6 min. Reference equations are available for calculating the percentage of age- and sex-specific norms: predicted distance (m) = 868 − (age × 2.9) − (sex × 74.7) where age is in years and the value “1” is assigned for female and “0” for male. A recent paper has supported the validity of the 6MWT as a measure of surgical recovery. Initially developed for patients with chronic obstructive pulmonary disease, the 6MWT evaluates the ability of an individual to maintain a moderate level of walking for a period of time, reflecting activities of daily living (Enright and Sherrill 1998).

The test–retest reliability has been reported to range from 0.73 to 0.99 among a variety of populations, including the elderly. In community-dwelling elderly persons, measurement error was estimated at 20 m and this was used as the threshold value for determining meaningful change (Antonescu et al. 2014). General population values can be obtained through gender-specific reference equations and normative tables. The 2MWT is an indicator of muscle force. It correlates well with maximum oxygen consumption (VO_2max). General population values are extrapolated from those obtained for the 6MWT.

The 6MWT has been used as a tool to assess performance in patients that undergo total knee arthroplasty. Not only can it show exercise ability mainly with the lower limbs, but it can also accurately reflect change over time. The 6MWT has been used in the midterm follow-up at six weeks, while the 2MWT has been used in the acute postoperative phase (first three days after surgery). There is a significant decrease in the 2MWT in the immediate postoperative period, compared to preoperative values. The 6MWT has been validated in many medical and surgical contexts and found to be accurate (Moriello et al. 2008, Pecorelli et al. 2016).

These two walk tests correlated positively with CHAMPS, a self-reported indicator of weekly energy expenditure for physical activities. In this way, unlike range of motion measurement, it can link functional assessment with patient-reported measures.

Maximum Voluntary Isometric Contraction

Maximum voluntary isometric contraction is measured using an isometric force electromechanical dynamometer. The use of a dynamometer allows a more accurate and standardized means of measuring muscle strength and has been previously described and validated in the clinical setting. Charous et al. (2011) assessed quadriceps femoris and hip adductor muscles motor block in volunteers while comparing local anesthetic delivery by bolus versus infusion. They found no difference in muscle contraction with either method of delivery. Mizner et al. (2011) measured quadriceps strength, by assessing maximal isometric force in patients scheduled for elective total knee arthroplasty, and found this measure correlated well with values obtained from other tests such as the stair climbing test, TUG and 6MWT.

Timed Up-and-Go (TUG)

TUG is a simple measure used to assess lower body strength, power and overall mobility (Bennell et al. 2011). It measures the time to rise from sitting in a chair, walk 3 meters, turn around, walk back and sit. It involves mobility, strength and agility. Reference values by age and characteristics have been established (Bohannon 2006). This test is easy to perform and to interpret, and requires simple equipment. However, it has been described to have a floor and ceiling effect in some subgroups, especially in comparison to tests such as the stair climb test and 6MWT. In the context of a clinical pathway for knee arthroplasty, TUG has been shown to deteriorate during the first 48 hours, with an inverse correlation with pain intensity. This tool is an indicator of recovery, as body composition data shows that the lower extremities are affected the most by lean body mass changes after surgery. This tool can also be used as a means for assigning surgical risk to elderly patients. Patients identified as slower (indicative of frailty) have an increased risk of postoperative complications, longer hospital stay and are more likely to suffer disability. Participants are classified according to their surgical risk: fast (completed test in 10 s or less), intermediate (completed test in 11–14 s) and slow (greater than or equal to 15 s to complete test). This outcome measure is evaluated as a change in TUG pre- and postoperatively with a clinically meaningful change in the preoperative period considered to be a change of four seconds (the minimal detectable clinical difference in an elderly population based on measurement error), or improvement in classification. Postoperatively, a return to baseline classification is considered to be recovered.

Sit-to-Stand (STS) Test

The STS test is a measurement to assess lower-body strength. The test is administered by having the individual sit on a chair and attempt to stand as many times as possible in a maximum of 30 seconds. The STS test requires minimal instrumentation: the necessary equipment includes the use of a stopwatch and a standard armless chair. The back of the chair should be placed against a wall to maintain stability and prevent slipping. Patients are encouraged to have their arms crossed on their chest during the test to avoid the use of the upper limbs. They are instructed to rise from the chair to a full stand on the signal “go” and then return to a fully seated position. The individual is allowed to have at most two practice trials and a demonstration should be given. The demonstration should be done slowly at first to show proper form and another demonstration should be given at a faster pace to show that the objective of the test is to do as many sit to stands as possible in the given amount of time.

Cumulated Ambulation Score

The Cumulated Ambulation Score assesses basic mobility. It rates how independent patients are in performing three tasks: getting in and out of bed, sitting down and getting up from a chair, and the ability to walk, with the correct walking aid if needed. The score for each activity is added up. It can be measured on each postoperative day, giving a cumulative value. Aside from reflecting patients’ ability to move independently, in hip fractures it has been shown to predict the length of hospital stay, time to discharge, 30-day mortality and postoperative medical complications (Foss et al. 2006).

Self-Reported Outcomes

Besides assessments of interventions impacting on patients’ physiological functions, it is necessary to determine what is the effect on the patient’s perception of what such an intervention will have on his/her quality of life. Therefore, self-reported outcomes have been incorporated into studies. Considering that the concept of health is much broader than just physical capability, these outcomes allow information to be collected on the patients’ independence, overall wellbeing and psycho-social functionality. Self-reported measures are used mainly in healthcare facilities under supervision, in person or by telephone. Self-reporting is useful to measure constructs that cannot be observed by performance (e.g. pain, energy level). Although these measures require few resources, there is a high likelihood of a low response rate and missed items. Interviewer-administered measures result in a high response rate but cost more to run.