Clinical exercise testing

Chapter 29


Clinical exercise testing



Exercise testing has been used routinely for the past 50 years in human medicine to evaluate fitness and the significance of a range of diseases on exercise capacity. Testing usually has been performed in laboratories equipped to perform cardiovascular and respiratory measurements by using either bicycle ergometers or treadmills to vary the intensity of exercise. More and more information from field tests is available to athletes, with well-documented cases from cycling and triathlon, where athletes are able to determine work output by using a combination of heart rate (HR) meters, speed, and inputs of other variables.


Exercise testing to evaluate the physiologic responses of athletic horses to exercise has followed that in humans. The Swedes were the first to describe standardized protocols for investigation of exercise capacity, with research performed both on the track and using a treadmill, in Swedish Standardbred Trotters. This research began in the 1960s and became more routine in the 1970s and established normal responses of HR, oxygen uptake, blood lactate, and total red blood cell volume in this breed. However, in the exercise tests performed on the treadmill, horses were not exercised at maximal exercise intensities. In other studies using track testing, horses with lower airway disease were studied by other groups in Europe.


During the 1980s and 1990s, a dramatic increase occurred in the number of studies investigating cardiorespiratory and metabolic findings in athletic horses by using exercise tests undertaken either on the track or treadmill. Such studies provided important information on expected normal physiologic responses to exercise and data on the effects of some diseases. However, measurements regarding the physiologic responses of elite athletic horses to exercise are still few. The majority of studies have been performed using experimental horses of moderate to poor athletic ability. More recently field testing of racehorses has become more common and as such the depth of available information has increased. Despite these limitations, exercise testing has reached a point where important conclusions can now be drawn. This chapter will review material on exercise testing in athletic horses. It is a primer and in no way attempts to provide specifics on all specific methodologies available for exercise testing in health and disease.



Indications for exercise testing


Exercise testing provides a mechanism for evaluating a range of body systems under standard exercise conditions. Measurements of cardiorespiratory and metabolic functions during an exercise test provide information about the capacity and efficiency of key body systems involved in energy production. Thus, some conclusions may be drawn about the athletic potential, or lack thereof, of the horse based on measurements of oxygen transport or estimates of anaerobic capacity, depending on the duration and intensity of the competitive event. Additionally, changes in levels of fitness may be evaluated by using exercise testing because resting measurements of hematology or biochemistry provide little or no indication of improvements in fitness. Exercise testing is probably of most use from a clinical point of view to assess the effect on performance of abnormalities found on physical examination or to determine the reason(s) for reduced athletic capacity in horses that have no abnormalities on resting examinations. Whatever the reason for the testing, one important premise is that standardized procedures are followed so that the data derived from each test can be compared against subsequent tests for the same horse or with measurements from other horses of similar age and fitness level.



Track versus treadmill exercise tests


Measurements from horses at the track are obviously much simpler and can be performed more readily without access to sophisticated equipment than investigations using treadmills. Track testing is not only more easily performed than treadmill testing but also has the advantage of being undertaken in an environment similar to that in which the horse has to perform. Originally, it was considered that these advantages were outweighed by the disadvantages of track testing, which included the relatively limited range of measurements that can be performed, variations in track and environmental conditions, and the influence of the rider or driver. This has changed in recent years with the advent of sophisticated telemetric HR meters, global positioning system (GPS) to track speed and changes in elevation, horse-side lactate analyzers, more durable and accurate portable respiratory masks, and dynamic, horse mounted video-endoscopy allowing real-time appreciation of upper respiratory function during intense exercise.


Measurement of lactate in blood may be performed on either whole blood or plasma. Plasma values will be about one third higher than whole-blood values, although the relationship between plasma and blood lactate is variable from horse to horse. If samples are not to be analyzed immediately, it may be best to collect blood into tubes containing fluoride or oxalate as an anticoagulant so that glycolysis is inhibited and lactate values do not continue to increase after collection. However, it has been found that provided the blood samples are kept refrigerated and the samples are analyzed within 48 hours of collection, sodium or lithium heparin is a suitable anticoagulant.


One of the areas where great advances have occurred in the past decade is related to pulmonary function testing (Evans, 2007). Tests of lung function are particularly important in racehorses because minor compromise of oxygen transport may have profound effects on performance. A great advance now available to the practicing veterinarian is dynamic video-endoscopy (DVE), which is now more commonly used for assessment of possible upper respiratory limits to performance. With the development of these systems, exercise horses may be examined at training sites under more normal conditions. In general, DVE systems consist of a semi-rigid yet malleable insertion tube (approximately10 mm diameter) with light-emitting diodes (LEDs) in the tip (lower power needs). The tube is attached to a purpose-made bridle. This fits over standard tack. Key electronic components are housed in a permanent virtual circuit box. This interacts with a remote receiver or video display allowing real-time visualization of the upper airways. The electronics and lavage system are stored in a backpack, on the sulky or saddle blanket. Newer versions have remote control to allow adjustment of the head of the scope during exercise such that the best image is maintained. Digital images are readily downloaded and transmitted for review.


Breath-by-breath measurement of pulmonary ventilation with a suitable spirometer is the ideal method of assessing the likely clinical impact of a respiratory problem on athletic performance or for assessment of response to treatment. Likewise, tests of function in resting horses are unlikely to be helpful unless the horse has a severe respiratory problem such as recurrent airway obstruction. Clearly, the future of exercise testing will be based around accurate and sensitive pulmonary function tests in horses when exercising under field conditions emulating those occurring during racing. Studies of the maximal breathing capacity (the product of maximal tidal volume and respiratory frequency during maximal exercise) and the ratio of pulmonary ventilation to oxygen consumption during exercise (ventilatory equivalent for oxygen) are likely to have potential as tests of lower airway function in horses. Studies of pulmonary ventilation and flow volume loops during the recovery period after maximal exercise may also have potential diagnostic use. A byproduct of development of these lightweight masks is the accurate measurement of maximal oxygen consumption under field conditions.


Thus, the gap between what can be determined in the field versus the treadmill has narrowed substantially in the past two decades. One advantage of treadmill testing, however, is that if the machine is situated in a room, climate control is possible, as are standardized conditions for testing and the opportunity to perform a range of measurements during exercise as well as before and after exercise. Previously, it was considered that these advantages outweighed the disadvantages of the artificial nature of the treadmill environment and the fact that energy expenditure during exercise on the treadmill is quantitatively different from that during exercise on the track. Proponents suggested that treadmill exercise testing allowed more precise identification of disturbances to particular body systems if a wide range of measurements of the function of key body systems is undertaken. As mentioned above, with the advent of more accurate, lightweight technologies, a huge amount of useful information can be garnered from horses exercising in the field. This, of course, has huge advantages, as the investigator can go to the horse as opposed to the horse being shipped to a laboratory. Also, the horse can be exercised in its normal environment under ambient conditions similar to those in which it is expected to perform. Both field testing and treadmill testing have a place in the assessment of performance in horses.



Track exercise testing


Track exercise tests involve various measurements undertaken during or after a standardized bout of exercise. The simplest track test is assessment of exercise capacity by timing the horse over the competition distance. A fast track time and good recovery are good evidence that the horse is fit for a particular race or event distance. This approach has the advantage of simplicity, the only piece of equipment required being a means to accurately determine exercise duration the simplest being a stop watch. However, in most cases where exercise testing is considered, more information is usually required.


One of the key issues in track exercise testing is the measurement of speed. For ridden horses, linear and temporal markers need to be located on the track for the rider to regulate the velocity and cadence of the horse. Velocity is then measured with a stopwatch. Also, velocity may be measured by the use of a GPS carried by the rider. In driven horses such as Standardbred Trotters or Pacers, velocity may be measured by using a tachometer placed on the wheel of the sulky connected to the meter or the GPS. HR response to exercise is an important indication of metabolic capacity. It may be easily measured and registered by means of two electrodes placed on the horse and connected to a heart rate monitor. HR response to graded exercise is linear, between 120 and 210 beats per minute (beats/min−1), as shown in Figure 29-1. Many factors such as exogenous factors (e.g., geometry and length of the track, environmental conditions), stage of training, and disease may influence the regression line of HR on work speed. However, the regression of HR on speed is very precise and reproducible when measured during a standardized exercise bout.



Blood lactate concentration may be measured by taking blood samples at the end of the exercise period, from the jugular vein into tubes containing fluoride-oxalate. Lactate is a product of muscular metabolism and accumulates in muscle and blood at high intensities of exercise. The concept of anaerobic threshold, extrapolated from plotted curve of blood lactate concentration against speed has been defined as the level of work just below that at which metabolic acidosis occurs. The aerobic–anaerobic transition or onset of blood lactate accumulation (OBLA) has been defined empirically as 4 millimoles per liter (mmol/L−1) blood lactate concentration. At this level of activity, the initial increase in lactate production is followed by a steady state in which lactate utilization and lactate production are equal. At higher levels of activity, lactate production exceeds its utilization, and so it accumulates in blood (Figure 29-2).




Overview of testing procedures


Numerous and varied testing procedures (Table 29-1) have been described for horses involved in different disciplines such as 3-day eventing, endurance, show jumping or racing (flat and Standardbred). Whatever the horse’s discipline, field exercise test protocols should always be rigidly defined to calculate meaningful fitness measurements and to limit variability. Following standardized procedures is of great importance as the data derived from each test can be compared with those from subsequent tests for the same horse or with measurements from other horses of similar age and training status. Results may vary according to the methodology used and with factors such as number and duration of steps, resting time between steps, and increment from one step to the next.



Therefore, the following points are important to consider when designing a possible testing procedure:




Standardized track-based exercise test for standardbred trotters


The experience of one of the chapter authors, A. Couroucé-Malblanc, relates mainly to the testing of Standardbred Trotters in Europe. Given this, we provide an example herein widely used by this author.




Calculation of indices of exercise capacity


From the measured variables HR, V (speed, or velocity), and blood lactate concentration, derived variables may be calculated to permit simple comparison of test results. Examples are described below.



Velocity and blood lactate concentration


For comparison of blood lactate values between horses or in the same horse during training, the velocity at a blood lactate concentration of 4 mmol/L−1 (V4) generally has been used. V4 is considered a reference value for horses, as it is considered by many to be a good predictor of aerobic capacity. A high value for V4 (see Figure 29-2) is an indication of superior exercise capacity and is related to racing performance. In the 1990s, Evans et al. (2007) studied the relationship between the blood lactate response to exercise and performance in Thoroughbreds during a submaximal exercise test on a 5% inclined treadmill. These authors showed that the blood lactate concentration 2 to 5 minutes after exercise was correlated to racing performances as assessed by Timeform rating (r = −0.68 ; p<0.01). In another study undertaken in Sweden, it was shown that in a small group of Trotters, those first two to begin their racing careers were the ones that had the lowest lactate concentration after submaximal tests. In another study, Casini and Greppi (1996), studying 20 Trotters completing a field exercise test were able to compare 10 good performers and 10 poor performers, on the basis of best time. These authors found significantly higher V4 values in the first group and a negative correlation between V4 values and best time (0.61). Couroucé et al. (1997) also showed that 96% of horses with low V4 values according to their age were poor racing performers. Finally, Davie et al. (2002) studied the relationships between V4 values and earnings in a population of 16 pacers. A significant correlation was found between V4 values and log earnings and log earnings or start. Overall, the higher the V4, the fitter is the horse and the greater is its exercise capacity.



Velocity and heart rate


A useful reference point for comparison of cardiovascular capacity in Standardbred horses, is the V200 (see Figure 29-1), which represents the velocity at an HR of 200 beats/min−1. According to many, even though individual variations may be found, at an HR of 200 beats/min−1, most Standardbreds are close to the point of onset of blood lactate accumulation (OBLA, blood lactate concentration of 4 mmol/L−1). A retrospective study carried out with 194 French Trotters that performed 1105 field standardized exercise tests on a sand training track, permitted calculation of V4 and V200 mean values (± standard deviation [SD]) according to age groups (Table 29-3). It was also shown that in most cases, the workload carried out at V200 is close to V4. For 1-, 3-, 4-, and 5 year-olds, no significant difference was seen between mean V4 and V200 values. In contrast, for 2- and 6-year-olds, a significant difference was seen between mean V4 and V200 values, V200 being lower for 2-year-olds and higher for 8-year-olds and those older.



For saddle horses, the velocity for an HR of 170 beats/min−1 has been suggested to be more appropriate, as it is often difficult for these horses to reach an HR of 200 beats/min−1.




Reproducibility


On the track, a number of the conditions may vary, for example, track quality and temperature, humidity, or other weather related factors. However, standardized field exercise tests should represent reference data for trainers in the evaluation of the fitness level of their horses and in evaluating their response to training. For this standardized field exercise test for French Trotters, we have found that results are reproducible, at least on the same track. Under standardized conditions, we have found that no significant differences exist between V4, HR4, and V200 measurements from one test to the other.



Interpretation of v4 and v200


It is particularly important to be able to compare data obtained from one test with measurements from other horses of similar age and level of training. In this author’s experience with French Trotters, V4 is the most important measurement to assess the fitness level because of the relationship between this variable and racing performance. In fact, it seems that a horse with a low V4 measurement according to its age and state of training is likely to be a poor performer because of a low aerobic capacity.



V4, V200, and track testing: The track is an important variable to consider, as it may influence the calculation of V4 and V200. In a previous study, five French Trotters performed standardized exercise tests at two different tracks (a 1250-m sand race track and a 720-m sand training track) and on an uninclined treadmill during the same week to determine the influence of exercise surface on different measured variables such as V4 and V200 (Couroucé et al., 1999). No significant differences were found for the physiologic variables between the two tracks. In contrast, significant differences for these variables were observed between those occurring on the two tracks versus the those on testing on the treadmill, with horses showing lower HR and blood lactate responses on an uninclined treadmill (Table 29-4).


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Related posts:

  1. Training the racing quarterhorse
  2. Hematology and biochemistry
  3. Training standardbred trotters and pacers
  4. Energetic considerations of exercise

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Clinical exercise testing

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