Following my post last week on lessons learned on genetic testing of thoroughbreds, it was suggested by a well known equine exercise physiologist that I was "down" on cardio's based on the tone of what I had written. Nothing could be further from the truth so I think I should take the time to explain both what a good Thoroughbred cardio looks like, and why there are limitations to cardiovascular testing alone.
Firstly, what does a good cardio look like.
Well, they say a picture is a thousand words so I have below the m-mode images of three horses. The first image is of the horse Sunofwolf. I actually misspelt his name when I was imaging him but you can take a look at his race record by clicking here. For those that don't know Australian form, he's a maiden in four starts and can't win a race at a country racetrack so he is pretty useless (edit 3/13/13 - Sunofwolf has won 1 race at a country track in Australia in 5 starts) . Below that are images for Lightinthenite and Take Charge Indy. Lightinthenite is a Group 3 winning gelding than has earned just over A$400,000 for his owners Champion Thoroughbreds. Take Charge Indy is a Grade One winner on the Florida Derby who retired this year to stand at WinStar Farm.
M-Mode cardiovascular dimensions of a thoroughbred racehorse
So what are we looking for? Well without giving away the house, the images are m-mode intersects of the heart. Effectively you are looking at the heart as if it were sliced in half and you were looking at it from the top down. The ridges that you see are the heart beats of the horse with the heart contracting at the point where the two ridges get closer, and widening right after that. The grey areas on the top (below the white lines) and bottom (above the white lines) are the cardiac walls.
So what are the differences that separate elite horses from non-elite? Well right after the heart contracts we take a measurement of the Internal Diameter of the heart. If you look you can see that Sunofwolf's internal diameter measures 8.98cm while Lightinthenite's diameter is 13.1cm. The image I have of Take Charge Indy hasn't got the measurement on it but it is 12.95cm. The bigger this measurement the better (within reason). You will also notice that the walls (the grey part above the line and below it) of Take Charge Indy and Lightinthenite are both relatively even, quite clear, and not too thick or thin while you will see with Sunofwolf that his cardiac walls are uneven, rough edged and one (the bottom wall) is particularly thick.
There are two other raw cardiac measurements that we take, which develop a number of cardiac calculations, but the long and the short of it is that there are only a couple of measurements that are related to performance in the Thoroughbred. Back in 2002 Young et al, found that heart size estimated by echocardiography correlates with maximal oxygen uptake. They found that LVIDd (r = 0.57; P = 0.01), which is the measurement that you see on the Sunofwolf and Lightinthenite images, Mean Wall Thickness (r = 0.44; P = 0.07), Left Ventricular Mass (r = 0.78; P = 0.0002) and Left Ventricular short-axis area (r = 0.69; P = 0.003) were correlated to maximal oxygen uptake. A couple of years after that Young and colleagues published another paper where they found that Left ventricular size and systolic function in Thoroughbred racehorses was related to race performance. This paper found that left ventricular ejection fraction and left ventricular mass combined were positively associated with race rating in older flat racehorses running over sprint (<1,408 m) and longer distances (>1,408 m), explaining 25–35% of overall variation in performance
The last part of that last sentence is where the trick to cardio's lie. On our data, and on the study that Young and colleagues did above, cardio's only explain about 25% of the variation in performance, leaving 75% explained by other factors (genetics, biomechanics, etc). You cannot rely on cardio's alone as they don't explain enough of the picture. That said when you have what are "bullseye" cardio's like you see with Lightinthenite and Take Charge Indy, they stand out. If any lay person took to an ultrasound and went to the November Sales and tested a lot of off the track fillies you would see a "bullseye" from an average cardio, they are that easy to pick. But....and this is where it gets tricky, just because these measures are correlated to performance, doesn't mean that they explain the variance (R2) of performance well. Cardio's are less predictive in sprinters and less predictive in fillies, which is why they only explain 25% of the picture in a population of horses.
A good sized and strong cardio is a good start for any racehorse but as another noted exercise physiologist once said to me "where is the speed"? The cardio is responsible for the oxygenated blood to get to the muscle so the more effective the pump, the better the blood flow to the muscle, but once it gets there it is the responsibility of the muscle as to how well it is used. That is where genetics comes into play - how well does the muscle use the blood? This is why you also see a lot of horses with good cardio's that are slow, they don't have the rest of the parts working for them, they have no speed to start with. The cardio doesn't make them fast, muscle does, which is why genetic markers of exercise relevant genes are important.
To clarify, I am not "down" on cardiovascular evaluation as a tool for performance evaluation. It is a very useful tool, but it needs to be augmented with other data points like genetic markers otherwise you get in to trouble with prediction, especially with sprinters where a slightly above average cardio is still good enough to compete at the highest level, especially if it is coupled with biomechanical efficiency.