The heart - general

What is stroke volume

The volume of blood ejected from each cardiac ventricle with each contraction

How much blood is ejected per cardiac contraction in a trained horse?

Over/about 1 liter of blood is ejected per cardiac contraction

What happens to blood flow with each contraction of the left ventricle?

One stroke volume of blood is pumped into the aorta

What does the pumping of blood into the aorta cause?

A wave of accelerated blood flow through the systemic arteries.

What is cardiac output

output of blood from the heart = HR x Stroke volume

What happens to cardiac output during dehydration?

A decrease in stroke volume so a decrease in cardiac output, so horse must increase HR to maintain CO

Why is blood flow through arteries continuous?

Elastic recoil of the arterial walls compresses the blood vessels, keeping blood flowing forward from the heart toward the tissues.

What does blood pressure depend on?

Cardiac output, arterial resistance, venous blood flow, changes in air pressure in thorax

What is arterial resistance?

arterioles are the main regulators of resistance so they can constrict or dilate (muscle in arteriolar walls)

What does venous blood flow depend on?

external muscular action compressing thin-walled veins

How does changes in air pressure in thorax affect venous return?

because inhalation causes negative pressure which leads to faster flow of blood into the heart

What is the heart mass?

~1% of body weight (4 - 5 kg in Thoroughbred). A trained horse may have a slightly increased heart mass.

How does heart mass vary between horse breeds?

Heart mass-to-body mass ratio is a function of breed. Thoroughbreds have a greater relative heart mass than Arabians or draft horses.

What is cardiac cycle

sequence of events that occurs during contraction (systole) and relaxation (diastole)

Heart rate

1 heart beat = 1 palpable pulse. Count the pulses for 15 seconds and multiply by 4.

what is the resting HR?

25-40 bpm

what is the trotting HR?

70-120 bpm

what is the cantering HR?

120-150 bpm

What is the maximum velocity HR?

220-240 bpm

What can HR raise to during sudden excitement, fear, or exercise or food anticipation?

rapid rise to 100 bpm

What happens to the HR at start of exercise?

Reaches a steady state in 2-3 min and stays constant during submaximal work

What are recovery heart rates used to assess in horses?

Recovery heart rates are used to assess fitness in horses competing in endurance rides, eventing, etc.

What do recovery heart rates indicate 30 minutes after exercise?

Horses with HR less than 60 bpm at 30 min after exercise have less dehydration and myopathy. Horse with a HR over 65-70 bpm at 30 min recovery time during midpoint of an endurance ride are more likely to develop severe dehydration and exhaustion if allowed to continue.

What is poor performance and exercise intolerance due to?

usually respiratory and/or musculoskeletal disorders