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Your pets pulse is governed by the the rate and strength with which its heart is beating. The special muscle (cardiac muscle) that makes up your pet’s heart doesn’t contract or beat the way ordinary muscle does. Instead, a system of nerves extend through the walls of its four chambers firing timed electrical impulses that originate from a group of specialized cells in in the upper right corner of your pet’s heart (the sino-atrial or SA node).
A second node, the atrioventricular or AV node conveys the electrical impulse to the two, more muscular, lower heart chambers (ventricles). The time during which you pet’s heart is contracting is called systole. The time it is relaxed is diastole.
The SA node receives messenger chemicals and nerve impulses that govern the rate at which the heart beats; but the heart will continue to beat (irregularly) without them. Those impulses come via the autonomic nervous system and they are of two kinds: sympathetic impulses, which increase heart rate and parasympathetic impulses, coming by way of the pet's vagus nerve, which decrease heart rate.
Whenever the sympathetic nerves that connect your pet’s heart to its spinal cord predominate in activity over the vagus nerve, the pet’s heart beat and cardiac function increase. The chemical that is released to cause this change in the SA node’s firing rate is norepinephrine (noradrenaline). Calcium plays an important role in this process as well.
All causes of sympathetic nerve stimulation will increase your pet’s heart rate. That stimulation could be excitement. It could be fear, apprehension or simply physical activity. The more strenuous the exercise the faster your pet's heart rate and pulse are likely to be. Your pet's heart rate will also increase when its body is starved of oxygen due to failing heart or lungs or a circulatory system obstruction. It will also increase when exhaustion depletes your pet's muscle glycogen stores.
Dehydration can also increases your pet’s heart rate due to a lack of sufficient blood volume to keep the body well oxygenated. Significant anemia or the sluggish circulation of congestive heart failure (CHF) will have the same effect.
It is normal for sympathetic nerve stimulation to slightly increases heart and pulse rate during inspiration and to decreases it slightly during expiration due to changes in vagal nerve tone.
Increased blood pressure in the left upper chamber (the left atrium) of your pet's heart also increases heart
rate (the Bainbridge reflex).
Medications can increase your pet’s heart rate too. Drugs that have that effect include Allerest antihistamine (chlorphenamine) , caffeine and decongestant medications that contain compounds like phenylephrine, phenylpropanolamine or pseudoephedrine. Chocolate toxicity in dogs increases their heart and pulse rate as well.
Atropine eye drops, given in excess, can cause your dog or cat's heart rate and pulse to go up or down. Anti-nausea medications that contain scopolamine (hyoscine) can have the same double effect. Theophylline, given to ease difficult respiration, can also increases heart rate.
The chemical that is released that does that is acetylcholine, which shifts potassium out of the heart muscle cells that form the SA node. That decreases the rate at which the cells that form the node send their beat impulses to the rest of the heart. These parasympathetic effects predominate in your dog or cat when it is emotionally tranquil, resting or sleeping.
Stimulation of the blood pressure sensors (baroreceptors located in your pet’s arterial walls) decrease its heart rate. These receptors respond to stretched blood vessel walls by increasing vagal nerve parasympathetic “tone” which releases acetylcholine, slowing your pet’s heart rate. The opposite is also the case - decreased vagal tone decreases pulse frequency and heart rate.
Medications can slow your pet’s heart rate too (eg ACE inhibitors for heart disease such as benazepril and enalapril or ARBs. So can other heart medications such as digoxin and clonadine. Tranquilizers such as acepromazine can also slow pulse and heart rate.