EngradeWikisCardiology EMT 1825 › Heart Rates

Heart Rates

Heart Rate

(1) The heart is affected by both the sympathetic and parasympathetic divisions of the autonomic nervous system
(2)The sympathetic division prepares the body to function under stress
(3) The parasympathetic division conserves and restores body resources

Chronotropic Effect

(1) Refers to a change in heart rate
(2) A positive chronotropic effect refers to an increase in heart rate
(3) A negative chronotropic effect refers to a decrease in heart rate

Inotropic Effect

(1) Refers to a change in myocardial contractility
(2) A positive inotropic effect results in an increase in myocardial contractility
(3) A negative inotropic effect results in a decrease in myocardial contratility

Dromotropic Effect

(1) Refers to a change in the speed of conduction through the AV junction
(2) A positive dromotropic effect results in an increase in AV conduction velocity
(3) A negative dromotropic effect results in a decrease in AV conduction velocity



(1) Baroreceptors (pressoreceptors) are specialized nerve tissue (sensors). They are found in the internal carotid arteries and the aortic arch
(2) These sensory receptors detect changes in blood pressure
(3) When they are stimulated, they cause a reflex response in either the sympathetic or the parasympathetic divisions of the autonomic nervous system
(4) For example, if the blood pressure decreases, the body will attempt to compensate by:
a. constricting peripheral blood vessels
b. increasing heart rate (chronotropy)
c. increasing the force of myocardial contraction (inotropy)
(5) These compensatory responses occur because of a response by the sympathetic division. This is called a sympathetic or adrenergic response.
(6) If the blood pressure increases, the body will decrease sympathetic stimulation and increase the response by the parasympathetic division. This is called a parasympathetic or cholinergic response
(7) The baroceptors will be "reset" to a new "normal" after a few days of exposure to a specific pressure


(1) Chemoreceptors in the internal carotid arteries and aortic arch detect changes in the concentration of hydrogen ions (pH), oxygen and carbon dioxide in the blood
(2) Decreased pH or oxygen levels or increases in carbon dioxide levels in the blood cause a sympathetic response, resulting in increased heart rate, contractility and vasoconstriction
(3) Increased pH or decreased carbon dioxide levels in the blood causes a decrease in vasoconstrictor effects, leading to a general vasodilatory effect.
(4) Nerve impulses are carried from the sensory receptors to the brain by means of the vagus and glossopharyngeal nerves (afferent pathways)
(5) The medulla of the brain serves as the integration center and interprets the sensory information received
(6) The medulla determines what body parameters need adjustment (if any) and transmits that information to the heart and blood vessels by means of motor nerves (efferent pathways).

Parasympathetic Stimulation

(1) Major parasympathetic (inhibitory) nerves are the vagus nerves (one on each side of the body)
(2) Vagus nerve fibers supply the SA node, atrial muscle, and the AV junction of the heart
(3) Acetylcholine is a chemical messenger (neurotransmitter) released when parasympathetic nerves are stimulated
(4) Acetylcholine binds to parasympathetic receptors
(5) The two main types of cholinergic receptors are nicotinic and muscarinic receptors
a. Nicotinic receptors are located in skeletal muscle
b. Muscarinic receptors are located in smooth muscle
(6) Parasympathetic stimulation has the following actions:
a. Slows the rate of discharge of the SA node
b. slows conduction through the AV node
c. decreases the strength of atrial contraction
d. can cause a small decrease in the force of ventricular contraction

NOTE: There is little effect on the strength of ventricular contraction because of minimal parasympathetic innervation of these chambers.

Sympathetic Stimulation

(1) Sympathetic (accelerator) nerves supply specific areas of the heart's electrical system, atrial muscle, and the ventricular myocardium
(2) When sympathetic nerves are stimulated, norepinephrine (a neurotransmitter) is released
(3) The job of the sympathetic division is to prepare the body for emergency or stressful situations. So, the release of norepinephrine results in the following predictable actions:
(a). increased heart rate, force of contraction, conduction velocity, blood pressure, and cardiac output
(b). dilation of smooth muscles of bronchi to improve oxygenation
(c). shunting of blood from skin and blood vessels of internal organs to skeletal muscle
(d) mobilization of stored energy to ensure an adequate supply of glucose for the brain and fatty acids for muscle activity
(e). dilation of pupils
(f). increased sweating

Adrenergic Receptor Sites

(1) Sympathetic (adrenergic) receptors are located in different organs and have different physiologic actions when stimulated
(2) There are five main types of sympathetic receptors:
(a) alpha 1
(b) alpha 2
(c) beta 1
(d) beta 2
(e) dopamine (also called dopaminergic)
(3) Alpha 1 receptors are found in the eyes, blood vessels, bladder and male reproductive orgains. Stimulation of alpha1receptor sites results in constriction
(4) Alpha 2 receptor sites are found in parts of the digestive system and on presynaptic nerve terminals in the peripheral nervous system. Stimulation results in decreased secretions, peristalsis and suppression of norepinephrine release
(5) Beta receptor sites are divided into beta1 and beta2
(a) Beta1 receptors are found in the heart and kidneys. Stimulation of beta1 receptor sites in the heart results in increased heart rate, contractility, and ultimately, irritability of cardiac cells.Stimulation of these receptor sites in the kidneys results in the release of renin into the blood. Renin promotes the production of angiotensin, a powerful vasoconstrictor.
(b) Beta2 receptor sites are found in the arterioles of the heart, lungs and skeletal muscle. Stimulation results in dilation. Stimulation of beta2 receptor sites in the smooth muscle of the bronchi results in dilation
(6) Dopamine receptors are found in the renal, mesenteric, and visceral blood vessels. Stimulation results in dilation