Syncope causes refer to the reasons people faint. They explain the physiologic processes resulting in fainting even though they don’t usually explain why the processes occur. Knowledge of the different causes is important though, for recognizing, treating and preventing it. Axiomatically, learning the language explaining a cause is the first step to acquiring the knowledge and the understanding of the effect.
Atrioventricular node (AV node) – It is a small mass of tissue in the wall of the right side of the partition between the right atrium and left atrium of the heart. Electrical impulses from the sinoatrial node pass through it before reaching the right and left ventricles. It serves as a relay station.
Atrioventricular block (AV block) – Is the delayed or total inability of electrical signals originating from the SA node to pass through the AV node and reach the ventricles of the heart.
Autonomic reflex – Is any of the vast related actions between autonomic nerves and structures or organs of the body in response a change. The change can be in the body’s internal or external environment. The process is involuntary and serves to maintain homeostasis or to perform a needed function. Some examples are a reflex increase or decrease in the heart rate, increase or decrease in the force of contraction of the heart, sweating, urination, and saliva production.
Baroreceptors – Are stretch receptors of afferent nerves located in the carotid sinuses and arch of the aorta. Increases in blood pressure cause stretching of the receptors. Once stretched, they send signals to the part of the brain that regulates blood pressure and heart rate. That portion of the brain does so by stimulating efferent sympathetic and parasympathetic nerves to send signals to the blood vessels and heart. The signals to the blood vessels regulate peripheral vascular resistance. Those to the heart regulate heart rate and the force of contractions.
Increased stretching of baroreceptors from a rise in blood pressure causes decreased sympathetic and increased parasympathetic nerve stimulation. In blood vessels these actions result in vasodilation and a drop in blood pressure. In the heart they cause a decrease in the heart’s force of contraction and a decrease in the heart rate.
Decreased stretching of the receptors from a drop in blood pressure causes increased stimulation of sympathetic nerves and decreased stimulation of parasympathetic nerves. In blood vessels these actions cause vasoconstriction and a rise in blood pressure. In the heart they cause an increase in the heart’s force of contraction and an increase in the heart rate.
Baroreflex – A reaction in the body triggered by the stimulation of a baroreceptor.
Cardiac output (CO) – Is the volume of blood the left or right side of the heart pumps out in a unit of time, usually one minute. It is equal to the stroke volume multiplied by heart rate. The formula is CO = Stroke Volume x Heart Rate.
Carotid massage – Is a test for determining if the cause of fainting is activity of the carotid sinus. It consists of applying firm pressure over each carotid sinus (just above the angle of the jaw) consecutively for 10 seconds. The person performing the test then checks for a decrease in the heart rate and/or drop in the blood pressure of the person tested. A positive test can be any of the following:
- A drop in the systolic blood pressure of at least 50 mmHg
Hypovolemia – Is a decreased volume of blood in the body. Since blood is contained in vessels the volume decrease is intravascular – within blood vessels. In addition to water, blood is composed of dissolved particles which help hold the fluid in the vessels. Those particles are blood cells, electrolytes and proteins. The volume decrease can be in any or a combination of the components. It is rarely due to a decrease in free water (dehydration) alone unless it is severe.
Orthostatic hypotension (Postural hypotension) – Is a temporary lowering of blood pressure which occurs upon sudden standing from a sitting or lying position. By definition it is a drop in systolic blood pressure of 20 mmHg or more or a drop in diastolic blood pressure of 10 mmHg or more.
Orthostasis – Is an unofficial term that health care professionals sometimes use in reference to the drop in blood pressure and/or increase in heart rate which occurs during a change in posture from sitting or lying to standing. They sometimes use it interchangeably with orthostatic hypotension.
Peripheral vascular resistance – Is the resistance of blood flow through peripheral vessels, mainly arteries. It is inversely proportional to the diameter of the lumen of the vessels. The degree of tone or contraction of the smooth muscles in the walls of the vessels determines the diameter and thus the resistance. Total peripheral vascular resistance (TPVR) is resistance for the entire cardiovascular circuit. It along with cardiac output is a determinant of blood pressure as expressed in the formula BP = CO X TPVR.
Sinus node (Sinoatrial node) (SA node) – Is a cluster of cells in the upper part of the wall of the right atrium from which electrical pulses originate. It is the heart’s natural pacemaker.
Tilt table test – Is a test to determine if the cause of syncope is a vasovagal reaction or orthostatic hypotension. The test involves strapping a patient to a table inclined to nearly vertical for up to 45 minutes while continuously recording blood pressure and heart rate. The administration of certain medication(s) likely to induce a drop in blood pressure might be a part of the test. Because of risks it is not a test option for patients with known or suspected heart disease as the cause of syncope.
Vagus nerve – vagal – adj. – It is the 10th cranial nerve. It supplies many regions of the body and is responsible for parasympathetic control of the heart and many internal organs.
Vasoconstriction – Is a decrease in the lumen of an artery due to contraction of the smooth muscle in its wall. Because it causes the blood vessel’s diameter to decrease, pressure within the vessel increases for a given volume of fluid within it. – Vasoconstrictor – n.
Vasodilation – Is an increase in the lumen of an artery due to relaxation of the smooth muscle in its wall. Because it causes the blood vessel’s diameter to increase, pressure within the vessel drops for a given volume of fluid within it. – Vasodilator – n.
Vasovagal reaction – Is a reflex involving the vagus nerve, heart and blood vessels causing a slowing of the heart rate and a drop in blood pressure. The cause of the fall in blood pressure is mainly parasympathetic stimulation of the blood vessels by the vagus nerve. Some refer to it as a vasovagal attack. It is the basic mechanism of vasovagal syncope.
Volume depletion – It is an extracellular water and salt deficit. Thus, it is a state of diminished fluid volume in the space between cells and in blood vessels.