Cardiovascular Emergencies Part I (1.5 Hours)

Electrical Conduction In The Heart

Electrical signals in your heart muscle cause your heart to contract. The electrical signals begin in the sinoatrial (SA) node (at the top of the right atrium). The SA node is sometimes called the heart’s natural pacemaker.  The electrical signals travel through the muscle fibers in the atria and ventricles, causing them to contract.
The heart works for itself, however the body can signal to the heart that more cardiac output is needed.  When the body wants more cardiac output, it stimulates the sympathetic nervous system to dump epinephrine and norepinephrine into the bloodstream.  When those neurotransmitters reach the heart, the effect is faster rate, greater contraction strength, and greater electrical activity (to accomplish the faster rate and increased strength).   The sympathetic branch of the autonomic nervous system has two types of effects:

• Alpha - vasoconstrictive effect on systemic blood vessels
• Beta – A combination of inotropic, dromotropic, and chronotropic effects
• Inotropic –  Greater Force of Contraction
• Dromotropic – Greater electrical activity
• Chronotropic – Faster rate

Conversely, the parasympathetic branch of the autonomic system has the opposite effect on the heart than the sympathetic branch does.  The parasympathetic nervous system affects the body with the ‘feed or breed’ response.  The parasympathetic branch of the autonomic nervous system utilizes acetylcholine as its’ sole neurotransmitter.

Starling's law states that the more the ventricle is filled with blood during diastole (preload), the greater the volume of ejected blood will be during the resulting systolic contraction.  This law give us great insight into the dynamics of blood pressure.  Anything that will decrease preload will decrease stroke volume (amount of blood ejected during systolic contraction).   This is why fluid boluses are sometimes effective at treating hypovolemia.