Heart Conduction System

Conduction System of the Heart

• Electrical impulses initiate cardiac contractions through specialized myocardial fibers
• The cardiac muscle lacks nerves
• Intercalated discs are present at the junctions between cardiac muscle fibers, a unique characteristic
• Desmosomes connect muscle cells, preventing their separation
• Gap junctions facilitate ion passage, which ensures rapid impulse transmission
• Specialized structures ensure the atria contract together, followed by the ventricles

Control of Heart Rate and Contraction

• Stimulation of the sympathetic nervous system (SNS), such as during stress, increases epinephrine secretion
• Epinephrine stimulates beta receptors
• Increased stimulation of beta receptors increases both heart rate and contractility
• Tachycardia is increased heart rate
• Bradycardia is decreased heart rate
• Cardiac muscle needs constant oxygen and nutrients for efficient impulse conduction and contraction
• Cardiac muscle has limited oxygen storage

The Cardiac Cycle

• At the cycle's end, the atria begin to refill, the ventricles relax, and the aortic/pulmonary valves close to prevent backflow
• The same blood volume is pumped from both sides of the heart to balance systemic and pulmonary circulation
• Heart sounds ("lubb-dupp") come from valve closures, heard via stethoscope (mediate auscultation)
• AV valve closure during ventricular systole causes the "lubb" sound
• Semilunar valve closure during ventricular diastole causes the "dupp" sound
• Pulse indicates heart rate
• Pulse is felt using fingers (not thumb) over an artery near bone, usually at the wrist
• The pulse is described by strength, regularity, and BPM

Cardiac Output

• This is the volume of blood ejected by ventricle(s) in one minute
• It depends on heart rate and stroke volume

Stroke Volume

• Stroke volume is the volume of blood pumped from one ventricle in a single contraction
• Cardiac output, blood pressure, and running from a lion require good levels
• Hypovolemia results in low stroke volume

Cardiac Reserve

• Cardiac reserve is the heart's ability to increase output as needed

Preload

• Preload refers to the mechanical state of the heart at the end of diastole
• It is said that at this point, the ventricles are at their maximum volume

Afterload

• Afterload is the force needed to eject blood from the ventricles and depends on peripheral resistance
• It is determined by the opening of the semilunar valves
• An example of increased afterload is high diastolic pressure from vasoconstriction

Blood Pressure

• Blood pressure is the pressure of blood against systemic arterial walls
• Normal blood pressure is about 120/70 mm Hg when resting
• Systolic pressure (higher number) is the pressure from blood ejected from the left ventricle
• Diastolic pressure (lower number) is the pressure sustained when the ventricles are relaxed
• Pulse pressure is calculated by diastolic pressure subtracted from the systolic pressure

Blood Pressure Correlation to Heartbeat

• Blood pressure relies on cardiac output and peripheral resistance
• Peripheral resistance is the force opposing blood flow corresponding to friction from blood vessel walls
• Decreasing blood vessel diameter (or lumen) correspondingly increases blood flow resistance

Cardiovascular Assessment and Diagnostic Tests

ECG (Electrocardiogram)

• ECG records electrical activity of the heart for irregularities, myocardial infarctions, and conduction abnormalities
• Has leads through monitor, ECG on strip

Echocardiography

• Echocardiography uses ultrasound to image the heart and assess structure, function, and valve abnormalities

Exercise Stress Tests

• These tests assesses cardiovascular response to exertion by monitoring ECG and BP during physical activity

Chest X-Rays

• Allows one to provides images of heart size and shape, as well as lung congestion

Nuclear Imaging

• It uses radioactive tracers to highlight blood flow and heart function, used to identify ischemia

Single Photon Emission Computed Tomography (SPECT)

• It is a nuclear imaging that provides 3D blood flow images of the heart

Cardiac Catheterization

• It uses a catheter inserted into a blood vessel and guided to the heart
• This measures pressures and assess coronary arteries

Angiography

• Angiography uses dye and X-rays to visualize blood flow in the coronary arteries and detect any blockages

Troponin Blood Test

• This test measures troponin levels released during heart muscle damage, indicating myocardial infarction

Doppler

• Doppler uses ultrasound to assess blood flow and detect blockages or clots

Blood Tests

• These measure cholesterol, triglycerides, cardiac enzymes, and markers of inflammation like C-reactive protein (CRP)

Arterial Blood Gas (ABG) Determination

• ABGs measure oxygen, carbon dioxide, and pH arterial levels to assess respiratory/metabolic function in patients

Cardiovascular Illness Treatments

• Four common forms of treatment include: dietary modifications, regular exercise, smoking cessation and drug therapy

Dietary Modifications

• Dietary changes include reduced fat and sodium intake
• General weight reduction may be recommended

Regular Exercise

• Exercise improves overall cardiovascular function and circulation
• It lowers lipids, raises HDL cholesterol, and reduces stress

Smoking Cessation

• Quitting smoking reduces coronary artery disease risk
• This also reduces vasoconstriction/ heart rate, platelet adhesion, clot formation risk, and CO displacing oxygen

Drug Therapy

• Vasodilators (e.g., nitroglycerin) reduce peripheral resistance and heart workload by dilating blood vessels improving oxygen supply, but may lower blood pressure
• Digoxin is used for heart failure and atrial dysrhythmias
• Digoxin slows heart rate, increases contractility improving efficiency
• Antihypertensive drugs lower blood pressure (adrenergic-blocking agents, calcium channel blockers, ACE inhibitors, and angiotensin II receptor blockers/ARBs)
• Diuretics (e.g., hydrochlorothiazide, furosemide) remove excess sodium/water via kidneys, reducing blood volume
• Anticoagulants (e.g., warfarin, aspirin, apixaban, rivaroxaban) reduce blood clot risk by inhibiting platelet aggregation

Angina Pectoris

• Angina pectoris occurs when the heart muscle lacks oxygen
• The heart works harder and needs more oxygen, can lead to ischemia and chest pain

Angina Pectoris Signs and Symptoms

• Recurrent- intermittent episodes of chest pain
• Substernal pain, pallor, diaphoresis, and nausea

Angina Pectoris Treatment

• Rest
• Administration of vasodilators (nitroglycerin), reducing systemic resistance

Myocardial Infarction (MI)

• MI (heart attack) occurs from total coronary artery obstruction
• This leads to prolonged ischemia, oxygen loss, and cell death

Myocardial Infarction (MI) Causes

• Most common cause is atherosclerosis with thrombus formation
• Thrombus buildup obstructs artery
• Atheroma breaks off to form an embolus
• Vasospasms also results in total obstruction

Myocardial Infraction (MI) Signs and Symptoms

• Sudden chest pain extending to left arm, shoulder, jaw (women), or neck
• Severe and unrelieved pain (pale), along with diaphoresis and nausea

Myocardial Infraction (MI) Treatment

• Oxygen, anticoagulants (eg. Heparin), analgesics(morphine), and thrombolytic agents

Congestive Heart Failure (CHF)

• CHF is when the heart can't pump properly to meet the body’s metabolic demands, often is a complication of another condition

Congestive Heart Failure (CHF) Causes

• CHF results from impaired heart contraction
• It causes blood to back up, resulting in congestion in the lungs/systemic circulation

Congestive Heart Failure (CHF) Signs and Symptoms

• Left-sided CHF: pulmonary congestion, dyspnea, pink frothy sputum, and rales
• Right-sided CHF: manifests as edema in feet and legs, hepatomegaly, splenomegaly, distended neck veins, ascites

Congestive Heart Failure (CHF) Treatment

• One should reduce workload on the heart
• Taking ACE inhibitors, diuretics, and digoxin helps too

Hypertension

• Idiopathic primary hypertension (140/90) warrants treatment
• Secondary hypertension is due to something in the body
• Malignant BP over 200 causes end-organ damage

Hypertension Causes

• Hypertension is high blood pressure
• Hypertension leads to arterial damage and complications like stroke, heart failure, and kidney disease
• Increased peripheral resistance due to vasoconstriction
• Increased renin, angiotensin, and aldosterone secretion leading to increased blood volume
• Over time, arterial walls thicken and harden reducing blood supply

Hypertension Signs and Symptoms

• Often asymptomatic in early stages, but can include fatigue, malaise, morning headache, and consistently elevated blood pressure

Hypertension Treatment

• Treatment includes lifestyle changes, weight reduction, stress management, diuretics and ACE inhibitors

Shock

• Shock or hypotension results from decreased circulating blood volume
• This leads to reduced tissue perfusion and hypoxia

Shock Causes

• Hypovolemic shock is caused by loss of blood from hemorrhage, burns, or dehydration
• Cardiogenic shock is caused when the heart can’t effectively pump blood as a result of myocardial infarction or arrhythmias
• Vasogenic shock is a widespread vasodilation
• Anaphylactic shock stems from systemic vasodilation
• Septic shock results from severe infection

Shock Signs and Symptoms

• Anxiety, restlessness, tachycardia, cool and pale skin, oliguria, thirst, rapid respirations
• Metabolic acidosis

Shock Treatment

• Place patient supine and keep them warm
• Administer oxygen if possible
• Identify the underlying cause, and treat