Homeostatic imbalances• Pericarditis inflammation of the

pericardium • Endocarditis bacterial infection of the

endocardium can lead to valvular stenosis

• Myocardial infarction heart attack/coronary

• Heart block any damage to the AV node results in irregular beating of ventricles

• Murmurs abnormal or unusual hearts sounds

ECG: normal and abnormal

20-3

Alterations in Electrocardiogram

Clinical Cardiology Concepts for Vets

• Cardiac Output - the amount of blood pumped by a ventricle per minute. Units may be in milliliters or Liters per minute.

• Heart Rate - number of cardiac cycles per minute. Average for males = 64-72/min. Average for females = 72-80/min.

• Stroke Volume - amount of blood pumped out of a ventricle each beat. Average resting stroke volume = 70 ml.

Cardiac Output

Cardiac Output• CO (mL/min) =SV (70 mL/beat) X HR (75

beat/min) – Cardiac output = stroke volume X heart rate =

5250ml/min

• SV= EDV - ESV– Stroke volume = end diastolic volume – end systolic

volume

• Cardiac reserve is the difference between a person’s maximum cardiac output and cardiac output at rest.

Frank Starling Law of the Heart

• The more cardiac muscle is stretched within physiological limits, the more forcibly it will contract.

• Rubber band analogy• Increasing volumes of blood in ventricles

increase the stretch & thus the force generated by ventricular wall contraction.

• Greater stretch means more blood volume is pumped out, up to physical limits.

Regulation of stroke volume

• Preload is the degree of stretch on the heart.

• Contractility is the force of ventricular contractions.

• Afterload is the pressure that must be exceeded to eject blood from the ventricles.

• The Frank-Starling law of the heart—preload is the critical factor controlling SV (stroke volume).

Regulation of heart rate

• When blood volume drops or the heart is weakened, SV drops and the HR increases.– Positive chronotropic factors increase HR.– Negative chronotropic factors decrease HR.

• Extrinsic regulation by the ANS – Sympathetic nervous system increases firing

of the pacemaker and enhances Ca2+ entry into heart cells.

– Parasympathetic nervous system reduces heart rate through ACh which opens K+ channels.

– Under resting conditions the PNS is dominant

Chemical Regulation• Hormones– Epinephrine enhances HR and contractility– Thyroxine enhances effects of Epi and NE and

leads to slower more sustained increase in HR

• Ions– Hypocalcemia (Ca2+)depresses the heart;– Hypercalcemia prolongs the plateau phase

and lead to heart irritability– Hyperkalemia (K+) may lead to heart block

and cardiac arrest; hypokalemia leads to arrhythmia and weakened contractions.

– Excess Na+ blocks Ca2+ inflow, decreasing contractile strength

Other Factors• Exercise leads to a reduced HR when at

rest.• Working large body muscles for at least

20 minutes elevates cardiac output and accelerates metabolism.– Athletes have a larger heart and a resting HR

of 40-60 beats per minute• Increased temperature increases heart

rate; decreased temperatures reduce heart rate and oxygen needs.

Homeostatic imbalances• Congestive heart failure– Caused by Coronary artherosclerosis– Caused by Persistent high blood pressure– Caused by Multiple myocardial infarcts

– Left side failure Pulmonary congestion– Rights side failure Peripheral congestion

Angina Pectoris• Medical term for chest pain due to

coronary heart disease.• It occurs when the myocardium doesn’t get

as much blood (Oxygen) as it needs.• Insufficient blood supply is called ischemia.• May initially occur during physical exercise,

stress, or extreme temperatures.• It is a sign of increased risk of heart attack.

Cardiac Arrhythmias• Arrhythmias irregular heart rhythms that may

lead to fibrillation

• Tachycardia: Heart rate in excess of 100bpm• Bradycardia: Heart rate less than 60 bpm• Sinus arrhythmia: Heart rate varies 5% during

respiratory cycle and up to 30% during deep respiration

• Premature atrial contractions: Occasional shortened intervals between one contraction and succeeding, frequently occurs in healthy people

Aneurysm• Weakness of the wall of an artery causing

an abnormal enlargment or bulge.• The aorta or the arteries that supply the

heart, brain, legs or kindeys are most commonly affected.

Atherosclerosis (Arteriosclerosis)

• Narrowing and hardening of arteries and impairment of blood flow due to the deposition of fatty materials and calcium in their walls.

• Risk factors include:– smoking– inactivity– diabetes– high blood cholesterol– personal or family history of heart disease– Atherosclerosis Animation

Resources

• Heart Sounds & Cardiac Arrhythmias • Live Cardiac Exam Video • Anatomy Links • Cardiac Cycle • Cardiac Cycle Graphics• Interactive Physiology Review • AP II Notes Homepage

Conducting System of Heart