EKGs and the Cardiac Cycle Flashcards
Action Potentials of Contractile Heart Cells
- Most of the heart is made of contractile cells
- contractile cells are responsible for the heart’s pumping function
- like skeletal muscle, depolarization precedes contraction
- unlike skeletal muscle, action potentials in cardiac muscle cells have a characteristic hump/plateau
non contractile cells
pacemaker cells
3 steps of cardiac action potential
- depolarization
- voltage change opens Ca2+ channels, influx of extracellular Ca2+
- repolarization
depolarization
opens fast sodium (na+) channels, extracellular Na+ enters
- rising phase of action potential (-90mV to +30mV)
- influx of Na+ will stop quickly
voltage change opens ca2+ channels, influx of extracellular ca2+
- ca2+ influx prolongs depolarization - the plateau
- cells will contract as long as ca2+ is entering
repolarization
results from inactivated ca2+ channels; the opening of potassium (k+) channels - an efflux of K+
- resting potential (-70mV) is restored
- ca2+ is either pumped out of the cell or into the sarcoplasmic reticulum
function of the plateau
- both the action potential and the contraction phase are longer in cardiac muscle than skeletal muscle
- sustained contraction ensures efficient ejection of blood from the ventricles
- longer absolute refractory period avoids tetany
electrocardiography
detects the electrical currents generated in and transmitted through the body
ECG/EKG
graphic recording of the heart’s activity
- composite of all action potentials generated by nodal and contractile cells
- typically has 3 distinguishable waves or deflections
P wave
EKG wave
- lasts 0.08 seconds
- results from movement of the depolarization wave from SA to AV node
- the atria contract 0.1s after the P wave begins
QRS complex
EKG
- lasts 0.08s
- results from ventricular depolarization
T wave
EKG
- lasts 0.16s
- results from ventricular repolarization
atrial repolarization
occurred during ventricular depolarization - the resultant wave was obscured by the qrs complex
intverval
duration of time that includes 1 segment and 1+ wave
segment
a region between 2 waves
P-R interval
0.16s - the beginning of atrial depolarization to the beginning of ventricular depolarization
S-T segment
- action potentials of ventricular myocytes are in plateau, the entire ventricular myocardium is depolarized
- an elevated or depressed ST segment can indicate cardiac ischemia
depolarization and repolarization
always precede mechanical events of the heart
Intrinsic conduction with EKG
- Atrial Depolarization
- Completed by SA Node, causes P Wave - Atrial Depolarization Complete
- Impulse delayed at AV Node - Ventricular Depolarization
- Begins at apex, causes QRS complex
- Atrial repolarization occurs, but is obscured - Ventricular Depolarization Complete
- Ventricular Repolarization
- Begins at apex, causes T Wave - Ventricular Repolarization Complete
cardiac cycle
mechanical events of the heart
systole
contraction
- blood is forced out of the hearts chambers
diastole
relaxation
- blood refills the heart’s chambers
1 cardiac cycle
all events associated with blood flow through the heart in 1 complete beat
(atrial systole + atrial diastole + ventricular systole + ventricular diastole)
steps of the cardiac cyclel
- ventricular filling
- isovolumetric contraction
- ventricular ejection
- isovolumetric relaxation
ventricular filling
- pressure is low, blood flows from atria to ventricles
- av valves open, sl valves are closed
- responsible for 80% of ventricular filling
- following atrial depolarization (p wave), atrial systole occurs, and blood is compressed into the ventricles
- EDV
- atrial diastole and the start of ventricular depolarization
End diastolic volume (edv)
the maximum volume of blood that the ventricles will contain in the cardiac cycle
isovulmetric contraction
- atria relax, the ventricles start contracting, pressure in the ventricles rises
- AS valves close
- for a moment, the ventricles are closed chambers with a constant blood volume
- pressure continues to rise until it exceeds rhe pressure in the great vessels
- sl valves open
ventricular ejection
- blood moves from the ventricles to the great vessels (pulmonary trunk vs aorta)
- pressure in the aorta is typically 120mmHg
isovolumetric relaxation
- follow the t wave, the ventricles relax
- ESV
- ventricular pressure drops, blood slides from the great vessels back towards the ventricles - sl valves close
- ventricles are again closed chambers
- dicrotic notch
End systolic volume (ESV)
the un-ejected blood remaining in the ventricular chambers
dicrotic notch
a brief rise in aortic pressure caused by blood rebounding off the newly closed aortic valve
restarting the cardiac cycle
- flow of blood through the heart is governed by pressure changes - blood always follows down a pressure gradient (high-low)
- while the ventricles are contraction in systole, the atria are in diastole
- when the pressure in the filling atria exceeds the pressure in the. ventricles, the av valves open, and the ventricular filling begins again
1 cardiac cycle
0.8s
- atrial systole = .1s, ventricular systole = .3s, quiescent period = .4s
pulmonary circulation
low pressure circuit - pressure in the pulmonary arteries = ~24/10 mmHg
systemic circulation
higher pressure circuit - pressure in the aorta + ~120/80 mmHg
- despite difference in pressure, both sides of the heart eject the same volume of blood with each beat
heart sounds
- 2 sounds can be hear with each heart beat “lub-dup”
- sounds heard are the sounds of the valves closing
- the pause between sounds is the quiescent period
1st sound
av valves closing
- start of ventricular systole
- longer, louder sound
2nd sound
sl valves closing
- start of ventricular diastole
- shorter, sharper sound
valve closure
typically, the mitral valve closes slightly before the tricuspid valve, and the aortic valve closes slightly before the pulmonary valve
murmur
an abnormal heart sound secondary to turbulent blood flow
- more common in children and elderley people; thin-walled hearts allow more vibration
- can be indicative of a valve problem - insufficient or incompetent valves allowing regurgitation or backflow
stenotic valves
fail to open all the way - the narrowed opening restricts blood flow, and a high-pitched sound (or click) can be hear
