EKG and the Cardiac Cycle Flashcards
3 steps of Cardiac Action Potential (1st step)
Depolarization- opens fast sodium (Na+) channels, extracellular Na+ enters
- rising phase of action potential (-90mV to +30mV)
- influx of Na+ will stop quickly
3 steps of Cardiac Action Potential (2nd step)
Voltage change open calcium Ca2+ channels, influx of extracellular Ca2+
-Ca2+ influx prolongs depolarization - the plateau
- cells will contract as long as Ca2+ is entering
3 steps of Cardiac Action Potential (3rd step)
repolarization- results from inactivated Ca2+ channels, the opening of potassium K+ channels - an efflux of K+
- resting potential (-70mV) is restored
- Ca 2+ is either pumped out of the cell or into the sarcoplasmic reticulum
Function of Plateau
- Action potential and contraction phase are longer in cardiac muscle
- sustained contraction ensures efficient ejection of blood from the ventricles
- longer absolute refractory period avoids tetany
P Wave
- Last .08s
- results from movement of the depolarization wave from SA to AV node
- the atria contracts .1s after the P wave begins
QRS complex
- lasts .08s
- results from ventricular depolarization, precedes ventricular contraction
T wave
- lasts .16s
- results from ventricular repolarization
Why is atrial repolarization not seen on a typical EKG?
Atrial repolarization occurred during ventricular depolarization – the resultant wave was obscured by the QRS Complex
Interval
duration of the time that includes 1 segment and 1+ wave
Segment
a region between two waves
What is the possible clinical significance of an elevated or depressed ST segment?
Cardiac ischemia
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 repolarization
Begins at apex, causes T wave
Systole
- Contraction
- Blood is forced out the heart’s chambers
Diastole
- Relaxation
- Blood refills the heart’s chambers
Cardiac Cycle (1st step)
Ventricular Filling
- pressure is low, blood flows from atria to ventricles
- AV valves open, SL valves close
- 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
Cardiac Cycle (2nd step)
Isovolumetric Contraction
- atria relax, ventricles start contracting, pressure in ventricles rises
- AV valves close
- for a moment, the ventricles are closed chambers with a constant blood volume
- pressure continues to rise until it exceeds the pressure in the great vessels
- SL valves open
Cardiac Cycle (3rd step)
Ventricular Ejection
- blood moves from the ventricles to the great vessels (pulmonary trunk vs aorta)
- pressure in the aorta is typically - 120mmHg
Cardiac Cycle (4th step)
Isovolumetric Relaxation
- following 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
End Diastolic Volume (EDV)
the maximum volume of blood that the ventricles will contain in the cardiac cycle
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 (isovolumetric Relaxation)
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
Closure of the ____ Valves is the 1st Sound (“lub”)
AV
Closure of the ____ Valves is the 2nd Sound (“dup”)
SL
Murmur
an abnormal heart sound secondary to turbulent blood flow
