EKG - Ch 1 & 2

which layer of the heart contains the cardiac conduction system

endocardium

semilunar valves

control blood flow to the major arteries

- pulmonic (R) and aortic (L) valves

atrioventricular valves

control blood flow from atria to ventricles

- tricuspid (R) and mitral (L) valves

cardiac cycle

mechanical events that occur to pump blood

- two phases: diastole and systole

phases of diastole

- rapid-filling phase: AV valves pop open (high pressure in atria) + ventricles fill rapidly

- diastasis: flow into ventricles slows (pressures are equalizing)

- atrial kick: atria contract to force remaining blood into ventricles + valves shut (higher pressure in ventricles)

phases of systole

- isovolumetric contraction: all valves shut + ventricles start to contract (no blood flow yet)

- ventricular ejection: semilunar valves open + blood fills arteries

- protodiastole: blood flow slows (pressures are equalizing)

- isovolumetric relaxation: ventricles relax + valves close (higher pressure in arteries)

blood flow through systemic circulation

aorta --> arteries --> arterioles --> capillary bed --> venules --> veins --> vena cava

when is the heart muscle able to receive its own blood supply (through coronary arteries)

diastole

left main coronary artery (and branches)

provides blood flow to the walls of left ventricle

- perfuses about 60% of myocardium

- has two main branches: left anterior descending + circumflex

left anterior descending artery supplies

anterior wall of left ventricle

circumflex artery supplies

lateral wall of left ventricle

right coronary artery supplies

right ventricle and inferior wall of left ventricle

posterior descending artery arises from

- right coronary artery (in 70% of people)

- circumflex artery (in the other 30% of people)

types of heart cells

- contractile cells

- conduction system cells

control of the heart conduction system

- conduction system cells CREATE and conduct impulses to regulate cardiac cycle

- influenced by the autonomic nervous system

sympathetic NS

mediated by norepinephrine

- released by adrenal gland

- increases heart rate and blood pressure

- causes pupils to dilate

- slows digestion

parasympathetic NS

mediated by acetylcholine

- secreted as result of stimulation of vagus nerve

- slows heart rate and decreases blood pressure

- enhances digestion

cardiac cells at rest are electrically

negative on the inside (compared to outside)

cardiac cell steps of depolarization and repolarization

- negatively charged resting cardiac cell is polarized (Na+ primarily outside cell, K+ inside)

- depolarization: when stimulated by electricity, cell becomes positively charged (lots of Na+ rushes in, some K+ leaks out)

- electrical wave courses from cell to cell spreads throughout the heart (positive ions move to adjacent cells)

- repolarization: during cell recovery, Na+ and K+ shift back to their original places (via Na+/K+ pump)

depolarization should result in

heart muscle contraction

repolarization should result in

heart muscle relaxation

electrical stimulus precedes

mechanical response

- no heart beat without first having had depolarization

- however, electrical stimulus does not guarantee mechanical response

action potential phases

- phase 4: cardiac cell at rest; resting membrane potential is -90mV

- phase 0: cardiac cell is stimulated; depolarization occurs

- phase 1: Ca2+ released to cause ventricular contraction; early repolarization occurs

- phase 2: plateau phase

- phase 3: Na+ and K+ return to normal; rapid ventricular repolarization

what is flatline

indicates electrical silence

what EKG finding corresponds with phase 0

QRS complex

what EKG finding corresponds with phase 1

ST segment

what EKG finding corresponds with phase 2

where waveform levels off (before T wave)

- part of ST segment

what EKG finding corresponds with phase 3

T wave

absolute refractory period

no stimulus can cause depolarization

relative refractory period

strong stimulus can cause depolarization

supernormal period

even a weak stimulus can cause depolarization

- cell is "hyper"

when does absolute refractory period occur

from beginning of QRS to beginning of T wave

when does relative refractory period occur

top of T wave

when does supernormal period occur

end of T wave

P wave

- atrial depolarization

- normal P wave is small, rounded, and upright

Ta wave

- atrial repolarization

- not usually seen, as it occurs simultaneous with QRS

QRS complex

- ventricular depolarization

- normal QRS spiked in appearance, consisting of one or more deflections from the baseline

T wave

- ventricular repolarization

- normal T wave is broad and rounded

U wave

- late ventricular repolarization

- not normally seen

- should be shallow and rounded, the same deflection as the T wave

PR segment

flat line between P wave and QRS complex

ST segment

flat line between QRS complex and T wave

baseline

flat line between the T wave of one beat and the P wave of the next beat

- aka isoelectric line

Q wave

- negative deflection (wave) that occurs before a positive deflection

- can only have one (if present, must be first wave of QRS complex)

R wave

- any positive deflection

- can be more than one (second one is R prime or R')

S wave

- negative deflection that follows an R wave

QS wave

- negative deflection with no positive deflection at all