qt

also some drugs

cardiac output

amount of blood pumped out by either ventricle

5-6L/min

stroke volume SV

amount of blood pumped out by either ventricle in a single contraction

60-100ml

heart rate HR

number of cardiac contractions per minute

60-100 bpm

cardiac output formula

cardiac output = stroke volume x heart rate

preload

the pressure under which a ventricle fills.

influenced by:

• volume of blood returned by veins to heart

afterload

the load or pressure against which the heart contracts to eject blood

• rise in systemic vascular resistance increases after load

• when after load is high, cardiac output falls or heart works harder to overcome increased pressure

contractility/inotropy

strength of the hearts contraction

• influenced by mediations that have inotropic effect

• regulated by nervous system

sinoatrial node

dominant pacemaker of heart located in right atrium

• receives blood from right coronary artery

• impulses travel btwn atria via internodial pathways

• impulse is delayed by 0.12s here to allow atria to empty

AV junction: contains AV node and surrounding tissue along with bundle of his

impulses travel via AV node to bundle of his, into right and left bundle branches, into purkinje fibres which contract ventricles simultaneously

depolarization

when muscle fibers are stimulated to contract

sodium ions rush into cell making it positive » calcium enters to maintain depolarization and supply calcium ions for contraction

repolarization

sodium and calcium channels close » potassium channels open » potassium rushes out » sodium potassium pump restores proper balance

refractory period

absolute refractory period: cell is highly depolarized » new action potential cannot be initiated

• from q wave to peak of t wave

relative refractory period: partially depolarized » new action potential inhibited but not impossible

• second half of T wave

secondary pacemakers

SA node is damaged » any other component of conduction system takes over

the farther the conduction tissue is from the SA node, the slower its intrinsic rate of firing

SA node: 6-100 bpm » AV junction: 40-60 bpm » purkinje fibres: 20-40 bpm

electrolytes function in cardiac

sodium: flows into cell to initiate depolarization

potassium: flows out of cell to initiate repolarization

• hypokalemia: increased myocardial irritability

• hyperkalemia: decreased atomicity/conduction

calcium: maintains depolarization, contracts heart tissue

• hypocalcemia: decreased contractility and increased myocardial irritability

• hypercalcemia: increased myocardial contractility

magnesium: stabilizes cell membrane, opposes actions of calcium, acts in concert with potassium

• hypomagnesemia: decreased conduction

• hypermagnesemia: increased myocardial irritability

P wave

caused by depolarization of the atria » followed by pause as conduction is slowed by AV junction

• are the P waves regular?

• is there one P for every QRS?

• is the P wave in front of the QRS or behind?

• is the P wave normal and upright in lead I?

• are there more P waves than QRS

• do all the P waves look alike?

• are the irregular P waves associated with ectopic beats

measure from first positive deflection from baseline to where wave returns to baseline

<0.10s, 0.5 - 2.5mm

QRS complex

caused by depolarization of the ventricles

• a QRS less than 0.12s means the impulse was supra ventricular

• amplitude 5 - 15 mm

• are all the QRS complexes of equal duration?

• what is the measurement?

• within normal limits?

• do they all look alike?

• are the irregular QRS complexes associated with ectopic beats

wide QRS can be caused by:

• supra ventricular impulse that reaches obstruction in bundle branches

• supra ventricular impulse that cannot be conducted normally via ventricles because they are in refractory

• irritable focus on the ventricles assumes pacemaking responsibility

T wave

caused by the repolarization of the atria and ventricles

• atrial repolarization wave is not visible

• ventricular wave follows the QRS complex

• deep symmetrically inverted T waves may suggest cardiac ischemia

• elevated more than half the height of QRS complex may indicate new onset of myocardial ischemia or hyperkalemia

leads

a lead provides an electrical image of a certain vantage point of the heart

• electricity moving away from SA node towards lead causes a positive line (upwards)

PR interval (PRI)

distance from beginning of the P wave to the beginning of the QRS complex

• time required for impulse to traverse atria and AV junction

• usually 0.12-0.20s (3-5 squares)

• includes all atrial and nodal activity

• are all the PRIs constant?

• is the measurement within normal range?

• is there a pattern to the changing measurements?

ST segment

line from end of QRS complex to the start of the T wave

• should be at same level as baseline

• elevated or depressed could mean myocardial ischemia, injury or infarction

• findings are significant if viewed in 2 or more leads looking at the same or adjacent area of heart

• indicates refractory phase

RR interval

time between two successive QRS complexes

• interval between two ventricular depolarizations

• gives indication of HR

parasympathetic slowing heart

brain senses heart should slow » electrical impulse travels down vagus nerve to SA node » ACh is released » heart rate slows

sympathetic increasing heart

muscles send signal to brain » brain sends message via sympathetic nerves to SA node » norepinephrine is released » heart speeds up increasing cardiac output

blood pressure formula

BP = cardiac output x systemic vascular resistance

alpha beta receptors

alpha:

• arteries: constriction

• lungs: mild bronchoconstriction

beta 1:

• heart: increased dromotropy, inotropy, chronotropy

beta 2:

• lungs: smooth bronchial muscle dilation

• arteries: dilation

blood pressure formula

blood pressure = cardiac output x systemic vascular resistance

alterations in one variable bring about compensatory changes in the other to restore BP

cardiac dysrhythmia

disturbance in normal cardiac rhythm

• can be caused by ischemia, electrolyte imbalances or disturbances/damage in electrical conduction system

  • resulting in escape beats, circus reentry, or enhanced automaticity

• develop after AMI for 2 reasons

  • irritability of ischemic heart muscle surrounding an infarct may cause damaged muscle to generate abnormal electrical impulses

  • infarct damages conduction tissues

• very slow HR <50bpm lead to inadequate cardiac output and often precede electrical instability of the heart

  • produces escape beats to assist in maintaining cardiac output

• very rapid HR >140bpm lead to decreased cardiac output and decreased stroke volume since ventricles have less time to fill

  • hypoxia, hypokalemia, metabolic alkalosis, hypocalcemia can lead to electrical instability causing cells that usually do not have automaticity to fire

    • causes potential for tachycardias, flutters and fibrillations in the atria or ventricles, heralding grave rhythms (VT and VF)

    • impulses get stuck in a pattern of repetition causing multiple ectopic beats or VF

ECG leads placement

white: right upper chest near shoulder (arm for 12 lead)

black: left upper chest near the shoulder (arm for 12 lead)

red: left lower abdomen (leg for 12 lead)

green: right lower abdomen (leg for 12 lead)

left leg (red) is positive terminal

lead 1 is formed between left and right arm electrodes

lead 2 is formed between right arm and left leg

lead 3 is formed between left arm and left leg

bipolar leads

contain leads 1,2,3

leads that contain a positive and negative pole

• impulses moving toward positive terminal make positive deflection on ECG

• impulses moving toward negative electrode causes negative deflection

• perpendicular to lead causes isoelectric tracing or very little inflection in any direction

which segment for isoelectric line (0 mV)

TP segment since PR depression can occur with pericarditis and ST depression, and elevation can occur with cardiac ischemia, infarction and other causes.

two types of cells

electrical (conductive) cell: initiate electrical activity and conduct it through heart

mechanical (contracting) cell: respond to electrical stimulus and contract to pump blood

automaticity

the ability of cardiac cells to initiate electrical impulses on their own

primary electrolytes involved in creating hearts electrical stimulus

sodium (Na) and potassium (K)

• both have positive charge

• Na outweighs K making K relatively negative to Na

• difference in potential allows electrolytes to move through cell membrane

• movement of electrolytes through the cell membrane generates electrical impulse

electrical conduction through heart

sinoatrial node »inrernodal and intraatrial pathways » AV junction » bundle of His » left and right bundle branches » purjinje fibers

inherent rate ranges

SA node: 60-100 bpm

AV junction: 40-60 bpm

ventricle: 20-40 bpm

parasympathetic vs sympathetic effect on heart

sympathetic: effects both atrium and ventricles

• increased rate

• increased conduction via AV node

• increased irritability

parasympathetic: effects only atrium

• decreases rate

• decreases irritability

• slows conduction via AV node

height of deflection indicates

voltage or amplitude

how much distance between one vertical line

.04 sec

between thick lines: 0.20 sec

sodium role in cardiac function

flows into the cell to initiate depolarization

potassium role in cardiac function

flows out of the cell to initiate repolarozation

• hypokalemia» increased myocardial irritability

• hyperkalemia» decreased myocardial contractility

calcium role in cardiac function

major role in depolarization of pacemaker cells (maintains depolarization) and in myocardial contractility (contraction of heart muscle tissue)

• hypocalcemia » decreased contractility and increased myocardial irritability

• hypercalcemia » increased contractility

magnesium role in cardiac function

stabilizes cell membrane

• acts in concert with K

• acts in opposition with Ca

• hypomagnesemia » decreased conduction

• hypermagnesemia » increased myocardial irritability

PR segment

indicates the delay of the impulse as it travels through the SA node

Q wave

first negative deflection following the P wave but before the R wave

• represents depolarization of the interventricular septum

R wave

the first positive deflection following the Q wave

• indicates ventricular depolarization

• should correspond to the patients pulse

S wave

second negative deflection following the P and R waves

• R and S waves represent the sum of electrical forces from depolarization of L and R ventricles

T wave

positive deflection following S wave

• represents ventricular repolarization

QRS segment

normal range: 0.06s - 0.11s or <0.12s

• indicates ventricular depolarization

analyzing regularity and rate

looking at the R-R interval

• is it regular?

• is it irregular?

• are there any patterns to the irregularity?

• are there any ectopic beats? early or late?

determine the rate

• what is the exact rate?

• is the atrial rate the same as the ventricular rate?

normal sinus rhythm

pacemaker impulse originates from the sinus node and travels through normal conduction pathways within normal time frames

• uniform, upright, P waves; one in front of every QRS

• 60-100bpm

• PR interval 0.12-0.20s and consistant

• QRS less than 0.12s

sinus bradycardia

rate lower than 60bpm with impulse originating from the sinus node

• normal upright P wave in front of every QRS

• normal PRI and QRS

• regular

fits all rules for NSR except for HR>60bpm

sinus tachycardia

rhythm that fits all the rules for NSR except for HR<100

• regular, uniform P wave in front of every QRS

• normal and constant PRI and QRS

• rate > 100bpm

indications for ECG

• risk for dysrhythmias

• suspected cardiac patients

• suspected OD

• electrical injuries

• syncope

• elderly patients feeling unwell

• issues concerning sympathetic NS

everybody

horizontal plane leads

v1: forth intercostal space, right sternal border

v2: forth intercostal space, left sternal border

v3: midway between v2 and v4

v4: fifth intercostal, left midclavicular line

v5: fifth intercostal, left anterior axillary line

v6: fifth intercostal, left midaxillary line

3 lead what nodes for each lead

lead 1: RA is negative, LA is positive

lead 2: RA is negative, LL positive

lead 3: LA is negative, LL is positive

QT interval

period from beginning of ventricular depolarization (QRS) until the end of ventricular repolarization (end of T)