lab 4 - excercise 28: electrical conductivity of the heart

systole

contraction of the heart muscle; better described as atrial systole and ventricular systole

diastole

relaxation of the heart muscle; atrial diastole or ventricular diastole

sinoatrial (SA) node

initiation of the electrical impulse in the heart; known as the pacemaker

where is the sinoatrial (SA) node located?

superior portion of the right atrium

how does the sinoatrial (SA) node conduct the electrical impulse?

sodium potassium pump! used to generate a difference in electrical voltage across the cells membrane

what does it mean to be polarized?

more negative ions on the inside than the outside

what causes the action potential in the SA node?

spontaneous depolarization - occurs approximately 72 times per minute

what does the SA node action potential cause?

contraction of the atria

where does the SA node action potential travel to?

the atrioventricular node (AV) node

what’s cool about the SA node impulse?

it has a slight delay (about 0.1 second) in the AV node before being conducted further to allow the atrial cardiac muscle to contract prior to ventricular firing

where do the action potentials from the AV node go to? (hint: three places)

the atrioventricular bundle (bundle of His), to the right and left branches, and to the Purkinje (conduction) fibers

what do the purkinje fibers do?

stimulate ventricle contraction; starting at the apex to the base

when do depolarization versus repolarization occur?

contraction of the heart muscle occurs just after depolarization of the heart muscle, while relaxation of the heart occurs just after repolarization

what type of impulse does the heart use?

low-voltage electrochemical impulses

what’s the average potential difference during a heart contraction?

-90 millivolts (mV); so a little less than 0.1 volts

what is special about impulses from the heart?

they can travel through the saline of the medium of the body and be picked up by sensors (electrode plates) attached to the skin

why can the impulses do this?

the cells of the body are bathed in a saline solution of a little less than 1% salt, an excellent conducting medium

what does an ecg do?

measured the slight changes in voltage related to cardiac activity

what’s the actual paper recording of an ecg called?

an electrocardiogram (ecg or ekg)

what do the axis of the ekg represent and what are the measurements for each?

horizontal: time (each millimeter equals 0.04 seconds)

vertical: voltage (each millimeter equals 0.1 mV)

how does an ecg work?

ecg has a baseline known as the isoelectric line and deflections from that line record the electrical activity of the heart

what does lead one of einthoven’s triangle measure?

connects the right arm to the left arm (RA-LA) and it measures the potential voltage across the horizontal axis of the heart

what does lead two of einthoven’s triangle measure?

connects the right arm and left leg (RA-LL) and it records the potential voltage from the base to the apex of the heart

what does lead three of einthoven’s triangle measure?

connects the left arm and the left leg (LA-LL) and records the potential voltage across the left side of the heart

what’s the purpose of the lead on the right foot for a 4 lead ecg?

serves as an electrical baseline and is not used for measurement

what’s the first event measured by the P wave?

also called a deflection wave; represents atrial depolarization

what’s the second event measured by the QRS complex?

ventricular depolarization

what’s the third event measured by the T wave?

ventricular repolarization

when does atrial repolarization occur?

during ventricular depolarization but is masked by the larger QRS complex

pr interval (pq intervals)

normally about 0.16 seconds, the time between the beginning of atrial depolarization and the beginning of ventricular depolarization

what indicates a heart block?

if the PR interval is longer than 0.2 seconds; results from reduced conduction between the atria and ventricles

what causes heart block?

damage to the AV node or decreased transmission in the AV bundle

what happens in a complete heart block?*

atria does not stimulate the ventricular depolarization at all; therefore, the atria fire independently from the ventricles

• p waves may be spaced 0.8 seconds apart, the SA node depolarization rate, but the ventricles fire at a much lower rate (1.5 to 2.0 sec apart)

how long is the QRS complex and what does it indicate when it’s longer?

0.08 to 0.10 average; if longer than 0.12 sec it may indicate a right or left bundle branch block

• the two ventricles contract at slightly different times, increasing the length of the QRS complex

QT interval length

0.36 sec average; direct relationship with heart rate