Lecture 8 - Heart Rhythmicity and Normal ECG Flashcards
What is duration of the PR interval and what is the significance of this timing?
(L8 S20)
-0.16 seconds; this is the delay of SA node AP conductance to the Purkinje fibers
What is a monophasic action potential?
- depolarization and repolarization of tissue
- in the ventricle it would be the QT interval
What is the duration of the QT interval?
-0.35 secs
What does the P wave and QRS complex represent in the atrial and ventricular APs respectively?
(L8 S16)
-phase 0
What are the connections of lead I?
How does it look at the heart?
What is the angle of its axis?
- negative on R arm
- positive on L arm
- look from R to L
- 0 degrees
What are the connections of lead II?
How does it look at the heart?
What is the angle of its axis?
(L8 S32)
- negative on R arm
- positive on L leg
- looks at heart from upper right to lower left
- 60 degrees
What are the connections of lead III?
How does it look at the heart?
What is the angle of its axis?
- negative on L arm
- positive on L leg
- looks at heart from upper left to lower left
- 120 degrees
What is the name of the triangle around the heart formed by the limb lead placements?
-Einthoven’s triangle
A __________ charge causes a __________ deflection on an EKG.
Negative; positive/upward
The length of an vector represents __________ and the direction represents __________.
Magnitude of voltage; direction of current flow
What leads make up a standard 12 lead EKG?
L8 S39
Bipolar limb leads:
-I, II, and III
Augmented limb leads:
-aVR, aVF, aVL
Precordial leads (standard leads): -V1-V6
What is the mean electrical axis of the heart and what is responsible for this?
- 59 degrees
- the apex of the heart remaining positive with respect to the the base of the heart during ventricular depolarization (QRS)
What conditions result in an abnormal QRS axis?
- change in heart position
- hypertrophy
- bundle branch blocks
- fluid in the pericardium
- pulmonary emphysema
What conditions result in increased or decreased QRS voltages?
Increased:
-hypertrophy
Decreased:
-cardiomyopathies
What is the normal QRS duration?
-0.06-0.08 seconds
What conditions result in prolongation of the QRS complex?
- destruction of cardiac tissue
- blocks in the Purkinje system
What is the J point and conditions can alter the position of the J point?
(L8 S49-50)
- the end of the QRS complex
- represents when all ventricular tissue is depolarized
- used to analyze current of injury
Causes of alterations:
- mechanical injury
- infectious processes
- ischemia
What is a current of injury?
L8 S49
- cardiac abnormality causes a region to remain partially or fully depolarized
- current flows away from the site of abnormality toward regular cardiac tissue
- current flows even between heart beats
Describe the timing of action potential conduction in the heart and what is responsible for delays.
- AP generated in the SA node and arrives at the AV node 0.03 secs later.
- Signal is delayed in the AV node for 0.09 secs
- delay is caused by small size of cells, low magnitude of AP, and slow rate of depolarization
- AP is delayed again in the penetrating bundles for 0.04 secs
What is the resting membrane potential and threshold of the SA node?
Resting potential:
- -55 to -60mV
Threshold:
-40mV
How does vagal nerve stimulation affect heart rate?
- distributed to SA and AV nodes
- releases acetylcholine which affect muscarinic receptors
- increases the permeability to potassium ions, hyperpolarizing the cells to -65 to -70
- hyperpolariziation decreases rate of SA node (negative chronotropic effect)
How does sympathetic stimulation affect heart rate?
- distributed to entire heart
- releases norepinephrine which affects beta-1 adrenergic receptor
- may increase the permeability to sodium and calcium ion
- increases depolarization rate (positive chronotropic effect)
What does the P wave represent?
-atrial depolarization
What does the QRS complex represent?
-ventricular depolarization
What does the T wave represent?
-ventricular repolarization
