07. Physiology

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Questions and Answers

What is the correct formula to calculate heart rate from the R-R interval?

HR = RR / 60

HR = 60 / RR (correct)

HR = 60 / (RR * 0.2)

HR = RR * 60

How many beats per minute correspond to an R-R interval of one box on the ECG grid?

300 BPM (correct)

150 BPM

75 BPM

120 BPM

What information does the rhythm of an ECG primarily provide?

Source and speed of pacing events (correct)

Heart rate only

QRS duration analysis

Ventricular volume changes

What is indicated if the heart rate calculated from the ECG is between 60 and 75 BPM?

The R-R interval is more than 4 boxes but less than 5 boxes apart. Signup and view all the answers

Which of the following statements about the Nernst/Goldman equation outputs is true regarding extracellular ion concentrations?

Increasing extracellular potassium will increase resting membrane potential. Signup and view all the answers

What feature of the ECG does the P-wave represent?

Atrial depolarization Signup and view all the answers

Which of the following best describes the Axis of an ECG?

The direction of electrical activity during heartbeats. Signup and view all the answers

If an R-R interval is measured at 4 boxes on the ECG, what should be the heart rate approximately?

75 BPM Signup and view all the answers

What is the first step in estimating the Mean Electrical Axis (MEA)?

Identify the largest QRS amplitude. Signup and view all the answers

The MEA is characterized as normal when it falls within which range of angles?

-30º to 90º Signup and view all the answers

What should be true about the MEA in relation to the axis with the largest QRS vector?

The MEA must be within 30-60º of it. Signup and view all the answers

What type of axis deviation is characterized by an MEA between 90º and 180º?

Right Axis Deviation. Signup and view all the answers

Which lead is likely to show the most isoelectric amplitude when estimating MEA?

Lead III. Signup and view all the answers

In the context of electrical axis estimation, what can result from a pathological axis?

Movement of the MEA. Signup and view all the answers

Which of the following is NOT a category for MEA characterization?

Inferior Axis. Signup and view all the answers

What is the angular relationship of the MEA to the most isoelectric segment?

It is perpendicular by 90º. Signup and view all the answers

What does the P wave on an ECG primarily represent?

Atrial depolarization Signup and view all the answers

Which of the following options accurately describes the R-R interval in an ECG?

Heart rate calculation Signup and view all the answers

What does the QT interval represent on an ECG?

Ventricular depolarization to repolarization Signup and view all the answers

Which wave corresponds to the depolarization of the ventricles?

QRS complex Signup and view all the answers

Why does the PQ interval reflect a delay in conduction?

Due to decremental conduction in the AV node Signup and view all the answers

What physiological principle underlies the action potentials measured in an ECG?

Movement of depolarization across tissues Signup and view all the answers

What does the T wave indicate in the ECG cycle?

Ventricular repolarization Signup and view all the answers

Which component of the heart's electrical activity is reflected by the S wave?

Last part of ventricular depolarization Signup and view all the answers

What defines bradycardia according to heart rate ranges?

Less than 60 BPM Signup and view all the answers

What does Bazzett’s formula correct for in cardiac assessments?

QT interval duration Signup and view all the answers

Which aspect of the ECG indicates a positive deflection?

Depolarization from negative to positive electrode Signup and view all the answers

What does the Mean Electrical Axis (MEA) represent?

The direction of depolarization around the chest Signup and view all the answers

Which of these rhythms is NOT considered a pathophysiologic rhythm?

Bradycardia Signup and view all the answers

When depolarization moves perpendicular to the electrodes in an ECG, what occurs?

No signal is detected Signup and view all the answers

What is a characteristic of tachycardia?

Heart rate greater than 100 BPM Signup and view all the answers

What is the direction of Lead I in Einthoven's Triangle?

Horizontal (right to left) across the chest Signup and view all the answers

What is the measurement angle of Lead II relative to the horizontal axis?

60º below the horizontal Signup and view all the answers

Which augmentation lead points at -150º away from Lead I?

aVR Signup and view all the answers

If the conduction of repolarization is moving along the axis of the lead, what will be the deflection observed in the ECG?

Negative deflection Signup and view all the answers

Which lead represents a vector that indicates the movement from the left arm to the left leg?

Lead III Signup and view all the answers

What defines one side of Einthoven’s Triangle as positive?

It must be specified in the lead configuration. Signup and view all the answers

Which of the following statements regarding augmented leads is correct?

They take two leads at a time to make one negative. Signup and view all the answers

During the depolarization initiated by the SA Node, which wave is indicated as positive?

P-wave Signup and view all the answers

Study Notes

Isoelectric Period

Refers to the flat line on the ECG that occurs between waves, indicating no electrical activity.

### Reference Period

ECG measures the electrical activity of the heart by comparing the potential difference between two points.

ECG Waves

P Wave: Represents atrial depolarization, initiated by SA node.
QRS Complex: Represents ventricular depolarization.
- Q Wave: Depolarization of the ventricular septum.
- R Wave: Represents depolarization of the major portion of the ventricles.
- S Wave: Represents depolarization of the terminal portion of the ventricles.
T Wave: Represents ventricular repolarization.
U Wave: Not pictured in the text.

ECG Intervals and Segments

R-R Interval: Represents the time between two R-waves, used to determine heart rate.
PR Interval: Represents the time taken for the electrical impulse to travel from the SA node to the ventricles.
QRS Duration: Represents the time taken for the ventricles to depolarize.
ST Segment: Represents the period between ventricular depolarization and repolarization, indicating the early phase of ventricular repolarization.
QT Interval: Represents the total time taken for ventricular depolarization and repolarization.

ECG and Action Potentials

ECG does not directly measure action potentials.
ECG measures the potential differences generated by the movement of electrical charges (depolarization and repolarization) along the cardiac axis.
ECG waves represent the conduction of action potentials over time.

Mean Electrical Axis (MEA)

MEA indicates the direction of depolarization through the heart's electrical axis.
It provides a measure of the heart's overall electrical activity in the frontal plane.
MEA can be estimated using the following steps:
- Identify the lead with the largest QRS complex magnitude (most parallel to MEA).
- Identify the lead with the most isoelectric RS segment (most perpendicular to MEA).
- MEA lies within a 30-60° range from the lead with the largest QRS complex magnitude.
MEA is 90° away from the lead with the most isoelectric segment.
MEA can be characterized into four categories:
- Normal QRS Axis: -30° to 90°.
- Left Axis Deviation (LAD): -30° to -90°.
- Right Axis Deviation (RAD): 90° to 180°.
- Extreme Axis Deviation: -90° to -180°.

Chest Leads

Chest leads (V1-V6) provide information about the heart's electrical activity in a plane perpendicular to the frontal plane.
Chest leads are placed on the chest to create a three-dimensional view of the heart's electrical activity.

Factors Affecting MEA

MEA can change due to a pathological axis deviation.
Extracellular ion concentrations can also affect the MEA, as changes in ion concentrations alter the electrical activity across the cell membrane.

### Rate, Rhythm, and Axis

Rate: Heart rate calculated as HR= 60/RR Interval.
- Can be assessed quickly by counting the amount of time between R-waves:
  - 300 BPM if R-R Interval is one large box apart.
  - 150 BPM if R-R Interval is two large boxes apart.
  - 100 BPM if R-R Interval is three large boxes apart.
  - 75 BPM if R-R Interval is four large boxes apart.
  - 60 BPM if R-R Interval is five large boxes apart.
  - 50 BPM if R-R Interval is six large boxes apart.
Rhythm: Refers to the regularity and timing of heart electrical activity.
- Sinus Rhythm: Normal rhythm, starting from SA node with a normal PR interval.
- Bradycardia: Slow heart rate (< 60 BPM).
- Tachycardia: Fast heart rate (> 100 BPM).
Axis: Represents the direction of depolarization through the heart.

Cardiac Rhythm

Rhythms may be described as source + speed.
Source: Typically reflects the origin of the electrical impulse.
Speed: Refers to the regularity or rate of the heart's rhythm.
Atrial Flutter: Rapid and regular atrial rhythm.
Ventricular Tachycardia: Rapid and regular ventricular rhythm.
Atrial Fibrillation: Irregular and chaotic atrial rhythm.
Ventricular Fibrillation: Irregular and chaotic ventricular rhythm.
Ventricular Premature Contraction (VPC): Early heartbeat originating from the ventricles.
Bundle-Branch Blocks: Delay or blockage in the conduction of electrical impulses through the ventricles.

### Automaticity

Refers to the ability of cardiac cells to generate their own electrical impulses.
Important for maintaining a regular heartbeat.

### Re-entry

Refers to a phenomenon where electrical impulses circulate through the heart in a loop, causing abnormal rhythms.

ECG Leads

Standard limb leads create Einthoven's Triangle, which comprises:
- Lead I: Left arm (+) to right arm (-), oriented horizontally across the chest.
- Lead II: Left leg (+) to right arm (-), oriented 60° from the horizontal.
- Lead III: Left leg (+) to left arm (-), oriented 120° from the horizontal.
Augmented Leads: Point to specific regions of the chest:
- aVR: Right arm (+), used to assess depolarization moving toward the right arm.
- aVL: Left arm (+), used to assess depolarization moving toward the left arm.
- aVF: Left leg (+), used to assess depolarization moving toward the left leg.

Understanding ECG Waveforms

Positive waveform indicates that the depolarization is moving towards the positive electrode.
Negative waveform indicates that the depolarization is moving away from the positive electrode.

### Bazzett's Formula

Used to correct the QT interval for heart rate variations.
QTc=QT / square root of RR.

12-Lead ECG

Provides a comprehensive assessment of the heart's electrical activity.
Utilized for diagnosis and monitoring of various cardiac conditions.
Consists of 10 electrodes attached to the body.
Limb leads are placed on the arms and legs.
Chest leads are placed on the chest.

Studying That Suits You

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Description

Test your knowledge on the different waves and intervals of the ECG. This quiz covers essential concepts such as the P wave, QRS complex, T wave, and their significance in measuring heart activity. Dive into the fascinating electrical rhythms of the heart!

07. Physiology - ECG

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