AC, Frequency Response and Interference

Question 1

What is DC

Answer 1

Electrical current that flows in pneumonia constant direction; voltage is steady and does not change with time

Question 2

DC in ECG systems

Answer 2

DC components include baseline offset and electrode half-cell potentials

Question 3

What is AC

Answer 3

Continuously changes direction; voltage varies sinusoidally

Question 4

Different waveforms

Answer 4

• Sinusoidal
• Sawtooth
• Square wave

Question 5

Sinusoidal waveform

Answer 5

Smooth periodic oscillations

Question 6

Sawtooth waveform

Answer 6

Rises linearly then falls sharply

Question 7

Square wave

Answer 7

Rapid switching between two levels

Question 8

Amplitude definition

Answer 8

Maximum height of a waveform

Question 9

Peak-to-peak voltage definition

Answer 9

Difference between maximum positive and negative amplitude

Question 10

Period definition

Answer 10

Time for one complete cycle; measured in seconds

Question 11

Frequency definition

Answer 11

Number of cycles per second; measured in Hertz

Question 12

Relationship between frequency and period

Answer 12

f = 1/T

Question 13

Capacitor structure

Answer 13

• Two metal plates separated by an insulating material (dielectric)
• Stores energy as an electric field

Question 14

Capacitance (C) definition

Answer 14

Ability to store charge

Question 15

what does capacitance depend on

Answer 15

• Plate area
• Plate separation
• Dielectric permittivity

Question 16

Capacitive Reactance (Xc)

Answer 16

Resistance-like opposition to AC

Question 17

Frequency and capacitive reactance

Answer 17

As frequency increases Xc decreases

Question 18

What is an inductor

Answer 18

Coil of wire storing energy in a magnetic field

Question 19

What is inductive reactance (XL)

Answer 19

Opposition to changing current

Question 20

Inductive reactance and frequency

Answer 20

Higher frequency, higher inductive resistance

Question 21

What is impedance

Answer 21

Total opposition to AC

Question 22

What is impedance a combination of

Answer 22

• Resistance
• Capacitive reactance
• Inductive reactance

Question 23

Impedance related to ECG

Answer 23

Input impedance of ECG must be high; prevents current flowing from pt into device

Question 24

Time domain

Answer 24

Waveform shown as voltage vs time (normal ECG)

Question 25

Frequency domain

Answer 25

Shows what frequency components make up the waveform

Question 26

Fourier analysis concept

Answer 26

Any periodic waveform can be broken into sine waves

Question 27

Fundamental frequency

Answer 27

Lowest frequency in the series

Question 28

Harmonics

Answer 28

Integer multiples of the fundamental

Question 29

High-pass filter effect on ECG

Answer 29

Removed low frequencies (baseline wander)

Question 30

Low-pass filter effect on ECG

Answer 30

Removes high frequencies (muscle artefact)

Question 31

Notch filter effect on ECG

Answer 31

Removes mains interference

Question 32

Noise definition

Answer 32

Any unwanted signal that interferes with desired ECG

Question 33

Types of noise

Answer 33

- Mains interference - Muscle artifact - Baseline wander - Electrical equipment

Question 34

Signal-to-noise Ratio (SNR)

Answer 34

SNR = signal amplitude / noise amplitude

Question 35

Why high SNR matters

Answer 35

- Improves diagnostic accuracy - Reduces artefacts that mimic arrhythmias

Question 36

What are late ventricular potentials (LVP)

Answer 36

- Small, low-frequency signals occurring at end of QRS - Indicate risk of ventricular tachyarrhythmia

Question 37

Why LVP need averaging

Answer 37

- LVP amplitude is tiny - Noise amplitude is larger - Signal must be enhanced by averaging many heartbeats (up to 20 mins)

Question 38

Principle of signal averaging

Answer 38

- Signals occur at same time each beat = add constructively - Noise is randome = averages out