If limb lead III has tiny waves (i.e. nearly a flat line), while leads I and II have large waves, this is because:
a.the subject is not grounded
b. leads I and II have mirror image R waves and cancel out
c. the left arm electrode is not connected
d. the subject has a cardiac disorder
e. the mean electrical axis is perpendicular to lead III electrodes
If your MP100 is recording at 10 sps and your input analog signal is at 100Hz, how many waves would the data acquisition miss in a 5 s recording?
a. 50
b. 1000
c. 25
d. 500
e. 100
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For the first question, to determine the answer, we need to understand the normal electrical conduction through the heart and the role of limb leads in EKG recordings. Limb leads I, II, and III are commonly used in EKG recordings to measure the electrical activity of the heart.
Limb lead III records the electrical activity between the left leg (positive electrode) and the left arm (negative electrode). If limb lead III has tiny waves (nearly a flat line) while leads I and II have large waves, it suggests a specific situation. Let's go through the options one by one:
a. The subject is not grounded: This could be a possibility if the electrical connection between the subject and the recording device is not properly established. However, if this were true, all limb leads (I, II, and III) would likely show abnormal or missing waves, not just limb lead III.
b. Leads I and II have mirror image R waves and cancel out: Leads I and II do not have a mirror image relationship with each other. Lead III is derived from Lead II, and a similar relationship does not exist between Leads I and III. Therefore, this option is incorrect.
c. The left arm electrode is not connected: This could be a possibility. If the left arm electrode is not connected or is disconnected during the recording, it would result in a flat line or tiny waves in limb lead III. Therefore, this option might explain the situation.
d. The subject has a cardiac disorder: While a cardiac disorder could lead to abnormal EKG findings, it would not specifically result in tiny waves in limb lead III while leads I and II have large waves.
e. The mean electrical axis is perpendicular to lead III electrodes: The mean electrical axis represents the overall direction of electrical activity in the heart. If the mean electrical axis is perpendicular to the lead III electrodes, it could adversely affect the recording. However, this situation would likely affect all limb leads (I, II, and III), not only limb lead III.
Based on the explanations, the most likely answer to the first question is:
c. The left arm electrode is not connected
Moving on to the second question, we need to calculate the number of waves that would be missed in a 5-second recording when the EKG machine is recording at 10 samples per second (sps) and the input analog signal is at 100 Hz.
To calculate the number of waves missed, we need to find the difference in frequency between the recording rate (10 sps) and the input signal frequency (100 Hz).
The recording rate of 10 sps means that every second, the machine records 10 points of data. If the input signal is at 100 Hz, it means there are 100 cycles (waves) in one second.
Therefore, the number of waves missed in a 5-second recording can be calculated as:
(Number of waves in one second) - (number of recorded points in one second) x (recording duration)
= 100 waves - (10 points/second) x (5 seconds)
= 100 waves - 50 points
= 50 waves
So, the answer to the second question is:
a. 50 waves