Medical City · Device Lab 38 of 69
Cardiac Monitor
Five electrodes on the torso, three limb positions, a driven reference and one chest position, giving seven leads, of which three carry information and four are arithmetic. The passband runs from half a hertz to forty, and there is a mains notch filter available, because nothing here is going to be measured to a tenth of a millivolt.
- 10 parts
- 2 configurations in the Device Lab
- Also called: Patient Monitor, Bedside Monitor
Configurations
The versions in the Device Lab.
Bedside monitoring, five electrodes
Five electrodes on the torso, three limb positions, a driven reference and one chest position, giving seven leads, of which three carry information and four are arithmetic. The passband runs from half a hertz to forty, and there is a mains notch filter available, because nothing here is going to be measured to a tenth of a millivolt.
Diagnostic acquisition, ten electrodes
Ten electrodes, the same three limb positions and driven reference, plus six precordial positions at published anatomical landmarks, giving twelve leads, of which eight carry information and four are arithmetic. The passband runs from a twentieth of a hertz to a hundred and fifty, and there is no notch filter, because fifty hertz is inside the QRS.
How it works
Explore it in the Device Lab.
Where the rejection is actually lost
Set the amplifier rejection and the input impedance where they are, then move only the imbalance between two electrodes and watch what arrives at the input.
Skin preparation, as a number
Fix the allowance at one millimetre on the trace and solve the same relation backwards for the largest imbalance that fits inside it.
What a corner frequency costs
Put the standard impulse, three millivolts for a hundred milliseconds, through each high-pass corner in turn and measure the displacement it leaves behind.
What the top of the band costs
Change only the upper corner and watch a pacing pulse, a rise time and a rejection all move together.
Twelve leads and eight of anything
Write the leads out as weights on the electrodes and count the independent ones.
The parts
Part by part.
Electrode and the electrode-skin interface
A foam disc fifty millimetres across with a blob of conductive gel in the middle, a small disc of silver coated in silver chloride sitting in the gel, and a stud on the back for the wire to clip to. It costs pennies, it is thrown away after one patient, and it decides the performance of everything behind it.
Patient leadwires and trunk cable
A bundle leaving a keyed socket on the front of the monitor, running to a small block that lies on the bed, and fanning out from there into one thin wire per electrode. Each wire ends in a sprung clip.
Defibrillation protection and input current limiting
A row of series resistors and a clamp device per channel, sitting between the connector and everything else. They are the largest passive parts on the input board and they exist for an event that may never happen and must be survived if it does.
Lead selection, input impedance and lead-off detection
A multiplexer, a set of buffers, and a small source that injects a current far above the signal band into every electrode. Between them they decide which electrodes form which leads, how much the amplifier loads the skin, and whether an electrode is connected at all.
Instrumentation amplifier
One chip, a handful of resistors, and the component everybody names when asked what a monitor is. It takes two inputs and produces the difference between them multiplied by a few hundred, and it is not the thing that decides whether that works.
Driven reference electrode and common-mode feedback
A fourth electrode with no lead attached to it, and one operational amplifier. It is the only electrode on the patient that the monitor drives rather than listens to, and it is the reason the other electrodes work.
Sigma-delta digitiser
One converter per channel, twenty-four bits, running at a few thousand samples a second. It is unremarkable to look at and its specification is decided entirely by something that is not the signal.
Digital filtering and baseline restoration
A processor and some code. It is where the passband actually lives, where the baseline is pulled back to the middle of the screen, and where every decision about what to discard is made.
QRS detection and heart rate derivation
An algorithm, a threshold and a refractory period. It produces the largest number on the screen, and that number is the output of a detector rather than a measurement of a heart.
Arrhythmia analysis
Software that classifies beats and rhythms and raises the alarms that make people run. It is the most consequential thing on this device and the least inspectable.
Simulations
Where it appears in the simulations.
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