A Brief History of Ambulatory ECG Machines: Part 1
Pre-1970s: Event recording
In the old days, when memory was extremely limited and battery power in short supply, short recordings of the critical events were all that could be managed. Lugging around a suitcase recorder that weighed almost 65 pounds with reel-to-reel tape and a “portable” power supply may be nostalgic for some, but it was a real pain in the back for others.
In those days, the amount of memory and power were the limiting factors. Event recording was the answer. The machines would record a short snapshot of electrocardiogram (ECG) only when the patient felt a symptom. Asymptomatic patients need not apply.
1970s: Event recording becomes loop recording
Often, after looking at the event recording, the doctor would be most curious about what preceded the actual event. That curiosity led to the invention of loop recording.
These recorders actually had a loop of tape with the minute or so before the actual time recorded and enough tape to record the next minute or so after the patient activated the event button. The loop of tape was constantly feeding around the recorder heads, saving the previous minute and waiting to capture the next minute after the event button was activated. Sounds clunky now, but it was state-of-the-art technology in the ‘70s.
1980s: Multiple events
With the advent of less expensive storage media and the ability to digitize ECG data with analog-to-digital converters, more events could be stored. The loop of tape could be replaced by a chunk of digital memory and several events could be stored by writing those digitized event “loops” to a new type of memory that replaced the magnetic tape.
That was the cat’s meow! A record of several events, with a period of time before and after the symptom, were all stored in one ever-shrinking machine.
1980 to Now: Trans-telephonic monitoring (TTM)
Wouldn’t it be nice to see those events before the patient came back to the office? So let’s send those loops of event data to the doctor’s office before the monitor comes off the patient. In fact, let’s send them by telephone.
If you recall hearing a fax machine send or receive, that’s exactly what TTM is and sounds like. You just have to use a land line, not a cell phone, to send a series of tones representing the ones and zeros of the digitized ECG data from one place to another — just like a fax.
For decades, this was state-of-the-art monitoring technology. Unfortunately, it required the patient to recognize an event had been recorded, dial a phone, and then hold the recorder near the phone until the event was transmitted.
1990: Holter (continuous) recording for 24 hours
The technology revolution was in full swing. Digital signal processing, memory, and battery life capabilities were converging to make complete ambulatory recording a reality.
The physical size of the components had diminished to the point where the recorders had become vanishingly small and almost unobtrusive. The most inconvenient part was (and still is) the electrodes that contacted (and sometimes irritated) the patient’s body. Unfortunately, it is the placement of the electrodes around the patient’s body that gives the fidelity and stereotaxis that are the hallmarks of a good Holter recording. With other less separated and more narrow electrode configurations (think patch recorders or on-demand recorders, and see Part 2 of the article on this topic), the value of the data is reduced to only rhythm analysis. The recorder was no longer the inconvenient part! And a complete, 24-hour period of ECG could be stored in onboard memory. Imagine that!
2005: Multi-day Holter
A 14-day Holter with a single double-A battery? They said it couldn’t be done, but NEMon did it ahead of the pack by nearly a decade.
The DR200-Series recorder addressed the power drain on the battery, which was largely determined by the size of the display and the sample rate of the data.
NEMon engineers realized that there is little information to be gained above a certain rate, which is good for battery life. Giving the patients too much information on an overly complicated screen display is not a good idea, so simplicity rules there as well. A happy “marriage” of size and weight — of power density and convenience — drives the useful shape of Holter and Event recorders.
In Part 2, we will review ambulatory ECG Machines from 2005 to the present.