From Analog to Digital Technology: What Are the Clinical Benefits?

Benefits?

R. M

, G. C

Introduction

In 1995, Nicholas Negroponte, theorist and researcher at MIT (Massachusetts Institute of Technology), published ‘Being Digital,’ in which he examined new technological developments and their impact on the world [1]. The title of his book has now become a catch-phrase to describe a technological revo- lution that has substantially modified our social, economic, and intercultural conditions.

This evolving scenario is founded on the process that transforms analog- ic signals into digital signals. Digitalisation consists of translating data into a numerical sequence of 0s and 1s (binary system). Thus encoded, differing signals (static or moving images, sounds, written texts) become homoge- neous and can be handled simultaneously in a rapid and flexible manner, while maintaining their quality and stability.

The digital ‘breakthrough’ has brought ever greater interaction-integra- tion to sectors that for a long time developed separately. Information tech- nology, telecommunications, media, electronics, and mathematics are now converging to create a set of products and services that are radically chang- ing our way of living and working [2]. Mobile telephones, CDs, Internet, elec- tronic diaries, DVDs, MP3 players, video cameras, digital cameras, and satel- lite and cable TV are just a few of the numerous applications of this ‘synthet- ic’ technology.

The concept of synthesis accurately reflects the nature of these new instruments, which are both integrated and interactive, flexible and dynam-

1

Cardiovascular Department, Ospedale Regionale ‘S. Maria dei Battuti’, Treviso;

2

Vitatron Medical Italia S.r.l., Bologna, Italy

ic. Likewise, it embodies increased speed in handling and transmitting data, as seen in faster means of communication (sms, e-mail) and the greater quantity of information exchanged as a result of data compression (by means of specific algorithms).

The Internet is one of the greatest manifestations of the digital era.

Connecting through the web annuls distance, bringing together remote loca- tions in business (e-commerce, on-line shopping, on-line conferences) and in services for the citizen (outsourcing, tele-medicine, home banking).

In digital format, data are easy to handle and can be compressed without undergoing any alteration in their contents. Bits enable a vast wealth of infor- mation to be contained in small space, as in the case of digital libraries (on- line or stored in memory devices such as CD-ROMs). High resolution, as evi- denced by the ability to record or reproduce the finest details, is achieved in both music and images. Indeed, CD-ROMs offer clearer and more subtly nuanced sound reproduction than the old vinyl records; similarly, digital images achieve greater definition than traditional photography. Another par- ticular feature of digital applications is that of stability over time; CD-ROMs, for instance, conserve their performance quality longer than vinyl records do.

The History of Digital Technology

The digital revolution is rooted in the distant past. In the 18th century [3], French artisans were experts in weaving elaborately decorated cloth. The looms used in this process were ‘programmed’ by means of a method that was perfected by Joseph Jacquard at the beginning of the 19th century.

Jacquard used punched cards, in which the holes served to position the threads according to a precise design. Each hole allowed a hook with a thread attached to it to be inserted into the design. Where there was no hole in the card, the hook could not pass through, and the corresponding coloured thread could not therefore be inserted into the weave. A punched card was provided for each single operation, and the whole set of cards con- stituted the complete programme of the weaving process.

Jacquard’s punched cards were taken up by the manufacturers of calcu-

lating machines. In 1890, the census taken in the United States was carried

out with the aid of calculating machines designed by Hermann Hollerith,

who had adapted the system of punched cards to meet the specific needs of

the census. Hollerith added an electrical device to detect the presence of per-

forations in the cards. The data collected in the census were analysed in a

tenth of the time that the previously used method would have taken. This

success prompted Hollerith to seek other applications for this technology; he

founded a company for the production of accounting machines, which sub-

sequently became the International Business Machine (IBM) Corporation.

After 1937, punch-card technology, together with the electrical mecha- nisms designed by Hollerith, proved to be so efficient that many engineers suggested combining it with the more recent model of calculating machine that was being designed by Howard Aiken, a professor of applied mathemat- ics at the University of Harvard. Aiken drew up a plan to develop mechanical calculators that could work on mathematical problems in a sequential man- ner. Subsequently, with the support of the International Business Machine Corporation and the University of Harvard, and with the help of four co- workers from IBM, he built the first modern computer. Named Automatic Sequence Controlled Calculator (also know n as the Har vard Mark I computer), this machine was unveiled at Har vard in August 1944.

Instructions and data were fed into the computer by means of a punched paper tape. The digital logical components were constructed on the basis of electrical, electronic, and mechanical principles, while the operations were controlled by means of switches and relays. The Mark I was capable of per- forming 200 additions per minute, and could work out sines and cosines in about one minute. Compared with modern computers, this machine was huge and somewhat limited in terms of speed and flexibility. Nevertheless, it was the first machine that truly possessed all the characteristics of a modern computer.

The first electronic computer suitable for generic use was built in 1946 by Eckert and Mauchly of the University of Pennsylvania. The rapid-response electronic components used in their ENIAC (Electronic Numerical Integrator and Computer) enabled two 10-digit numbers to be multiplied in 30 ms – a hundred times faster than the Mark I. The ENIAC was made up of 18 000 valves and 6000 switches so that it could perform 5000 additions per second.

It was a huge machine, occupying the entire perimeter of a 9 x 15 m room, weighing 30 tons and requiring 80 fans to prevent its components from over- heating.

Technological progress in electronics continued to gather speed. In the 1960s and 1970s, when computers became capable of high-powered calcula- tion, digital signal processing (DSP) began to open new doors in the world of science.

In many cases, the signals that are of clinical and scientific interest are produced by sensors that detect seismic vibrations, images, sound waves, etc.

DSP is a fusion of mathematics, algorithms, and the techniques used to analyse these signals once they have been converted into digital form. This analysis may have various objectives, such as image enhancement, speech recognition and generation, or data compression for transmission and stor- age.

The sectors in which DSP technology now find application are many and

various, as are the objectives for which it is used:

• Space sector: enhancement of images from space, data compression, analyses of data from space probes using ‘smart’ sensors.

• Commercial sector: compression of sounds and images for multi-media presentation, special effects for films, video conferences.

• Telephone sector: compression of voice and other data, echo reduction, signal multiplexing, filtering.

• Military sector: radar, sonar, encrypted communication.

• Industrial sector: oil and mineral prospecting, process control and moni- toring, non-destructive testing, CAD and design instruments.

• Scientific sector: seismic recording and analysis, acquisition of various types of signal, frequency analysis, modelling and simulation.

• Medical sector: diagnostic imaging (computerised tomography, nuclear magnetic resonance, ultrasonography), image storage and retrieval, elec- trocardiographic and encephalographic analyses [47].

The Analog and the Fully Digital Pacemaker

The Analog Pacemaker

Standard stimulators are only concerned with establishing whether or not the endocavitary potential is present; no attempt is made to evaluate it quali- tatively. The signal that the pacemaker receives from the lead is an analog electrical signal; that is to say, it is a signal which varies over time. The ana- log signal can assume any of the infinite values encompassed within a cer- tain interval. Proper functioning of the pacemaker and the delivery of appro- priate stimulation therapy depend exclusively on the exact detection of the intrinsic cardiac signals – when they are present, of course.

In order to achieve good sensing, the hardware of the pacemaker is equipped with filters that screen out useless signals, while the software pro- vides a range of parameters, such as refractory period, blanking period, sen- sitivity and polarity of sensing, in order to mask or avoid the detection of undesired signals. The refractory periods and the blanking periods do not eliminate the problem of the disturbance which is added to the useful signal;

they simply prevent the device from ‘seeing’ it, which, in itself, is incorrect. In reality, the device should always be able to ‘see’ and to interpret the cardiac rhythm in real time. In spite of the filters and careful programming of the pacemaker, the difficulty of correctly detecting the signals remains the weak point of the system and reveals the limits of the technology currently in use in cardiac stimulators, which handles the signals analogically.

Pacemakers utilise very little of the information contained in the intracar-

diac signal. Indeed, illustration of how the input signal into the pacemaker is

processed in an analog system reveals that this is the only information utilised.

The signal passes through an analog amplifier, which increases its ampli- tude. The amplified signal passes through an analog filter that screens out the undesired components and lets the useful ones through. Thus amplified and filtered, the signal reaches the sensing circuit, where a comparator com- pares it with a threshold value determined by the sensitivity programmed.

The sensing circuit is of paramount importance, in that all decisions regard- ing the pacemaker’s stimulation therapy are based on the output from this unit. The device decides whether to deliver the impulse, to switch the mode of stimulation, or to activate one or more of the many functions available, solely on the basis of sensing. Without the sensing circuit, the pacemaker would simply be a generator of electrical impulses.

Once sensing has been carried out, the rest of the information contained in the signal is lost, that is to say, it is not utilised, except for periodic mea- surements of the amplitude of the atrial or ventricular signal. The signal provides information only when it exceeds the sensing threshold (pro- grammed sensitivity), thereby allowing the pacemaker’s microprocessor to calculate the cardiac cycle and to establish which pacing therapy to activate.

Of the morphology of the signal, nothing remains. Some pacemakers memo- rise intracardiac signals (EGM) only for diagnostic purposes. This memori- sation is ‘passive,’ in that the device does not utilise the signal to ‘decide’

which therapy to deliver.

Pacemakers based on analog technology do not have the necessary equip- ment to ‘read’ the morphology of the signal; they cannot therefore discrimi- nate between a sinus P wave and a retrograde P wave, a tachyarrhythmia or a ventricular far-field wave. With analog technology, cardiac events are classi- fied solely on the basis of the time lag between consecutive sensing events.

The Fully Digital Pacemaker

The term ‘digital’ must not be confused with ‘electronic.’ The pacemakers in current use receive analog signals, which are detected analogically.

DSP technology, as applied to cardiac stimulators, may be regarded as the digital processing of intracardiac signals [8-10]. And this is the great innova- tion: the input signals entering the pacemaker are converted into digital form before any decision is taken as to the appropriate therapy to be deliv- ered!

A digital signal is one that assumes a quantifiable number of values in a certain interval (meaning that the values can be counted). Unlike the analog signal, which is continuous, the digital signal is discrete.

In order to convert an analog signal into a digital one, its value needs to

be measured at regular time intervals (sampling). The result of this sampling

is a sequence of numerical values – graphically, a sequence of equidistant points along the time axis. The profile of the original signal is reconstructed by ideally joining up the points obtained. The number of points, and there- fore the degree of resolution, depends on the sampling frequency, i.e. on the number of measurements taken during a given unit of time.

Once the intracardiac signal has been converted into digital format, the pacemaker has to be equipped with the capability to ‘read’ the information it contains, just as a cardiologist is able to interpret the cardiac events recorded on an ECG. The task of enabling the pacemaker to read and put together the information yielded by the morphology, amplitude, and duration of the digi- tal signal is left to the software designers.

The first stage is a standard amplifier that serves to increase the ampli- tude of the signal in order to make it compatible with the digital conversion system: from this stage on, every analysis of the signal is based on software algorithms. The signal is converted into digital form by means of an analog- digital converter, which samples the input signal. The quality of the sampled signal depends on the sampling frequency, that is to say, on the number of samples taken per second. A high sampling frequency means high resolu- tion, high quality of the reconstructed signal, and therefore reliability.

With DSP technology, all data are in numerical format and can be used by a high-speed microprocessor in the pacemaker to process and bring together information of clinical and technical interest. The results obtained have a direct bearing on the optimisation of the follow-up in terms of quality, time, and reliability, in that the stimulator can provide more reliable diagnostic and therapeutic suggestions [11].

Clinical Benefits

With the increasing number of applications for devices for bradycardia, tachycardia, and non-conventional applications, the complex arrhythmias and signal morphologies present a challenge to analog-based systems. It become evident that digitising of these signals opens a new word of opportu- nities. Inappropriate classification of cardiac signals by a device leads DSP may offer reliable recognition of local versus remote signals. In ICD therapy, electrogram morphology discrimination offers an additional approach to improve discrimination of supraventricular tachyarrhythmias from ventric- ular tachycardia.

Further automation of many pacemaker functions can be realised. This is

especially true for automation of programmable parameters related to tech-

nical functioning of the device, relieving physicians from time-consuming

procedures.

Conclusions

The advent of digital technology in implantable cardiac stimulators will open up new frontiers for the automatic analysis and diagnosis of endocar- dial signals. This will dramatically increase the reliability of the therapies delivered and the amount of information that can be processed and stored for clinical and scientific purposes. In the next few years, we can expect spe- cific algorithms to be developed for morphological discrimination of the far- field R-wave in the atrium and the retrograde P-wave. The use of blanking periods will gradually be phased out, since the system will instantly classify what it receives from the implanted electrodes, without needing to mask undesired signals. Such devices will really and continuously monitor every cardiac event.

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