Apparatus for telemetering of ekg signals from mobile stations

[57] ABSTRACT

A system for the radio transmission of EKG signals to a hospital from an ambulance. After an initial limited transmission of EKG waves, no further signals are transmitted unless the EKG (R—R) rate departs significantly from the initial rate. The EKG signals modulate a voltage controlled oscillator. A code is associated with the transmission to identify patient, rescue vehicles and hospital.

Claims, 8 Drawing Figures

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Patented Nov. 27, 1973

3,774,594

Sheets-Sheet i

EKG

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<td> v+_________H ______ FIG .4 o u u FIG. 5 DIFFERENTIATOR FIG 5Λ -----/-----1&gt;----</td><td> INVENTOR ROBERT J. HUSZAR ATTORNEYS</td>

Patented Nov. 27, 1973

3,774,594

Sheets-Sheet 2

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Patented Nov. 27, 1973

3,774,594

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Sheets-Sheet 3

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FIG 6

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INVENTOR

ROBERT J. HUSZAR

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ATTORNEYS

3,774,594

APPARATUS FOR TELEMETERING OF EKG SIGNALS FROM MOBILE STATIONS

This is a continuation of application Ser. No. 819,226, filed Apr. 25, 1969, and now abandoned.

This invention relates to telemetering and more particularly to the transmission from rescue vehicles of information correlated to a physiological parameter. According to the invention, a system is provided for the monitoring and transmitting of electrocardiogram signals (EKG) from ambulances or other rescue vehicles while a heart attack victim is en route to hospitals. The invention comprehends the transmission of still other parameters, such as respiration rate, blood pressure, and the like.

A great number (estimated at 400,000) of persons die in the United States each year of heart attacks. Of this number, it is estimated that approximately fiveeighths of such deaths occur outside of a hospital and after one hour of the initial attack. It is further estimated that perhaps three-fourths of these deaths would be preventable, provided proper diagnosis and medical attention could be made available rapidly enough. Considering those instances where medical attention is obtained, the victim must be brought from the locale of the attack to a hospital for treatment. Such treatment requires expert analysis of the particular heart condition before proper ameliorative procedures can be administered. The more rapid the analysis of the heart condition, the more rapidly treatment can be provided. In the past, monitoring of the heart’s EKG signals was generally not possible until the victim arrived at a hospital. If cardiac arrhythmias (heart standstill or fibrillation) occurred during ambulance transit, they remained undetected until arrival. By the practice of the present invention, EKG signals are radioed for analysis by a physician at the hospital. In severe cases, the doctor can then radio the ambulance and direct the attendants in emergency external heart massage in order to keep blood flowing and prevent brain damage or death due to oxygen starvation.

Accordingly, the main purpose of the present invention is to reduce the time required for initial analysis of a heart condition. This is achieved by initiating transmission of EKG data from an ambulance to a hospital immediately upon placing the victim in the ambulance, as opposed to the general practice of having to wait until the patient is in the hospital before conducting EKG tests. Because of the great demand for limited space in the wireless spectrum, continuous transmission from a large number of rescue vehicles is not practical. The present invention, accordingly, permits the minimum time use of the airways for a single emergency. This in turn enables a greater number of (ambulance) transmission stations to use the same band for transmission.

IN THE DRAWINGS

FIG. 1 is a schematic view of the major portions of the transmission system of the invention.

FIG. 2 is a detailed schematic view showing the automatic transmission system of the invention, located in an ambulance.

FIG. 3 represents a normal potential v. time EKG.

FIG. 4 represents the potential v. time curve in a certain line in the system of FIG. 2.

FIG. 5 represents a voltage generated by a portion of the system of FIG. 2.

FIG. 5α represents the voltage applied to the base of transistor Q₆ in FIG. 2.

FIG. 6 shows the face of the meter M of FIG. 2.

FIG. 7 illustrates the receiving equipment located in a hospital.

Referring now to FIG. 1 of the drawings, the illustration relates to the taking of an electrocardiogram of a patient either immediately before or immediately after he has been placed into an ambulance or other rescue vehicle for transfer to a hospital. An electrocardiogram may be defined as the amplification and pictorialization of a naturally occurring ±3mv potential generated in a tissue adjacent the heart. It is often taken by coupling the left leg to the common terminal of a differential amplifier and connecting the remaining two leads, respectively, to the right and left arms. After being amplified by amplifier A, the EKG information is fed to a radio transmitter whenever switch SW is closed. As indicated in FIG. 1, the EKG signals modulate the output (carrier) of a voltage controlled oscillator. The thus modulated VCO oscillator signal passes to switch SW. The other portion of the EKG is fed to a signal squaring circuit. The output from the square wave generator actuates two relays, RL] and RL₂. The former relay is coupled to a ramp generator and functions to ground the potential generated by the ramp generator whenever the relay is actuated, resetting it to zero. The latter relay RL2 maintains its associated switch closed during the major portion of the EKG signals, and becomes deenergized to thereby open the switch upon the QRS portions of the EKG signals. When closed, a voltage +V is shunted to ground by the RL₂ switch. Relay RL₃controls a normally open switch which shunts +V to ground whenever the voltage generated by the ramp generator exceeds a predetermined amount, here four volts. A fourth relay RL₄ actuates radio transmission with SW whenever either one of two conditions occur. The first condition occurs when the voltage generated by the ramp generator exceeds a predetermined amount, here 6 volts. In the second condition neither of the switches associated with relays RL₂ or RL₃ are closed to thereby shunt +V to ground and +V then actuates RL₄. By this arrangement, as will be apparent from the more detailed discussion to follow, the EKG is only transmitted (during the automatic transmission mode of operation of the system) whenever the EKG heart rate departs from an initial rate. The initial rate is whatever heart rate the victim had when the EKG was first taken. The subsequent rate is any later rate the victim displays while en route in the ambulance to the hospital. It will be appreciated that this journey occurs during the initial and often the most critical portion of the heart attack.

Referring now to FIG. 2 of the drawings, the patient is connected to the system through three integrated circuit linear amplifiers (A,, A₂, A₃) connected in a modified differential amplifier, voltage follower configuration. The construction of such amplifiers A_n A₂, A₃ is well known and forms no part of the invention. The amplified EKG signals are applied over wire 1 to a voltage controlled oscillator (VCO). The EKG modulates, hcle by frequency modulation, the VCO output over a 1,400 Hertz (Hz.) band between 600 and 2,000 Hz.

The VCO converts the EKG signal to an FM subcarrier compatible with the transmission capabilities of the radio transmitter. The VCO is a complimentary symmetry astable multivibrator which is driven directly

3,774,594 from the output of the EKG amplifier. The transistors Q16, Q17 adjust the base bias voltage of the transistors Q14, Q18. This adjustment alters the voltage at which the transistors switch, hence the duration of their ONOFF cycle, hence the multivibrator’s frequency. Two adjustments are provided: R₄₃ for zero input or center frequency, the other, R₄₂, adjusts the slope of the VCO transfer function, input voltage to output frequency. These controls are not independent. The VCO square wave output is filtered by a single pi section to remove high frequency harmonics.

Amplified EKG signals are also applied over wire 2 to the base of transistor Q_P The combination of Qi and O₂ with resistors R_x-R₂-R₃ is proportioned such that Q_xis completely cut off while O₂ conducts fully. The drop across R₃ provides the cut-off level of Q_P

The QRS portion of the EKG wave is quite large as compared to the rest of the complex, note FIG. 3. This portion causes Q, to start to turn on which in turn lowers the base and emitter currents of Q₂. The regenerative action then switches Q, into strong conduction until the peak recedes; then Q, switches off and Q₂switches on providing a sharp square wave across R₂and R₃ for the period when the peak exceeds the value of voltage across R₃. This action produces square waves in the lower conductor 2α, note FIG. 4.

Elements D₃, R₈, C₃, Q₅ and Re provide an integrator fed from source V, whose period is adjustable by R₈. The output of this integrator is linear. The integrator is a ramp generator whose output is a potential which is employed as a reference potential.

When the square wave voltage across R₄ and R₅changes, condenser C_x differentiates this voltage (FIG. 5a) to provide a sharp positive pulse across R₉ to fire Q_e which discharges C₃ to reset the ramp to zero.

Diode Dj conducts the ramp voltage over wire 3 to condenser C₅, which retains the last peak voltage obtained from the ramp. Transistors Q₇ and Q₈ are Darlington connected to provide an emitter follower to operate meter M which indicates the last peak voltage of the integrator.

When the patient is first connected to the system, an operator holds push button PBj, PB₂ open to prevent the relay RL₄ from becoming energized. Radio transmission is thus precluded. He then adjusts R₈ so that the patient’s heart rate (R—R) causes the meter M (initial heart rate) to read 5 volts. This should only require several EKG signals. PB_t and PB₂ are released after which PB₃ is depressed. This causes monostable multivibrator MMV to start up, the action of which is to throw normally open transistor Q_n into conduction for about 1 minute. Radio transmission takes place for this initial timed period. During this time, the EKG is recorded at the hospital receiving station. After this time, with no change in R—R rate, the system is quiescent. PB₃, Q_e, Qio and Q_u form a 1 minute initial transmit circuit. When the circuit has not been operated for some time, Q₁₀ is full ON due to bias current through R_le. The low collector voltage of Q_lo prevents Q_n from receiving bias current so it is OFF. The ratio of R_1S and Ri₈ is such that 0₈ does not receive bias current and it is OFF. Capacitor C_e is charged to the supply voltage through R_h. Depression of PB₃ connects C₈ to the base of Q_e. C₈ discharges providing bias current for Q_e turning it ON. The collector voltage of Q_e drops, C₇ couples this drop to the base of Q,_o and it turns off. The collector voltage of Q₁₀ rises toward the supply voltage and Q_n turns on. Collector current of Q_u flows through wire 8 connected to relay RL₄. Relay RL₄ turns on operating the radio transmitter. Capacitor C₇ charges through R_le. When the voltage on C₇ reaches the emitter voltage of Qio, Qio turns ON. The collector voltage of Q_lodrops turning off Q_xl. When Q_n turns OFF, relay RL₄turns OFF and the transmitter turns OFF. Q₁₀ turning On also turns OFF Q₉. The circuit is ready for the next depression of PB₄.

During quiescent operation the rise time of the first EKG pulse fires Q, and the square wave voltage in line 2a and across R₄ and R_s falls, producing a negative pulse on the base of Q_e which is already cut off. When the EKG pulse falls, Q! cuts off, Q₂ fires and a positive pulse is applied to the base of Q₆ to reset the ramp to zero by shunting its voltage to ground.

If the R—R period between each succeeding EKG pulse remains substantially constant (permissible deviation to be described later) the ramp voltage will arrive at approximately 5 volts after each integrating period.

Two actions should be observed: (1) should the heart rate increase from the preset normal, the ramp voltage will reset to zero before it reaches 5 volts; and (2) should the heart rate decrease, the ramp voltage will build up and exceed 5 volts prior to the next R peak.

The ramp voltage is applied over wire 4 to two Zener diodes, Zi and Z₂, rated at 4 volts and 6 volts. This difference defines a zone within which the R—R rate can fluctuate without activating automatic transmission. Zener Z_x conducts the ramp voltage to R_lo when the ramp exceeds 4 volts (a heart rate of 120 per minute). Zener Z₂ conducts the ramp to R_n when the ramp exceeds 6 volts (a heart rate of 30 per minute).

The voltage across R₄ and R₅ during the peak of the QRS wave is reduced to the point that Q₃ cuts off. Current now flows from +V_X through R« over wire 5 to Q₄. However, since the voltage on the ramp (from the last QRS to QRS period) exceeds 4 volts, Zener conducts current through wire 7 to R_lo and through R₇and C₂ and the base of Q₄ holding it conducting. Condenser C₂ provides a slight delay before Q₄ cuts off. When the EKG main pulse (QRS) decays, Q₃ conducts, pulling current on wire 6 below the conduction point of D₂. The rise of the square wave on R₄ and R₅ is differentiated by C, to apply a positive pulse to the base of Q_e which now resets the ramp to zero. Zener Z_xopens and Q₄ cuts off. Thus, during quiescent operation, either Q₃ or Q₄ bleeds V_t to ground, preventing it from triggering the gate of SCR_V

Should an EKG pulse (QRS) appear prior to the ramp attaining the 4 volt level (quickening of the heart rate) Q! conducts on the rise of the QRS complex causing Qz to reduce the voltage across R₄ and R_s, cutting off Q₃. Zener Z_t has not yet conducted and hence Q₄is cut off. Current now flows from +V, through R_e, wire 6, through D₂, through R_t2, through PB₂, to R_la, and to the gate of SCR,, which now fires and stays conducting. SCR, conduction operates the transmitter relay RL₄ to close switch SW turning on the transmitter.

If the R—R period lengthens, due to a slowing or failure of the heart to produce a normal QRS (cardiac standstill), the ramp will continue to build up until Zener Zj fires at 6 volts which immediately places the ramp voltage through R₁₂ to the gate of SCR₍, which fires and initiates radio transmission.

The meter M provides visual means for monitoring the heart rate and also setting into the system the initial

3,774,594 heart rate in accordance with the patient’s cardiac status. When the heart rate departs from the preset value, it is extremely urgent that the hospital to which the ambulance is proceeding be immediately alerted, advised of the ambulance identity code and start receiving the EKG pattern. At this crucial period, time does not permit coding transmission. Therefore, this invention provides for simultaneous transmission of the EKG signals and identity codes. SW provides the option for continuous (C) or automatic (A) EKG transmission. Small bypass capacitors C_B of 2.3 mf placed, as indicated in FIG. 2, served to improve operation by shunting radio frequency oscillations to ground.

The audio spectrum may be divided into several bands, 300 Hertz to 600 Hertz, for identity coding, and the band from 600 to 2,000 Hertz for the EKG modulation. Q₁₂ and Q₁₃ operate a code oscillator circuit. Code in FIG. 2 designates a code for, say, a particular rescue vehicle, while Codes 2 and 3 (of corresponding circuitry) designate the hospital and the patient. Q_I2 and Q₁₃ form a high gain amplifier. Voltage fed from the collector of Q₁₂ through R₂₈ is in phase with the signal at the base of Q₁₃. The tuned circuit formed by L, and C_I5 provide the necessary frequency selection to cause oscillation. R₃₀ is a resistor whose resistance changes with its power dissipation. This provides ambient temperature compensation of the oscillator frequency, D₃, R₃₂, R_M form bias adjustment circuit that provides automatic output amplitude compensation.

A tone generator and a lamp connected in parallel and Q₁₉ provides an audible and visual signal synchronous with each QRS complex.

FIG. 6 illustrates one layout of the meter dial. Three zones are provided to facilitate initial set in and rapid readout.

Referring now to FIG. 7 of the drawings, the receiver/demodulator is shown. This device, which is usually located in the hospital, consists of a demodulator which recovers the EKG signal from the carrier and provides an output compatible with standard EKG monitoring devices. The demodulator input is a two-stage (Q^, Q₂₁), high gain, saturating amplifier. The amplifier output is a square wave at the input frequency. The square wave is differentiated (C_lo, R₃₃) and rectified (D_n) to produce a positive pulse train. These pulses trigger a monostable multivibrator (Q₂₂, Q^, Q ^,,) of 100 microsecond duration.

The monostable multivibrator output is integrated by a 200 Hertz low pass filter (F,). The filter output drives a differential amplifier (Q₂₄, Qk, Q_2e)· The amplifier output is attenuated 1,000 times to permit the use of standard EKG monitors. The inverting input to the differential amplifier transistor is used to adjust (Re₂) the output voltage to zero when the subcarrier frequency is at the EKG zero input value (center frequency).

Control of the monostable multivibrator pulse duration is also provided (R₅₉). This control adjusts the slope of the demodulator transfer function, input frequency to output voltage. These two controls are not independent.

A transmitter which has been found satisfactory is GE model “Royal Professional”, 30 w. solid state. Similarly, a suitable commercially available receiver is GE model DM76KCU.

A tone actuated relay and tape recorder are coupled to the receiver as illustrated at FIG. 7. A Bramco multichannel reed relay device is satisfactory. The various reeds are resonant to several distinct frequencies and are accordingly well suited to respond to the codes sent with the EKG information. It will be recalled that the filtered VCO (FIG. 2) output consists of square waves whose spacing (frequency) is determined by the EKG signals, When received, these waves contain the audio frequencies corresponding to the EKG signals, the output at R₇₀, R_7t of the demodulator being the fully reconstructed EKG, such as shown at FIG. 3.