IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, MARCH 1977
188
Digital Electronic Control of Automatic Ventilators FRANK A. ENGELMAN, JR., AND ALBERT M. COOK Abstract-We have developed a digital system for use in ventilator control. The basic principle of this system is that every respiratory cycle is divided into a fixed number of counts. The control of the ratio of inspiration time to expiration time is accomplished by selecting a variable time out of the fixed time interval and designating it inspiration time. The remaining counts become expiration time. Respiratory rate is adjusted by varying the total time interval, and the ratio is adjusted by selecting a different inspiration time. The output of a digital-to-analog converter driven by the counters is adjustable and proportional to tidal volume. By inserting read only memories
between the ratio scalars and the digital-to-analog converter
input, a large number of different inspiration waveforms are generated.
INTRODUCTION Respiratory therapy depends in large measure on the existence of various devices such as those intended for automatic ventilation of the lungs. These devices are typically used for either short term assist or prolonged artificial respiration. Electronic control has been used in varying degrees on ventilators in recent years. Of the 81 ventilators listed in Mushin et al., 15 employ some means of electronic control [1]. In addition, a number of electronically controlled devices have appeared since the publishing of this text. In most cases, electronic control is implemented to provide more accurate and repeatable control signals than gas-powered or electromechanical ventilators [2]. Both analog and digital control circuits have been employed in ventilators, with the latter providing increased accuracy. The subject of this paper is a digitally controlled ventilator which has an accuracy of 1 percent for rate, ratio, and sigh functions, and within 5 percent for volume controls (normal and sigh) with independence of rate, ratio, and tidal volume controls. OVERVIEW OF SYSTEM The basic principle of the system is that every respiratory cycle is divided into a fixed number of counts. This provides a stable time base on which the control of ratio is based by selecting a variable time out of the fixed time cycle and designating it inspiration time (Ti). The remaining time in the total cycle then becomes the expiration time (Te) as shown in Fig. 1. The respiratory rate is adjusted by varying the total time interval (T), and the ratio is adjusted by selecting a different inspiration time. Thus, the ratio of the system can be easily adjusted independently of rate. The tidal volume adjustment is also independent of the other controls. An additional feature, incorporated here as in many other ventilators, is a "sigh" function. This feature is designed to overcome the phenomenun of atelectasis by providing a larger, longer breath at selected tii..es. Examples of the independent control of rate and ratio are shown in Fig. 1. In this figure, the ratio is 1: n - 1 and /T is the respiratory rate. The value of T is varied independently of the value of n. For this reason, rate and ratio are independent. Fig. 2 is a block diagram which illustrates the implementation of the basic principles discussed above. This system is designed to be used with a linear driven piston [31. The output of the control system is a voltage which is varying during Ti. During Te this voltage is reduced to 0 V. The position of Manuscript received May 1, 1974; revised June 23, 1975 and October 15, 1975.
This work was supported
by the Sutter Hospitals Medical
Research Foundation, Sacramento, CA. The authors are with the Bioengineering Laboratory, Department of Electrical Engineering, California State University, Sacramento, CA 95819.
the piston is proportional to the voltage applied. In this manner, the system provides a volume versus time drive signal. It can be seen from Fig. 2 that the system consists of four major blocks or functions. The rate block, C-1, contains the master oscillator (MO), rate scalar, and operator controls. The MO provides a stable reference frequency for all the control functions. The rate scalar, in conjunction with the operator control S-l, provides the proper frequency into the ratio block. C-2 also consists of rate scalars controlled by the operator switch S-2. These scalars provide the variable time required by the volume block C-3 to develop an independent ratio. The output L provides a "reset" pulse to the volume block C-3 and the sigh block C-4, indicating the completion of a respiratory cycle and the start of a new cycle. Thus S-1 selects respiratory rate (1T) and S-2 selects respiratory ratio 1: n - 1 shown in Fig. 1. The volume block, C-3, provides operator adjustment of the analog drive signal through the use of a decade voltage divider operating on the output of a digital-to-analog converter (DAC). This adjustment is independent of all others. The remaining block C-4 provides operator control of sigh rate, sigh/hour, sigh/interval, and sigh volume through use of switches S-4-S-7, respectively. C-4 is interconnected with C-1 and C-3 using several of the same components, reducing overall system complexity. It should be noted that C-4 also contains a rate scalar to provide a sigh/rate function. DETAILED CIRCUIT DESCRIPTION The rate scalar is a device which provides an output frequency scaled by the preset inputs. If the input frequency is equal to the proportionality constant built into the scalar, the output frequency will be equal to the preset inputs. In this case, by cascading two scalars, the built-in proportionality constant becomes K/4096 and the output is equal to the input multiplied by this proportion where K is equal to the preset inputs. Since the input frequency is equal to 4096 Hz, K/4096 becomes simply K Hz as shown in Fig. 2. Since the range of respiratory rates provided is 1 to 60 breaths per minute (BPM), the ROM converts a rate of 60 BPM to a value of K = 4096. In this manner, 30 BPM is equivalent to K = 2048, etc. (see Table I). Thus, the ROM's and decoders provide an interface between clinical parameters and desired control settings. The method of operation is that BCD data corresponding to specific rates from the operator switch S-1 address the ROM, and the K codes stored at the appropriate memory locations are used to control the scalars. The decoders also provide for the use of the same scalars to determine length of sigh when this mode of operation is selected. It should also be noted that the rate input from S-1 is overridden by the sigh/normal control signal from C-4. Additional codes have been stored in the ROM, so that the proper K numbers are also available for sigh operation. C-2 provides operator control of ratio. This circuit is similar to C-1 in that it uses rate scalars and a ROM. The operator ratio switch (S-2) settings are decoded by the memory which then provides the proper codes to the preset inputs of the N scalars. The rate scalar also provides a fixed output which, in this case, is K/4096 = L. A one-shot is used to provide a uniform pulsewidth independent of frequency as a "start of inspiration signal" (SOI). Typical L values are also shown in Table I. The N circuit provides a variable output (KN)/4096 proportional to the preset input settings. If the K input is 4096 (60 BPM), the output will be N, and if the K input is 2048 (30 BPM), the output is 0.5 N. In a like manner, all rate settings will appear as a fractional N output (see Table I). Block C-3 is designed so that 256 pulses on the KN/4096 input line determine the inspiration phase. Therefore, if N= 512 (ratio= 1: 1) and the rate equals 60 BPM, the first 256 pulses will occur in 0.5 s. These will be used by C-3 to determine inspiration time. The rest of the pulses up to
COMMUNICATIONS
189
Volume (drive voltage)
256 pulses I
(n-1) 256 pulses
MC
-
3
Tidal Volume
I} time
Fig. 1. Volume versus time waveform illustrating independence of respiratory rate and inspiratory/expiratory ratio. Rate = 1/T, ratio = 1:n - 1.
I
Fig. 2. Overall block diagram of digital electronic control system showing major components (C-1-C-4) and detailed circuit implementation. TABLE I Rate
(BPM) K (Hz) Ratio N (Hz) L (Hz) KIV/4096 60 60 30 30 15 15
4096 4096 2048 2048 1024 1024
1:1
1:4
1:1 1:4 1:1 1:4
512
1280
512 1280 512 1280
1 1 0.5 0.5 0.25 0.25
512
1280
256 640 128 320
Insp. Time
(seconds) 0.5 0.2 1 0.4 2 0.8
pulse number 512 will be counted for expiration. From the previous discussion of C-2, at a rate of 60 BPM, an SOI pulse will appear at 1-s intervals. In this case, SOI occurs at the 512th pulse, thereby causing the end of expiration and the
start of inspiration. Other values of inspiration, corresponding to different rates and ratios are shown in Table I. The purpose of C-3 is to provide adjustable amplitude waveforms to the drive circuitry. The waveforms are equal to zero during expiration, allowing the piston to return to the start position. The flip-flop and gate enable the counters when the SOI pulse occurs. The flip-flop is reset after 256 pulses have
been counted. Note that the SOI signal is independent of ratio settings and of any functions in the C-3 block. The output of the counter is fed to a digital-to-analog converter (DAC) which provides a linearly increasing ramp from d to 10 V in 256 steps of 40 mV each. These steps are so small that the drive signal appears as a smooth analog ramp waveform. The output of the DAC is fed to two attenuators, one for tidal volume and one for sigh volume. The purpose of S-3, which is mechanically linked to the tidal volume attenuator, is to provide independent adjustment of tidal volume. Likewise, S-7 provides adjustinetit of sigh
190
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, MARCH 1977
volume. The attenuators are voltage dividers which are adjustable in discrete steps of 10 mV. Because the volumes are set to be 1 V = 1 1, the operator volume adjustments are calibrated directly in steps of 10 ml. The voltages out of the attenuators are fed to a programmable op-amp which allows the selection of either sigh or normal volume signals to be fed to the drive circuits. This device also allows magnitude scaling required by the drive circuits. -It should be noted that if a ROM were to be inserted between the counters and the DAC, waveforms other than a simple ramp could be obtained. By proper choice of codes in the ROM, it would serve as a "look up" table for any of several waveforms. C-4 is designed to provide a larger breath of long duration (artificial sigh) according to the operator-controlled switches S-4-S-7. Rate scalers provide an output proportional to the settings from the ROM which has been preprogrammed to decode operator switch S-5 and can select any sigh rate from 1 to 49 per hour in steps of one. These circuits provide a "set" function on the sigh/normal control line through a flipflop. This line is used by C-l to switch the ROM address lines from the normal rate switch S-l to the sigh length switch S-4. In this way, ratio is not affected. The sigh/normal control line is also fed to C-3 where it is used by the programmable op amp to select tidal or sigh volume signals. A 4-bit counter, in conjunction with a decoder, and operator switch S-6 provide a reset function on the sigh/normal line when the proper number of sighs has been achieved. Switch S-6 may be set for 1, 2, or 3 sighs per interval. An interval is defined as the active sigh time as selected by switch S-5.
SUMMARY The control system described here has several advantages over previous electronic systems used in ventilator control. The use of a fixed clock and variable rate scalars eliminates problems associated with variable RC oscillators such as nonlinearities, instabilities, and drift. This feature provides extremely stable and accurate control of respiratory rate. In addition, field maintenance is simplified since there are no trim adjustments or calibrations necessary and circuit replacements do not require initial adjustments. Further, the principle of designating a fixed number of counts as inspiration time and varying the pulse rate to control respiratory frequency coupled with a variable number of counts for expiration insures independence of rate and ratio controls, and the use of ROM's provides for easy interfacing between clinical parameters and control signals. The wide dynamic range necessary in ventilators (5-60 BPM) makes the use of RC waveshaping circuits for volume curves impractical when the waveshape must be the same for all rates. This problem has been solved by using ROM's as digital waveform generators capable of generating a large number of frequency-independent waveforms. This solution also avoids problems associated with setting break points and slopes in diode function generators. In addition, an entirely new set of waveforms may be obtained by the substitution of two integrated circuits. This feature is particularly useful in light of the current controversy regarding the choice of a volume time waveform which is "best" for a given clinical situation. (See [4], for example.) The subject of ventilator safety has been widely discussed in recent years. In particular, dependence of ventilation parameters on line voltage fluctuations during "brownouts" or electric power failures has been a major concern. In this regard, the design described herein represents an improvement over previous designs since the digital system is not affected by widely varying input (line) voltage. In addition, operation during short (30 min or less) power failure is assured by floating the entire system on batteries. In the design of systems such as the one described here, a major concern is the existence of single point failures. A
number of such points exist in our system in the master oscillator and associated counter chains. In the current prototype, alarms are included which indicate abnormally low or high pressure, battery discharge, fail to cycle, and minute volume and leak. In addition, a backup system is planned which would feature a highly reliable oscillator operating at a fixed rate and ratio. Such a simple system has been used in life tests of the basic piston/cylinder/drive system with a high degree of reliability [3]. It should be noted that none of the currently used ventilators employ such a backup system. Due to the basic design, the most likely (by a wide margin) failure mode is failure to cycle rather than cycling at too high a rate. The remainder of our system (motor drive) has redundancy -built in which provides for operation in the event of a partial failure. In summary, we have developed a control system for use on ventilators which is time-cycled, has independence of controls, and is stable for variations in patient resistance and compliance. The overall accuracy of all digital controls is ±1 percent and analog control (volumes) is within 5 percent compared with the proposed ISO standards of ±10 percent
[5].
ACKNOWLEDGMENT
We wish to thank J. Fallon, J. Hathaway, and Drs. D. H. Gillott and J. G. Simes for assistance with this work, and J. Vaszary for typing the manuscript.
REFERENCES [1] W. W. Mushin, L. Rendell-Baker, P. R. Thompson, and W. W. Mapelsen, Automatic Ventilation of the Lungs, 2nd ed. Oxford, England: Oxford Blackwell, 1969. [2] D. W. Hill, "Recent developments in the design of electronically controlled ventilators," Der Anaesthesist, vol. 15, pp. 234-238, 1966. [3] D. H. Gillott and J. G. Simes, "Translational drive systems to facilitate ventilator control," in Proc. 27th ACEMB, vol. 139, 1974. [41 L. Jansson and B. Jonson, "A theoretical study of flow patterns of ventilators," Scand. J. Resp. Dis., vol. 5 3, pp. 237-246, 1972. [51 International Standards Organization, Sectional Committee ISO/TC 121/WG3. a) Standards for Anesthesia and Breathing Equipment, Proposed ISO. b) Standard Specifications for Breathing Machines for Medical Use.
A Sample/Long-Term-Hold Digital Circuit for Physiological Signal Monitoring RICHARD L. MASON Abstract-An uncomplicated circuit to sample a signal and digitally store its value is given. A modification to store the maximum or minimum value attained by the signal is also presented. The drift for these circuits is small, and the input signal may be obtained in
digital form.
INTRODUCTION A recent article by Johnson [I1 indicates the need to store a voltage for a long period of time and gives an analog circuit to perform that function. The circuit is stated to have a drift rate of 1 to 7 mV/h and to be simpler and less expensive than a digital circuit. Manuscript received December 12, 1975; revised February 27, 1976. The author is with the New London Laboratory, Naval Underwater Systems Center, New London, CT 06320.
188
Digital Electronic Control of Automatic Ventilators FRANK A. ENGELMAN, JR., AND ALBERT M. COOK Abstract-We have developed a digital system for use in ventilator control. The basic principle of this system is that every respiratory cycle is divided into a fixed number of counts. The control of the ratio of inspiration time to expiration time is accomplished by selecting a variable time out of the fixed time interval and designating it inspiration time. The remaining counts become expiration time. Respiratory rate is adjusted by varying the total time interval, and the ratio is adjusted by selecting a different inspiration time. The output of a digital-to-analog converter driven by the counters is adjustable and proportional to tidal volume. By inserting read only memories
between the ratio scalars and the digital-to-analog converter
input, a large number of different inspiration waveforms are generated.
INTRODUCTION Respiratory therapy depends in large measure on the existence of various devices such as those intended for automatic ventilation of the lungs. These devices are typically used for either short term assist or prolonged artificial respiration. Electronic control has been used in varying degrees on ventilators in recent years. Of the 81 ventilators listed in Mushin et al., 15 employ some means of electronic control [1]. In addition, a number of electronically controlled devices have appeared since the publishing of this text. In most cases, electronic control is implemented to provide more accurate and repeatable control signals than gas-powered or electromechanical ventilators [2]. Both analog and digital control circuits have been employed in ventilators, with the latter providing increased accuracy. The subject of this paper is a digitally controlled ventilator which has an accuracy of 1 percent for rate, ratio, and sigh functions, and within 5 percent for volume controls (normal and sigh) with independence of rate, ratio, and tidal volume controls. OVERVIEW OF SYSTEM The basic principle of the system is that every respiratory cycle is divided into a fixed number of counts. This provides a stable time base on which the control of ratio is based by selecting a variable time out of the fixed time cycle and designating it inspiration time (Ti). The remaining time in the total cycle then becomes the expiration time (Te) as shown in Fig. 1. The respiratory rate is adjusted by varying the total time interval (T), and the ratio is adjusted by selecting a different inspiration time. Thus, the ratio of the system can be easily adjusted independently of rate. The tidal volume adjustment is also independent of the other controls. An additional feature, incorporated here as in many other ventilators, is a "sigh" function. This feature is designed to overcome the phenomenun of atelectasis by providing a larger, longer breath at selected tii..es. Examples of the independent control of rate and ratio are shown in Fig. 1. In this figure, the ratio is 1: n - 1 and /T is the respiratory rate. The value of T is varied independently of the value of n. For this reason, rate and ratio are independent. Fig. 2 is a block diagram which illustrates the implementation of the basic principles discussed above. This system is designed to be used with a linear driven piston [31. The output of the control system is a voltage which is varying during Ti. During Te this voltage is reduced to 0 V. The position of Manuscript received May 1, 1974; revised June 23, 1975 and October 15, 1975.
This work was supported
by the Sutter Hospitals Medical
Research Foundation, Sacramento, CA. The authors are with the Bioengineering Laboratory, Department of Electrical Engineering, California State University, Sacramento, CA 95819.
the piston is proportional to the voltage applied. In this manner, the system provides a volume versus time drive signal. It can be seen from Fig. 2 that the system consists of four major blocks or functions. The rate block, C-1, contains the master oscillator (MO), rate scalar, and operator controls. The MO provides a stable reference frequency for all the control functions. The rate scalar, in conjunction with the operator control S-l, provides the proper frequency into the ratio block. C-2 also consists of rate scalars controlled by the operator switch S-2. These scalars provide the variable time required by the volume block C-3 to develop an independent ratio. The output L provides a "reset" pulse to the volume block C-3 and the sigh block C-4, indicating the completion of a respiratory cycle and the start of a new cycle. Thus S-1 selects respiratory rate (1T) and S-2 selects respiratory ratio 1: n - 1 shown in Fig. 1. The volume block, C-3, provides operator adjustment of the analog drive signal through the use of a decade voltage divider operating on the output of a digital-to-analog converter (DAC). This adjustment is independent of all others. The remaining block C-4 provides operator control of sigh rate, sigh/hour, sigh/interval, and sigh volume through use of switches S-4-S-7, respectively. C-4 is interconnected with C-1 and C-3 using several of the same components, reducing overall system complexity. It should be noted that C-4 also contains a rate scalar to provide a sigh/rate function. DETAILED CIRCUIT DESCRIPTION The rate scalar is a device which provides an output frequency scaled by the preset inputs. If the input frequency is equal to the proportionality constant built into the scalar, the output frequency will be equal to the preset inputs. In this case, by cascading two scalars, the built-in proportionality constant becomes K/4096 and the output is equal to the input multiplied by this proportion where K is equal to the preset inputs. Since the input frequency is equal to 4096 Hz, K/4096 becomes simply K Hz as shown in Fig. 2. Since the range of respiratory rates provided is 1 to 60 breaths per minute (BPM), the ROM converts a rate of 60 BPM to a value of K = 4096. In this manner, 30 BPM is equivalent to K = 2048, etc. (see Table I). Thus, the ROM's and decoders provide an interface between clinical parameters and desired control settings. The method of operation is that BCD data corresponding to specific rates from the operator switch S-1 address the ROM, and the K codes stored at the appropriate memory locations are used to control the scalars. The decoders also provide for the use of the same scalars to determine length of sigh when this mode of operation is selected. It should also be noted that the rate input from S-1 is overridden by the sigh/normal control signal from C-4. Additional codes have been stored in the ROM, so that the proper K numbers are also available for sigh operation. C-2 provides operator control of ratio. This circuit is similar to C-1 in that it uses rate scalars and a ROM. The operator ratio switch (S-2) settings are decoded by the memory which then provides the proper codes to the preset inputs of the N scalars. The rate scalar also provides a fixed output which, in this case, is K/4096 = L. A one-shot is used to provide a uniform pulsewidth independent of frequency as a "start of inspiration signal" (SOI). Typical L values are also shown in Table I. The N circuit provides a variable output (KN)/4096 proportional to the preset input settings. If the K input is 4096 (60 BPM), the output will be N, and if the K input is 2048 (30 BPM), the output is 0.5 N. In a like manner, all rate settings will appear as a fractional N output (see Table I). Block C-3 is designed so that 256 pulses on the KN/4096 input line determine the inspiration phase. Therefore, if N= 512 (ratio= 1: 1) and the rate equals 60 BPM, the first 256 pulses will occur in 0.5 s. These will be used by C-3 to determine inspiration time. The rest of the pulses up to
COMMUNICATIONS
189
Volume (drive voltage)
256 pulses I
(n-1) 256 pulses
MC
-
3
Tidal Volume
I} time
Fig. 1. Volume versus time waveform illustrating independence of respiratory rate and inspiratory/expiratory ratio. Rate = 1/T, ratio = 1:n - 1.
I
Fig. 2. Overall block diagram of digital electronic control system showing major components (C-1-C-4) and detailed circuit implementation. TABLE I Rate
(BPM) K (Hz) Ratio N (Hz) L (Hz) KIV/4096 60 60 30 30 15 15
4096 4096 2048 2048 1024 1024
1:1
1:4
1:1 1:4 1:1 1:4
512
1280
512 1280 512 1280
1 1 0.5 0.5 0.25 0.25
512
1280
256 640 128 320
Insp. Time
(seconds) 0.5 0.2 1 0.4 2 0.8
pulse number 512 will be counted for expiration. From the previous discussion of C-2, at a rate of 60 BPM, an SOI pulse will appear at 1-s intervals. In this case, SOI occurs at the 512th pulse, thereby causing the end of expiration and the
start of inspiration. Other values of inspiration, corresponding to different rates and ratios are shown in Table I. The purpose of C-3 is to provide adjustable amplitude waveforms to the drive circuitry. The waveforms are equal to zero during expiration, allowing the piston to return to the start position. The flip-flop and gate enable the counters when the SOI pulse occurs. The flip-flop is reset after 256 pulses have
been counted. Note that the SOI signal is independent of ratio settings and of any functions in the C-3 block. The output of the counter is fed to a digital-to-analog converter (DAC) which provides a linearly increasing ramp from d to 10 V in 256 steps of 40 mV each. These steps are so small that the drive signal appears as a smooth analog ramp waveform. The output of the DAC is fed to two attenuators, one for tidal volume and one for sigh volume. The purpose of S-3, which is mechanically linked to the tidal volume attenuator, is to provide independent adjustment of tidal volume. Likewise, S-7 provides adjustinetit of sigh
190
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, MARCH 1977
volume. The attenuators are voltage dividers which are adjustable in discrete steps of 10 mV. Because the volumes are set to be 1 V = 1 1, the operator volume adjustments are calibrated directly in steps of 10 ml. The voltages out of the attenuators are fed to a programmable op-amp which allows the selection of either sigh or normal volume signals to be fed to the drive circuits. This device also allows magnitude scaling required by the drive circuits. -It should be noted that if a ROM were to be inserted between the counters and the DAC, waveforms other than a simple ramp could be obtained. By proper choice of codes in the ROM, it would serve as a "look up" table for any of several waveforms. C-4 is designed to provide a larger breath of long duration (artificial sigh) according to the operator-controlled switches S-4-S-7. Rate scalers provide an output proportional to the settings from the ROM which has been preprogrammed to decode operator switch S-5 and can select any sigh rate from 1 to 49 per hour in steps of one. These circuits provide a "set" function on the sigh/normal control line through a flipflop. This line is used by C-l to switch the ROM address lines from the normal rate switch S-l to the sigh length switch S-4. In this way, ratio is not affected. The sigh/normal control line is also fed to C-3 where it is used by the programmable op amp to select tidal or sigh volume signals. A 4-bit counter, in conjunction with a decoder, and operator switch S-6 provide a reset function on the sigh/normal line when the proper number of sighs has been achieved. Switch S-6 may be set for 1, 2, or 3 sighs per interval. An interval is defined as the active sigh time as selected by switch S-5.
SUMMARY The control system described here has several advantages over previous electronic systems used in ventilator control. The use of a fixed clock and variable rate scalars eliminates problems associated with variable RC oscillators such as nonlinearities, instabilities, and drift. This feature provides extremely stable and accurate control of respiratory rate. In addition, field maintenance is simplified since there are no trim adjustments or calibrations necessary and circuit replacements do not require initial adjustments. Further, the principle of designating a fixed number of counts as inspiration time and varying the pulse rate to control respiratory frequency coupled with a variable number of counts for expiration insures independence of rate and ratio controls, and the use of ROM's provides for easy interfacing between clinical parameters and control signals. The wide dynamic range necessary in ventilators (5-60 BPM) makes the use of RC waveshaping circuits for volume curves impractical when the waveshape must be the same for all rates. This problem has been solved by using ROM's as digital waveform generators capable of generating a large number of frequency-independent waveforms. This solution also avoids problems associated with setting break points and slopes in diode function generators. In addition, an entirely new set of waveforms may be obtained by the substitution of two integrated circuits. This feature is particularly useful in light of the current controversy regarding the choice of a volume time waveform which is "best" for a given clinical situation. (See [4], for example.) The subject of ventilator safety has been widely discussed in recent years. In particular, dependence of ventilation parameters on line voltage fluctuations during "brownouts" or electric power failures has been a major concern. In this regard, the design described herein represents an improvement over previous designs since the digital system is not affected by widely varying input (line) voltage. In addition, operation during short (30 min or less) power failure is assured by floating the entire system on batteries. In the design of systems such as the one described here, a major concern is the existence of single point failures. A
number of such points exist in our system in the master oscillator and associated counter chains. In the current prototype, alarms are included which indicate abnormally low or high pressure, battery discharge, fail to cycle, and minute volume and leak. In addition, a backup system is planned which would feature a highly reliable oscillator operating at a fixed rate and ratio. Such a simple system has been used in life tests of the basic piston/cylinder/drive system with a high degree of reliability [3]. It should be noted that none of the currently used ventilators employ such a backup system. Due to the basic design, the most likely (by a wide margin) failure mode is failure to cycle rather than cycling at too high a rate. The remainder of our system (motor drive) has redundancy -built in which provides for operation in the event of a partial failure. In summary, we have developed a control system for use on ventilators which is time-cycled, has independence of controls, and is stable for variations in patient resistance and compliance. The overall accuracy of all digital controls is ±1 percent and analog control (volumes) is within 5 percent compared with the proposed ISO standards of ±10 percent
[5].
ACKNOWLEDGMENT
We wish to thank J. Fallon, J. Hathaway, and Drs. D. H. Gillott and J. G. Simes for assistance with this work, and J. Vaszary for typing the manuscript.
REFERENCES [1] W. W. Mushin, L. Rendell-Baker, P. R. Thompson, and W. W. Mapelsen, Automatic Ventilation of the Lungs, 2nd ed. Oxford, England: Oxford Blackwell, 1969. [2] D. W. Hill, "Recent developments in the design of electronically controlled ventilators," Der Anaesthesist, vol. 15, pp. 234-238, 1966. [3] D. H. Gillott and J. G. Simes, "Translational drive systems to facilitate ventilator control," in Proc. 27th ACEMB, vol. 139, 1974. [41 L. Jansson and B. Jonson, "A theoretical study of flow patterns of ventilators," Scand. J. Resp. Dis., vol. 5 3, pp. 237-246, 1972. [51 International Standards Organization, Sectional Committee ISO/TC 121/WG3. a) Standards for Anesthesia and Breathing Equipment, Proposed ISO. b) Standard Specifications for Breathing Machines for Medical Use.
A Sample/Long-Term-Hold Digital Circuit for Physiological Signal Monitoring RICHARD L. MASON Abstract-An uncomplicated circuit to sample a signal and digitally store its value is given. A modification to store the maximum or minimum value attained by the signal is also presented. The drift for these circuits is small, and the input signal may be obtained in
digital form.
INTRODUCTION A recent article by Johnson [I1 indicates the need to store a voltage for a long period of time and gives an analog circuit to perform that function. The circuit is stated to have a drift rate of 1 to 7 mV/h and to be simpler and less expensive than a digital circuit. Manuscript received December 12, 1975; revised February 27, 1976. The author is with the New London Laboratory, Naval Underwater Systems Center, New London, CT 06320.
