Method and apparatus for monitoring infants on assisted ventilation

An apparatus and method are described for measuring variables associated with the ventilation of infants during assisted ventilation. The infant is placed in a plethysmograph and various sensor means are used to measure flow of gas into and out of the plethysmograph and infant respiration. The outputs of the sensor means are supplied to a minicomputer system for processing. From this data, ventilator breaths are discriminated from infant breaths.

BACKGROUND OF THE INVENTION 
A portion of the disclosure of this patent document contains material to 
which a claim of copyright protection is made. The copyright owner has no 
objection to the electrophotographic reproduction by anyone of the patent 
document or the patent disclosure, as it appears in the Patent & Trademark 
Office patent file or records, but reserves all other copyright rights 
whatsoever. 
Mechanical ventilatory assistance is now widely accepted as an effective 
form of therapy for respiratory failure in the neonate. Mechanical 
ventilators are a conspicuous and fundamental part of tertiary neonatal 
care. When on assisted ventilation, the newborn infant becomes part of a 
complex interactive system which is expected to provide adequate 
ventilation and gas exchange. 
The overall performance of the assisted ventilatory system is determined by 
both physiological and mechanical factors. The physiological determinants, 
over which the physician has relatively little control, change with time 
and are difficult to define. These include the nature of the pulmonary 
disease, the ventilatory efforts of the infant, and many other anatomical 
and physiological variables. Mechanical input to the system, on the other 
hand, is to a large extent controlled and can be reasonably well 
characterized by examining the parameters of the ventilator pressure 
pulse. Optimal ventilatory assistance requires a balance between 
physiological and mechanical ventilation. This balance should insure that 
the infant is neither overstressed nor oversupported. Insufficient 
ventilatory support would place unnecessary demands on the infant's 
compromised respiratory system. Excessive ventilation places the infant at 
risk for pulmonary barotrauma and other complications of mechanical 
ventilation. 
Intelligent management of ventilatory assistance in the neonate requires 
that information about the performance of the overall system be available 
to the clinician. Instrumentation for continuous monitoring of infants on 
assisted ventilation, as well as certain component variables of 
ventilation are known, "Instrumentation for the Continuous Measurement of 
Gas Exchange and Ventilation of Infants During Assisted Ventilation", K. 
Schulze, M. Stefanski, J. Masterson, et al., Critical Care Medicine, Vol 
11, No. 11, pp. 892-896 (1983). However, at the present time, physicians 
rely largely on intermittent measurement of arterial blood gases to 
monitor the overall effects of the system on gas exchange. These 
measurements, while important in clinical care, have several limitations. 
Data acquired by such measurements provides little information about the 
separate contributions of the infant and the mechanical ventilator to 
overall ventilation and gas exchange of the infant. 
Absent this information, the effects of changes in ventilator support are 
not as readily observable. For example, it is frequently desirable to 
monitor how an infant responds to respiratory therapy such as positive end 
expiratory pressure ("PEEP") therapy. To administer this therapy, the 
ventilator increases resistance to expiratory gases, thus decreasing the 
burden on an infants lungs. 
In addition, arterial blood gas measurements are available only 
intermittently, which makes both trends and abrupt changes in clinical 
condition of the patient difficult to recognize. Continuous values are 
appreciably more helpful in describing the time course of changes in the 
patient's clinical condition. 
When acquiring measurements of infant ventilation for research purposes, it 
customary to place the infant in a container known as a plethysmograph. 
With the exception of openings used for respiratory support of the infant 
and quantitative measurement of the infant's respiration, the interior of 
the plethysmograph must be isolated from the external environment. Also, 
for these quantitative measurements to be useful in patient care, it is 
desirable to configure the plethysmograph such that the sensors are in a 
relatively stable environment and the infant monitored remains warm and 
undisturbed. At the same time, however, it is essential that the infant be 
accessable in a very short period in the event that some emergency arises. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a method and apparatus are 
described herein for providing continuous measurement of infant 
ventilation during assisted ventilation, which is particularly adapted for 
providing information about the respective contributions to ventilation by 
the infant and the ventilation mechanism. In the presently preferred 
embodiment, the apparatus comprises a heated plethysmograph in which an 
infant is placed, a pneumotachometer and a differential pressure 
transducer for detecting infant respiration, a pressure transducer for 
measuring pressure at an infant's airway in order to discriminate the 
infant's breaths from ventilator breaths, data recording means for storing 
data obtained by said pressure transducers and time data and a 
preprogrammed minicomputer system for processing and storing data acquired 
by the aforementioned components. In another embodiment, the invention 
comprises a second pneumotachometer and a second differential pressure 
transducer for determining when ventilator breaths occur. 
Details of the performance of this system have been described in Karl 
Schulze, et al., "Computer Analyses of Ventilatory Parameters For Neonates 
On Assisted Ventilation," IEEE Engineering In Medicine And Biology 
Magazine Vol. 3, No. 3, pp. 31-33 (Sept. 1984) which is incorporated 
herein by reference. 
In an alternate embodiment, data from the differential pressure transducer 
and from the airway pressure transducer or second differential pressure 
transducer are processed on-line and real-time monitoring data is 
displayed. 
An additional feature of the present invention is the isolation of the 
plethysmograph chamber by the use of a fluid seal. A reservoir of fluid 
fills a trough which extends around the perimeter of the plethysmograph. 
This trough receives the lower rim of the plethysmograph cover and an 
airtight seal is thereby formed with said fluid and lower rim of 
plethysmograph cover. Optionally, an extremely thin, elastic and pliable 
membrane covers the trough to prevent the fluid therein from escaping. In 
this embodiment, the seal is formed between the membrane and the lower rim 
of the plethysmograph cover. 
Still another feature of the invention is that the plethysmograph, which is 
ordinarily utilized as a highly specialized research tool, is adapted for 
clinical patient care. The plethysmograph is heated and ready access to 
the infant can be accomplished due to the fluid seal arrangement. 
Furthermore, the monitoring data obtained through use of the invention is 
of great clinical value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, the presently preferred embodiment of the apparatus 
comprises a plethysmograph 10 in which an infant to be monitored is 
placed, a ventilator 20 for ventilating the infant, a pneumotachometer 30 
and a differential pressure transducer 40 for measuring gas flow into and 
out of plethysmograph 10, and a pressure transducer 50 for detecting 
pressure in the infant's airway. A gas source 60 feeds ventilator 20 via 
pipe 70 with gas for ventilation of the infant. This gas, typically an 
oxygen-nitrogen mixture, is provided to the infant in the phethysmograph 
10 through pipe 75 and an endotracheal tube 80. Pipe 85 carries expiratory 
gases back to ventilator 20. When ventilator 20 fires to respirate the 
infant, the ventilator occludes pipe 85 so that gas provided to the infant 
by pipes 70, 75 will be forced through endotracheal tube 80 and into the 
infant's airway. 
Analog data provided by differential pressure transducer 40 and pressure 
transducer 50, together with timing data, is provided to recording device 
90 by lines 100, 110, 120. Minicomputer 130, digitizes this analog data 
and stores it on magnetic tape. The digital data is then processed by 
minicomputer 130 to obtain total tidal volume, volume due to infant 
respiratory efforts and volume due to the effects of mechanical 
ventilation. These values are then displayed by the minicomputer on a 
suitable display unit 150. This unit may provide both digital and analog 
displays as well as a continuous record in the form of a strip chart or 
circular chart recorder. Advantageously, all of the equipment depicted in 
FIG. 1 is mounted on a movable cart so that the infant can readily be 
moved, for example, for emergency treatment, without altering either his 
respiratory support or the monitoring thereof. 
In the presently preferred embodiment, plethysmograph 10 comprises an 
appropriately heated plexiglass box capable of containing an infant. The 
interior atmosphere of the plethysmograph is isolated from the exterior 
environment, except for one or more ports for respiration of the infant 
and coupling of pneumotachometer 30, described below. Illustratively, 
pipes 70, 75 are coupled together at port 77. Pneumotachometer 30 
illustratively comprises a chamber divided in half by a pliable and 
semi-permeable screen with one chamber having a tapered port 33 for 
receiving pressure and another chamber communication with the environment 
outside the piethysmoraph. Although many appropriately selected commercial 
pneumotachometers will be suitable for use in the practice of the 
invention, a description of the pneumotachometer used in the presently 
preferred embodiment may be found in "Pneumotechograph For Use With 
Patients During Spontaneous or Assisted Ventilation," G. Gregory, J. 
Kitherman, Journal of Applied Physics, Vol. 31, p. 766 (Nov. 1971). 
Pressure variations occurring on the interior of the plethysmograph, such 
as those resulting from expansion and contraction of an infant's chest, 
are reflected in the pneumotachometer. Pneumotachometer 30 and 
differential pressure transducer 40 are advantageously located inside the 
plethysmograph, with port 33 of the pneumotachometer open to the 
plethysmograph interior. Placement of pneumotachometer 30 and transducer 
40 inside the plethysmograph reduces the potential for inaccuracies in the 
data acquired due to temperature differences between said elements and the 
plethysmograph. Tubes 35, 36 are each coupled to one half of the 
pneumotachometer chamber. Differential pressure transducer 40 senses the 
pressure in the two halves of the pneumotachometer chamber via tubes 35, 
36 and outputs an analog signal to line 100 showing the amount and 
direction of gas flow into and out of plethysmograph 10. Illustratively, 
differential pressure transducer 40 comprises a Validyne DP45 pressure 
transducer. 
In the presently preferred embodiment, amount and direction of gas flow are 
reflected by the magnitude and polarity, respectively, of the output 
signal from transducer 40. Thus, inspiration and expiration result in 
different polarity outputs from the transducer. Pipe 75 is coupled to 
endotracheal tube 80, which is in turn inserted into the airway of the 
infant. Pressure transducer 50, preferably be a Novametrix airway monitor 
device, is located in the endotracheal tube and senses pressure in the 
infant's airway. The transducer 50 outputs an analog signal indicating 
said pressure to line 110. In an alternate embodiment the function of 
pressure transducer 50 is instead accomplished by a second 
pneumotachometer and pressure transducer 56. These components detect 
pressure changes in tube 58. 
To correlate in the time domain the signals produced by differential 
pressure transducer 40 and pressure transducer 50, a time-marking device 
140, preferably a Datum 1000 Time Code Generator device, continuously 
outputs data to line 120 showing the current time. Lines 100, 110, 120 are 
coupled to an analog data recording device 90, preferably an HP 3968 FM 
Analog tape recorder, which records the signals output by differential 
pressure transducer 40, pressure transducer 50 and time-marking device 
140. 
Minicomputer system 130, in the presently preferred embodiment, comprises 
an HP 1000 F-series minicomputer, an HP 2313 digital converter, a magnetic 
tape drive and other standard peripheral devices. Minicomputer system 130 
receives signals from analog data recording device 90 when said device is 
actuated to play back recorded data and utilizes the analog-to-digital 
converter to digitize this data. The magnetic tape drive stores the 
digitized data. 
In accordance with one embodiment of the present invention, digital data 
stored on the magnetic tape drive is processed by the minicomputer to 
obtain total tidal volume as follows. First, a calibration factor for 
pneumotachometer 30 is determined by injecting and withdrawing a 
premeasured volume of gas into and out of plethysmograph 10 and processing 
flow data obtained by penumotachometer 30 and transducer 40 during this 
procedure. 
The calibration factor is helpful in compensating for inaccurate flow 
measurements such as those resulting from thermal transients in the 
plethysmograph. Integration of inspiration flow data is then corrected by 
multiplying the value from each data point. 
In the presently preferred embodiment, the output signal of a differential 
pressure transducer 40 is positive during inspiratory flow and negative 
during expiratory flow. Ideally, the integration of positive value 
inspiratory flow data should equal the integration of negative expiratory 
flow data, since, over time, the volume of gas into and the volume of gas 
out of the infant's lungs are ordinarily the same. To obtain the 
calibration factor, a predetermined amount of flow data obtained during 
the sample injection and withdrawal of gas is integrated. A bias value, 
indicating the degree to which inspiration and expiration differ, is then 
obtained by dividing this integral by the product of the number of flow 
data samples integrated and the sampling rate. 
Next, the positive inspiratory data, only, are integrated, with the bias 
value being subtracted from each sample of flow data as it is included in 
the integration computation. The volume of gas which was injected in the 
calibration procedure, in cubic centimeters, is then divided by the 
adjusted integration to determine the calibration factor. The data 
obtained during monitoring is then processed. 
While in the presently preferred embodiment, a calibration factor is 
obtained one time, prior to the processing of data acquired during 
monitoring, it is also in accordance with the invention to calibrate 
periodically during monitoring. An important feature of this embodiment of 
the invention is to calibrate using data gathered over a sufficiently long 
period such that irregularities occurring within one or two breaths are 
not mistaken for a general inaccuracy in measurement which is susceptible 
to correction by calibration. In this embodiment, the predetermined volume 
of gas injected into and withdrawn from plethysmograph 10 is sufficiently 
large such that ventilator and infant breaths will comprise a relatively 
small percentage of total gas flow. A predetermined amount of flow data 
obtained from transducer 50 during injection and withdrawal of the gas is 
integrated and a real-time bias value is then obtained by dividing this 
integration by the product of the number of flow data samples obtained and 
the sampling rate. This real-time bias value is an index of the degree to 
which inspiration and expiration differ, and is useful to physicians as an 
estimate of such factors as endotracheal tube leakage and thermal 
transients. Optionally, the realtime bias value also serves as an alarm 
indicator when the endotracheal tube is slipping from the infant's airway. 
The bias value also gives the physician an indication of the stability of 
the monitoring system. 
The inspiratory flow data is then integrated using the real-time bias value 
as described above in connection with pre-monitoring calibration. A 
calibration factor is then obtained by first determining the sum of the 
injected volume and the average difference between said volume and the 
integration of the inspiratory data. That sum is then divided by the 
integration of the inspiratory data which was corrected by the real-time 
bias value, with the result being a real-time calibration factor. 
It is anticipated, however, that some flow data which appears to represent 
breaths will actually be the result of factors other than respiratory 
activity. Movement by the infant, for example, may generate such flow 
data. In order to obtain an accurate measurement of breath tidal volume, 
actual breath must be discriminated from noise. Illustratively, the flow 
data acquired during the relevant period is tested to determine whether it 
meets the following criteria: the data contains only two polarity changes, 
a negative-to-positive polarity change followed by a positive- to-negative 
polarity change; the flow data must indicate that the breath was of at 
least a minimum duration and a minimum tidal volume; and the highest value 
data point of the flow data must be of at least a minimum value. 
In the presently preferred embodiment, the criteria for a valid breath, 
either ventilator induced or infant, are that it must be at least 0.1 
seconds in duration and must result in a tidal volume of at least 0.5 
cubic centimeters. Furthermore, the amplitude of the flow data must be at 
least plus or minus three standard deviations of the breath amplitudes for 
the previous one minute period. 
Tidal volume of valid breaths is computed by integrating the inspiratory 
flow data obtained during those breaths and multiplying the result by the 
calibration factor. Illustratively, for each minute of monitoring, the 
tidal volume, duration and maximum amplitude of each inspiratory volume or 
breath and the total tidal volume and frequency of breaths are stored in 
an external memory file for minute total data. 
In accordance with the present invention, the minute total data is indexed 
to and processed in conjunction with time data and pressure data from 
pressure transducer 50 to determine whether each breath is the result of 
infant respiratory efforts or is due to mechanical ventilation. For each 
minute of monitoring, tidal volume of each breath is referenced to airway 
pressure data obtained from pressure transducer 50 during the same period. 
More particularly, under control of the computer program set forth in 
Appendix I, minicomputer system 130 compares each inspiratory volume, 
within a given minute, with the airway pressure data obtained from 
pressure transducer 40, or alternatively with the data output by second 
transducer 56, during the period that the inspiratory volume was 
collected. 
The data associated with a given breath is then copied into one of two 
different data files, preferably located in external storage memory, 
depending on whether the inspiratory volume associated with that data was 
due to an infant breath or resulted from mechanical ventilation. An 
inspiratory volume which occurs simultaneously with a change in airway 
pressure which exceeds a predetermined magnitude is stored in a file for 
mechanical ventilator breaths. In the presently preferred embodiment, the 
predetermined change in airway pressure is approximately 20 mm. of Hg., 
but this value is subject to change depending upon the ventilator output 
pressure. Conversely, a breath occurring without any such change in airway 
pressure is stored in a file for infant breaths. In the event that 
pressure data from pneumotachometer and pressure transducer 56 is used 
instead of data from transducer 50, comparison is made between the breath 
volume and the data output by transducer 50 during the period of the 
breath. In this case the program detects a drop in pressure in tube 58 
caused by ventilator 20 occluding tube 85. 
These two files, therefor, contain the number, duration and tidal volume of 
ventilator and infant breaths occurring within each minute as well as the 
maximum amplitude of the flow signal for each breath. 
In the presently preferred embodiment, the FORTRAN language computer 
program shown in Appendix I, suitable for execution on an HP 1000 
minicomputer, is used to process the digitized data stored on the tape 
drive of minicomputer system 130. It will be noted that the program calls 
various subroutines such as NUMR, OPENF, CLOCK, TIME, VSUM and other 
standard routines. These external routines are standard calls for the 
Hewlett-Packard standard Fortran compiler associated with the HP 1000 
F-series minicomputer. The program in Appendix I uses the term 
"respiration data" to refer to the data acquired by pneumotachometer 30 
and differential pressure transducer 40. "Flow data" in said program 
refers to data acquired by airway pressure transducer 50, or alternatively 
second transducer 56. 
Although the invention has been described as an apparatus and method 
wherein data is accumulated on a continuous basis for a predetermined 
period and is then loaded into a minicomputer system for processing, it 
will be apparent to those skilled in the art that an on-line data 
accumulation, processing and display is equally contemplated by the 
invention. In this embodiment lines 100, 110 are coupled to an 
analog-to-digital conversion means having at least two input channels 
which in turn outputs digital data to minicomputer system 130. Breath 
volume, duration, and frequency is determined in real-time and 
classification of ventilator versus infant breath is accomplished after 
each breath. Time marking device 140 is eliminated and timing functions 
are instead performed by standard minicomputer software. Display of the 
ventilation characteristics of the infant in real-time is similarly 
contemplated. 
One advantage to this embodiment is that changes in infant respiration can 
be rapidly perceived and appropriate action taken. It is thus further in 
accordance with this embodiment to continually determine a bias value, (as 
described above in connection with calibration of the pneumotachometer) 
using actual inspiratory and expiratory flow data obtained during 
monitoring of the infant. If the bias value exceeds a predetermined level, 
an alarm is sounded to alert the clinician to the possibility of an 
endotracheal tube leak or that some other potentially dangerous condition 
such as slippage of said tube, exists. 
An additional feature of the invention is shown in FIG. 3 and comprises a 
plethysmograph chamber having an upper rim with a trough filled with fluid 
extending around the upper rim and a cover having a lower rim extending 
downward around the perimeter of the cover. The lower rim of the cover is 
positioned such that the trough in the upper rim of the plethysmograph 
chamber receives the lower rim of the cover, thus forming a seal between 
the fluid in the trough and said lower rim. In an alternative embodiment, 
the fluid in the trough is covered by a very thin flexible and elastic 
membrane which prevents the fluid from escaping. 
With the plethysmograph sealed in this fashion, an infant therein can be 
accessed very quickly in the event some emergency arises. 
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