Improved Esophaqeal Cardiac Pulse Monitoring Apparatus and Method

Technical Field The present invention relates in general to devices and methods for monitoring the cardiac pulse. More particularly, the invention concerns esophageal stetho¬ scopes and devices and techniques for monitoring the cardiac pulse waveform using such stethoscopes.

Background Art

Esophageal stethoscopes were first described nearly three decades ago and have been in clinical use during anesthesia since then to enable the anesthesiologist to obtain a rather direct acoustic measure of the "heart's performance. Such acoustic esophageal stethoscopes typically have comprised a tube or lumen which is in¬ serted into the patient's esophagus to a location at which pressure variations due to heart and respiratory sounds are best transmitted to the interior of the lumen, by means such as a flexible diaphragm. The anesthesiolo¬ gist is provided with an earpiece connected to the lumen by a suitable conduit, so that an acoustic indication of heart activity is provided. In some instances rather than an acoustic earpiece, the stethoscope has been pro- vided with an electro-mechanical transducer, such as a piezo-electrical device, which produces a signal pro¬ portional to pressure variations in the lumen, the signal then being provided directly to a speaker or headset following amplification. In either case, determination of the actual cardiac pulse waveform, which is monitored for various reasons during surgery, has not been attempted

via the esophageal stethoscope. Rather, external pres¬ sure transducers applied to the carotid artery or electro- optical transducers applied to the fingers have been relied on to determine cardiac pulse waveform. For many reasons, these external devices are either- not useful on certain patients or are somewhat unreliable. Thus, a need has continued to exist for a simple, reliable cardiac pulse monitor which can be used on most patients while they are under anesthesia.

Disclosure of the Invention

A ^* primary object of the invention is to provide an improved esophageal stethoscope which can be used on most patients to provide an indication of cardiac pulse wave¬ form. Another object of the invention is to provide such a stethoscope which also will provide a direct, acoustic indication of heart sounds.

A further object of the invention is to provide such a stethoscope in which cardiac pulse waveform and acoustic indication of heart sounds are determined in isolated portions of the stethoscope probe.

Still another object of the invention is to provide such a stethoscope which produces an electrical signal proportional to cardiac pulse waveform, and includes means for conveying the signal to existing signal display devices such as oscilloscopes and strip chart recorders.

Yet another object of the invention is to provide such a stethoscope which produces an electrical signal pro¬ portional to cardiac waveform, the signal being free of high frequency components primarily due to heart noises and low frequency components primarily due to respiratory noises.

These objects of the invention are given only by way of example; thus, other desirable objectives and advantages inherently achieved by the disclosed invention may occur to those skilled in the art. Nonetheless, the scope of the invention is to be limited only by the appended claims. One aspect of the invention concerns an im¬ proved method of determining the cardiac pulse waveform by monitoring pressure variations in the esophagus. Means, such as a lumen, are used to convey these variations to an electro-mechanical transducer which produces an out¬ put signal proportional.to the frequency and intensity of the pressure variations. By filtering out low frequency components attributable mainly to respiratory noises and higher frequency components attributable mainly to heart noises, a sub-audible signal is obtained which is pro¬ portional to the cardiac pulse waveform. The sub-audible signal proportional to the cardiac pulse waveform has been found to be in the range of 0.1 to 30 Hz.

The probe used to convey pressure variations to the transducer may be configured in several ways. A simple, open-ended lumen can be used; however, its reliability is uncertain since liquids in the esophagus may block the open end and prevent transmission of pressure varia¬ tions up the lumen to the transducer. Improved performance is achieved if the single lumen is provided with radially extending holes adjacent its lower end and a flexible, cuff-like or tubular diaphragm surrounding and spaced from the lumen in the vicinity of the holes. The dia¬ phragm then expands upon internal application of fluid pressure to just touch the walls of the esophagus. This probe is similar to that used in the past for simple acoustic monitoring. Direct acoustic monitoring of heart sounds also is achievable using an acoustic earpiece

connected to the single lumen; however, an isolation diaphragm should be included in the earpiece to prevent leakage.

In the preferred probe according to the invention, two lumens are used, one located within the other. The outer lumen is relatively flexible and the inner one is considerably smaller and relatively stiff. At their lower ends, the two lumens are isolated from each other by a seal extending between them. The outer lumen is provided with radially extending holes below the loca¬ tion of the seal. A tubular diaphragm similar to that just mentioned surrounds and is spaced from the outer lumen in the vicinity of the holes, so that pressure variations in the esophagus are transmitted to the inner lumen and from there to the transducer. Where direct acoustic monitoring is also desired, the seal between the lumens is located between the ends of that portion of the outer lumen in which the holes have been provided; and the diaphragm is provided with a reduced diameter portion which engages the outer lumen in the vicinity of the seal. Thus, pressure variations in the esophagus are transmitted to both the inner lumen and the space between the inner and the outer lumens. The transducer again is connected to the inner lumen and an acoustic earpiece is connected to the outer lumen. In this instance, an isolation dia¬ phragm is not needed in the earpiece. Although the arrangement is not optimum, parallel single lumens may also be used, each having an associated diaphragm at its lower end, one being connected to the transducer and one being connected to an acoustic earpiece.

The signal processing circuitry according to the invention is simple and reliable. The piezoelectric transducers which are preferred in the invention have a certain inherent capacitance. When the output signal

^• of the transducer is fed to a buffer amplifier having a suitably high impedance, the resultant effect is to filter out certain low frequency components. In accord¬ ance with the invention, components below about 0.1 Hz are filtered out, since they are attributable primarily to respiratory noises, sometimes referred to as the respiration artifact. Applicants have found that by further filtering the amplifier output to eliminate com¬ ponents above approximately 30 Hz, a sub-audible signal proportional .to cardiac pulse waveform is obtained. The amplifier output may also be filtered to provide an audible signal proportional to heart sounds. By gating the signal proportional to pulse waveform at a preselected point in the output from a conventional electrocardiogram machine, synchronized display of the two signals may be achieved on an oscilloscope or a strip chart recorder.

Brief Description of the Drawings

Figure 1 shows a schematic, partially exploded view of an esophageal cardiac pulse monitoring apparatus according to the invention.

Figure 2 shows an enlarged, sectional view of an esophageal probe used in the apparatus shown in Figure 1.

Figures 3, 4 and 5 show enlarged sectional views of alternate forms of esophageal probes useful in the apparatus shown in Figure 1.

Figure 6 shows an adapter with an isolation diaphragm which is used to prevent leakage past the acoustic ear¬ piece.

Best Mode for Carrying Out the Invention The invention will be described with reference to the drawings, in which like reference numerals identify like elements of structure in each of the several figures.

Referring to Figures 1 and 2, the cardiac pulse monitoring apparatus according to the invention is seen to comprise an esophageal probe TO which is assembled from an outer tube or lumen 12 and an essentially concen- trie inner lumen 14. Lumen 12-is preferably 4 mm to 8 mm in outer diameter and 3 to 5 mm in inner diameter; whereas, lumen 14 is preferably 2 to 3 mm in outer diameter and 1 to 2 mm in inner diameter. To optimize gain and frequency response, the internal volume of lumen 14 preferably is in the range of 0.5 to 3 cc.

For lumens made from the preferred vinyl tubing, lumen 14 is comparatively stif er than lumen 12, which makes lumen 14 less sensitive to pressure variations acting on its walls. Concentric lumen 12 is more compliant to the shape of the esophagus, and also protects lumen 14 from some pressure variations that could pass through its walls, as well as undesirable contact with adjacent body elements or hardware. The lower end 16 of lumen 14 ex¬ tends through a seal block or fitting 18 which preferably is sealed to both lumens by means such as a suitable glue. Lumen 12 extends past seal block 18 to its lower end 20 and on either side of seal block 18 is provided with a plurality of radially extending holes 22 about its circumference and along a portion of its length. A tubular diaphragm or balloon 24 extends along and sur¬ rounds the portion of lumen 12 in which holes 22 are located. To maintain a radial spacing between diaphragm 24 and lumen 12, the diaphragm includes radially extending shoulder portions 26, 28 projecting inwardly at its op- posite ends and a reduced diameter portion 30 engaging lumen 12 in the region where seal block 18 is located. A cuff 32 extends upward along lumen 12 from shoulder portion 28; and a cuff 34 and spherical end closure 36 extend downward along lumen 12 from shoulder portion 26.

Diaphragm 24 is formed using conventional techniques from thin vinyl having a wall thickness of about 0.015 to 0.025 cm, and has a maximum diameter in the range of 5.8 to 12.2 mm. Cuff 32 preferably is glued to the exterior of lumen 12; however, reduced diameter portion 30 preferably engages lumen 12 with a simple tight fit, for purposes to be described subsequently.

As shown in Figure 2, diaphragm 24 and seal block 18 cooperate with lumens 12 and 14 to define a lower pressure responsive volume 38 and an upper pressure sensing volume 40, each responsive to pressure variations in the esophagus and each isolated from the other. In one actual embodi¬ ment, volume 40 was approximately 2.54 cm in length and volume 38 was approximately 1.91 cm in length. Volume 38 communicates with the interior passage of lumen.14; and volume 40, with the annulus between the two lumens. Lumen 14 is led out of probe 10 through a suitable fitting 42 which is inserted into the upper end of lumen 12. A three-way stop cock 44 is attached to the end of lumen 14 at one of the stop cock's ports and to electromechanical transducer 46 at another of its ports. The remaining port of stop cock 44 is adapted to receive the delivery conduit of a simple syringe 48. Fitting 42 also connects the annulus between lumens 12 and 14 to an acoustic earpiece 50, via an expansion bellows 52 and conduit 54. In use, syringe.48 is used to inject a small amount of air into lumen 14, such as approximately 3 cc, so that diaphragm 24 is certain to expand into light contact with the wall of the esophagus and yet to remain compliant. If excess air is injected into lumen 14 or lumen 14 becomes overpressurized for any reason, leakage from volume 38 into volume 40 past reduced diameter portion 30 will relieve the overpressure. To facilitate this

pressure relief, the tightness of portion 30 around lumen 12 may be adjusted as desired by appropriate selection of the inner diameter of diaphragm 24 at portio '30. The probe is inserted into the patient's esophagus prior to inflation of diaphragm 24 where it surrounds volume 38. The depth of insertion should be at least to the point where the aorta crosses the esophagus; however, experience has shown that better results are achieved when the lower portion of diaphragm is positioned where the left ventricle of the heart is closest to tiie esophagus. Following inflation, pressure variations are transmitted up lumen 14 to transducer 46, which preferably is a piezoelectric transducer such as the Model 1010C made by Transmed Scientific of San Luis Obispo, California Transducer 46 thus produces an electrical signal on line 56 which is proportional to the varying frequency and intensity of the pressure variations in the esophagus. These signals are fed to the input of a high impedance buffer amplifier 58 which is selected so that its input impedance is high enough, in view of the inherent capaci¬ tance of the transducer, effectively to filter out certain low frequency components of the signal, particularly those below about 0.1 Hz. Applicants have found that components below this level are attributable primarily to respiratory noises and may be ignored when determining cardiac pulse waveform. The output from amplifier 58 is fed in parallel to a low band pass filter 60 and a high band pass filter 62 Filter 60 is chosen to pass components of the signal below approximately 30 Hz. The resultant sub-audible signal has components in the 0.1 to 30 Hz range and has been found by applicants to be proportional to the desired cardiac pulse waveform. This signal is fed to a conventional D.C. restoration circuit 64 which may include an optional

gating feature controlled by the occurrence of a pre¬ selected portion of the output of a conventional electro¬ cardiogram unit 66. The output signals from both the esophageal stethoscope according to the invention and the electrocardiogram unit 66 thus may be synchronized as desired prior to visual presentation on a visual dis¬ play unit 68, such as an oscilloscope, strip chart recorder or both. Preferably, the apparatus according to the in¬ vention is provided with a connector plug 70 for conveying its output signal to display unit 68, via an input socket also intended for carotid artery pulse monitors or electro- optical pulse monitors. Synchronization between the cardiac pulse wave form produced by the invention and the electro¬ cardiogram waveform may be done in visual display unit 68, if desired.

The output from band pass filter 62 is limited to com¬ ponents in the audible 70 to 200 Hz range and is passed to means such as a phonocardiogram '72 which provides an audio output of actual heart sounds, in the known manner. Figure 3 shows an alternate esophageal stethoscope probe 74 according to the invention. Here, a pair of lumens 76, 78 are fed in parallel to the lower portion of the probe where they enter a third tubular section or lumen 80. Lumen 80 may also extend to the upper end of the probe to simplify handling and protect lumens 76, 78. An-upper seal block 82 extends between lumens 76, 78 and the inner wall of lumen 80. Lumen 76 extends just through seal block 82; whereas, lumen 78 extends on through a further seal block 84 located between the ends of lumen 80. A plurality of radially extending holes 86 are provided in lumen 80 on either side of seal block 84, the holes extending about the circumference of lumen 80 and along the portion of its length below seal block 82. A tubular diaphragm 24 surrounds the portion of lumen 80 in which

the holes 86 are located, in the manner previously de¬ scribed with respect to Figure 2. In use, lumen 78 is connected to earpiece 50; and lumen 76, to transducer 46. Figure 4 shows another esophageal stethoscope probe 90 according to the invention. In this case, only pressure responsive volume 38 is provided which is con¬ nected to lumen 14. Direct acoustic transmissions of heart sounds are not monitored with this probe since the annulus between lumens 12 and 14 does not communicate with tubular diaphragm 24. Otherwise, the probe is used just as in the case of the embodiments of Figures 2 and 3.

Figure 5 shows yet another esophageal probe 92 which may be used in accordance with the invention. In this case, all pressure variations to be monitored either electronically or acoustically are conveyed upward from the esophagus through a single lumen 94. To prevent leakage of air through earpiece 50, an adapter 96 is inserted in conduit 54, which includes a hermetically sealed isolation diaphragm 98. Pressure variations in conduit 54 thus cause diaphragm 98 to vibrate and trans¬ mit the variations to the doctor's ear, without allowing diaphragm 24 to depressurize.

Industrial Applicability

The invention is disclosed for use in esophageal stethoscopes in which a gas is used as the pressure transmitting medium. Those skilled in the art will appreciate, however, that a liquid could be used to transmit pressure variations, particularly through lumen 14 to transducer 46, without departing from the scope of the invention.

Having described our invention in sufficient detail to enable those skilled in the art to make and use it, we claim:

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