System and Method to Modulate Phrenic Nerve to Prevent Sleep Apnea

Abstract

An implantable medical device for treating breathing disorders such as central sleep apnea wherein stimulation is provided to the phrenic never through a transvenous lead system with the stimulation beginning after inspiration to extend the duration of a breath and to hold the diaphragm in a contracted condition.

Claims

1. A method of prolonging breath in a patient having a diaphragm innervated by a phrenic nerve and a vein near the phrenic nerve, the method comprising: implanting a stimulation electrode in a vein of a patient proximate the phrenic nerve; measuring respiration of the patient to find the inspiration phase of a breath; and stimulating the phrenic nerve via the stimulation electrode after the beginning of the inspiration phase, said stimulation magnitude being sufficient to still the diaphragm resulting in a substantial amount of air trapped in at least one lung.

2. The method of claim 1 further characterized by said stimulation beginning at a time after peak of the inspiration phase.

3. The method of claim 1 further characterized by said stimulation duration being sufficient to delay expiration.

4. The method of claim 1 further comprising: repeating measuring and stimulation steps for selected breaths.

5. A method of prolonging breath in a patient having a diaphragm innervated by a phrenic nerve and a vein near the phrenic nerve, the method comprising: implanting a stimulation electrode in a vein of a patient proximate the phrenic nerve; measuring respiration of the patient to find the inspiration phase of a breath; and stimulating the phrenic nerve via the stimulation electrode after the beginning of the inspiration phase, said stimulation duration exceeding a natural expiration phase of breath.

6. The method of claim 5 further characterized by said stimulation beginning at a time after peak of the inspiration phase.

7. The method of claim 5 further characterized by said stimulation duration being sufficient to prevent apnea.

8. The method of claim 5 further comprising: repeating measuring and stimulation steps for selected breaths.

9. A method of reducing breathing rate in a patient having a diaphragm innervated by a phrenic nerve and a vein near the phrenic nerve, the method comprising: implanting a stimulation electrode in a vein of a patient proximate the phrenic nerve; measuring respiration of the patient to find the inspiration phase of a breath; and stimulating the phrenic nerve via the stimulation electrode after the beginning of the inspiration phase, said stimulation magnitude being sufficient to still the diaphragm resulting in a substantial amount of air trapped in at least one lung, said stimulation duration being sufficient to reduce breathing rate of the patient.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] A preferred embodiment and best mode of the invention is illustrated in the attached drawings where identical reference numerals indicate identical structure throughout the figures and where multiple instances of a reference numeral in a figure show the identical structure at another location to improve the clarity of the figures, and where:

[0021] FIG. 1 is a schematic diagram;

[0022] FIG. 2 is a schematic diagram;

[0023] FIG. 3 is a schematic diagram;

[0024] FIG. 4 is a schematic diagram;

[0025] FIG. 5 is diagram showing experimentally derived physiologic data displayed in two panels A and B;

[0026] FIG. 6 is a schematic diagram showing physiologic data known in the prior art;

[0027] FIG. 7 is a schematic diagram showing physiologic data and device timing information; and

[0028] FIG. 8 is a schematic diagram showing physiologic data and device timing information.

[0029] FIG. 9 is a schematic diagram showing physiologic data and signal artifact rejection.

DETAILED DESCRIPTION OF THE INVENTION

[0030] FIG. 1 is schematic diagram showing an implanted medical device (IMD) 101 implanted in a patient's chest for carrying out a therapeutic stimulation of respiration. The patient has lungs shown in bold outline and indicated at 102 overlying the heart 103. The right phrenic nerve 104 passes from the head alongside the heart to innervate the diaphragm 106 at location 105.

[0031] In this embodiment a transvenous lead 107 passes from the IMD 101 and passes through venous vasculature to enter the cardiophrenic vein 108 on the right aide of the patient. The cardiophrenic vein 108 lies next to the phrenic nerve 104 along the heart. Electrical stimulation pulses supplied to the stimulation electrode 110 on lead 107 interact with the phrenic nerve to stimulate it and thus activate the diaphragm 106. In the figure a series of concentric circles 112 indicate electrical stimulation of the phrenic nerve. In this embodiment the stimulation electrode 110 lies far enough away from the heart 103 to avoid stimulating the heart 103. In this embodiment only one branch f the phrenic nerve 104 is stimulated and the other side of nerve is under normal physiologic control.

[0032] A respiration electrode 114 on lead 107 cooperates with an indifferent electrode on the can of the IMD 101 to source and sink low amplitude electrical pulses that are used to track changes in lung volume over time. This well known impedance plethysmography technique is used to derive the inspiration and expiration events of an individual breath and may be used to track breathing rate. This impedance measurement process is indicated in the diagram by the dotted line 116 radiating from the electrode site of respiration electrode 114 to the IMD 101. Transvenous stimulation of the phrenic nerve from a single lead carrying an impedance measuring respiration electrode is a useful system since it permits minimally invasive implantation of the system. However other architectures, are permissible and desirable in some instances.

[0033] FIG. 2 is a schematic diagram showing alternative electrode and lead placements for use in carrying out the stimulation regime of the invention. In some patients it may be easier or more suitable to access the phrenic nerve in the neck in the jugular vein at electrode location 200. In some instances it may be preferable to place electrodes in veins both near the right phrenic nerve as indicated by the deep location of a stimulation electrode 110 and in the left phrenic nerve at electrode location 202. Other potential locations for the stimulation electrodes are the large vessel (SVC) above the heart indicated by electrode 203. Unilateral stimulation is preferred but having multiple sites available may be used to reduce nerve fatigue. Non-venous placement is possible as well. For example, placement of a patch electrode in the pericardial space between the heart and within the pericardial sac is suitable as well, as indicated by electrode location 205. In this embodiment the insulating patch 206 isolates spaced electrodes 207 and electrode 208 from the heart. The lead 204 connects this bipolar pair of electrodes to the IMD 101.

[0034] Also seen in this figure is a pressure transducer 209 located in the pleural cavity and connected to the IMD 101 via a lead. The pressure transducer 209 tracks pressure changes associated with breathing and provides this data to the implanted device 101. The pressure transducer is an alternative to the impedance measurement system for detecting respiration. Such intraplueral pressure signal transducers are known in the respiration monitoring field.

[0035] FIG. 3 shows a schematic diagram of a system for carrying out the invention. The system has an implanted portion 300 and an external programmer portion 301.

[0036] The IMD 101 can provide stimulation pulses to the stimulation electrode 110. A companion indifferent electrode 306 may be used to sink or source the stimulation current generated in analog circuits 303. A portion of the exterior surface 302 of the IMD 101 may be used with respiration electrode 114 to form an impedance plethysmograph. In operation, logic 305 will command the issuance of a train of pulses to the respiration electrode 114 and measure the amplitude of the signal as a function of time in circuits 304. This well known process can measure the respiration of the patient and find the inspiration phase and the expiration phase of a breath. Respiration data collected over minutes and hours can be logged, transmitted, and/or used to direct the therapy.

[0037] When the therapy is invoked by being turned on by the programmer 301 or in response to high rate breathing above an intervention set point, the logic 305 commands the stimulation the phrenic nerve via the stimulation electrode 110 at a time after the beginning of the inspiration phase. Preferable the stimulation begins after the onset of exhalation. There is some flexibility in onset of stimulation. The shape of the stimulation pulses is under study and it may be beneficial to have the logic 305 command stimulation at higher amplitudes of energy levels as the stimulation progresses. It may also be desirable to have stimulation ramp up and ramp down during the therapy. It may prove desirable to stimulate episodically. The therapy may be best administered to every other breath or in a random pattern. The programmer may permit the patient to regulate the therapy as well. However in each case the stimulation of the diaphragm stills the diaphragm resulting in an amount of air trapped in at least one lung and extends the breath duration.

[0038] The duration of the stimulation is under the control of logic 305. It is expected that the therapy will be dispensed with a fixed duration of pulses corresponding to breathing rate. It should be clear that other strategies for setting the duration of stimulation are within the scope of the invention. For example the breathing rate data can be used to set the stimulation duration to reduce breathing rate to a fraction of the observed rate. The therapy may also be invoked in response to detected high rate breathing or turned on at a fixed time of day. In a device where activity sensors are available the device may deliver therapy at times of relative inactivity (resting or sleeping). Activity sensors may also help in the detection and rejection of artifacts. An accelerometer, such as those used in cardiac pacing, would be an exemplary activity sensor.

[0039] FIG. 4 shows a schematic diagram of an alternate partitioning of the system. In this implementation, the respiration sensing is carried out outside the patient with sensor 404, while the implanted portion 400 communicates in real time with an external controller 401 via coils 403 and 402. This respiration sensor 404 may be a conventional respiration belt or thermistor based system. Real time breathing data is parsed in the controller 401 and control signal sent to the IPG 101 to drive stimulation of the phrenic nerve via lead 107. This implementation simplifies IMD 101 portion for the system and may be useful for delivery of therapy to a resting or sleeping patient.

[0040] FIG. 5 is set forth as two panels. The data collected from an experimental animal (pig) is presented in the two panels and should be considered together. Panel 5B plots airflow into and out of the animal against time, while panel 5A plots volume against time. In the experiment the volume data was computed (integrated) from the airflow measurement. The two panels are two ways of looking at the same data collected at the same time. In each panel the dotted tracing 500 in 5B and 502 in panel 5A represent the normal or natural or not-stimulated and therefore underlying breathing pattern of the animal. In panel 5A the inspiration phase of tracing 502 is seen as segment 514. After tracing 502 peaks, the expiration phase begins as indicated by segment 516. The figure shows that along trace 502, the air that is inhaled is exhaled before 2 seconds has elapsed, as indicated by the dotted trace 502 returning to the zero volume level.

[0041] Trace 504 is associated with the unilateral delivery of stimulation 508 to a phrenic nerve. In the tracing the start of stimulation at time 518 is well after the start of inspiration and corresponds approximately to the reversal of airflow from inspiration to expiration as seen at time 518. Very shortly after the stimulation begins the animal inhales more air seen by the bump 520 in the tracing 504 in panel 5B. A small increment in the total volume corresponding to this bump is seen at the same time in panel 5A. Of particular interest is the relatively flat tracing 522 corresponding to no significant change in lung volume during stimulation. Once stimulation terminates the lungs expel air as seen at volume change 524 in panel 5A corresponding to outflow labeled 512 in panel 5B. Only after the exhalation outflow 512 was complete did the sedated experimental animal initiate the next breath (not shown). Thus duration of breath was extended in this case from approximately 2 seconds to approximately 6 seconds resulting in the breathing rate reduction from 30 to 10 breathe per minute. The data support the assertion that adequate phrenic stimulation initiated after inspiration and during expiration can prolong or hold the breath and thus regulate or regularize breathing which it the value of the invention.

[0042] FIG. 6 shows a bout 601 of rapid breathing 603 followed by or preceded by apnea 602 events. This waveform is a presentation of Cheyne-Stokes respiration (CSR) well known in the prior art. The corresponding tracing of blood gas 607 indicates that the rapid breathing drives off blood carbon dioxide (CO2) as indicated the slope of line 606. CSR begins with the rise of CO2 as indicated by ramp line 605 which triggers the rapid breathing. The ventilation drives the CO2 too low resulting in a loss of respiratory drive and an apnea event 602. During the apnea the level of CO2 rises as indicated by the slope of line 604. Once a threshold is reached the cycle repeats.

[0043] FIG. 7 shows a schematic diagram showing the delivery of the inventive therapy in the context of a patient experiencing CSR respiration. The patient experiences several quick breaths 701 and then the device is turned on as indicated by the stimulation pulses 709. The device looks for a natural inspiration and waits until about the turn from inspiration to expiration, then the burst 709 of stimulation is delivered to a phrenic nerve. As explained in connection with FIG. 5 the stimulation delays breath 706. This next breath is also a candidate for the therapy and stimulation burst 710 is delivered to the phrenic nerve delaying breath 707. In a similar fashion the device intervenes in breaths 707 and 708. It is expected that the lower rate breathing resulting from repeated application of the therapy will keep the CO2 level in a normal range 715 and prevent CSR. The therapy could also be invoked in response to a detected bout of CSR but this is not necessary and it is believed that keeping a patient out of CSR is the better therapy.

[0044] It may be noted that the stimulation waveforms vary in FIG. 7 with stimulation 710 rising in amplitude while stimulation 711 decreases in amplitude. Note as well that stimulation 712 ramps up and then down during the therapy. It is expected that the best waveform may vary from patient to patient or may vary over time. Also seen in the figure is a refractory period typified by period 730 that may be implemented in the logic 302 to prevent the device from issuing the therapy too close in time to the last intervention. In general the refractory period effectively disables the deliver of therapy until the refractory period expires. This places an effective low rate on stimulated rate of breathing. The refractory may be fixed, programmable or adjusted based on sensed breathing rate.

[0045] FIG. 8 illustrates the concept of expiratory period stimulation to expand a native breath. Panel 800 shows the inspiratory period 801 of the native breath 803, the peak native inspiration 803 and the extended expiratory period 814. For comparison, trace 805 shows the expiration period of the same breath without stimulation. Panel 810 shows the native phrenic nerve excitation burst 811 that causes spontaneous inspiration 801. Without additional stimulation, inspiration would be over, and expiration will begin shortly after the duration of the native excitation burst. The phrenic nerve electrode stimulation burst 809 begins approximately at the time with the natural excitation 811 stops. This time point also approximately coincides with the peak native inspiration 803. The respiratory signal on panel 800 can be a transthoracic impedance signal. Point 802 marks the inspiratory turn. Preferably, stimulation may begin after a delay following the inspiratory turn point or after the peak inspiratory point.

[0046] FIG. 9 illustrates the rejection of an artifact caused by cough or motion during sleep. Respiratory signal 900 shows disturbed breath 902 caused by cough. This signal pattern can be recognized and rejected by identification of: an unacceptably fast inhalation slop, an unacceptably high amplitude, and/or the presence of a high frequency component in the respiratory signal spectrum. In all cases the stimulatory logic will reject this breath and not apply a stimulation burst 910. The next breath may also be rejected to allow the system to come to a stead state by extending the blanking period window to the duration of two normal breaths. After the normal pattern of breathing is restored 903, stimulation is resumed 911 and 912. In addition, implantable stimulator may be equipped with an accelerometer. Acceleration trace 920 shows high acceleration (vibrations) 921 corresponding to the patient's cough or motion. The acceleration signal may be used to reject a breath or several breaths and delay stimulation.