Safety features for use in medical devices

Abstract

A therapy system for applying an electrical signal to an internal anatomical feature of a patient includes an implantable component and an external component. The medical device can be checked for safety issues by periodically initiating a sequence of tests of an H-bridge circuit, and, during each test, monitoring a current flow through a sensing resistor electrically connected between a sensing connection of the H-bridge circuit and a ground. Current flow through the sensing resistor indicates that both series electrical switches within at least one of the two pairs of series electrical switches are active during that test.

1 . A method of performing a safety check in an implantable medical device, during which electrical signals and therapy are not delivered to the patient, the method comprising:
 periodically initiating a sequence of tests of an H-bridge circuit of an implantable device, the implantable device comprising a H-bridge circuit, a field-programmable gate array (“FPGA”), a microprocessor, a current source, a voltage supply connection, a grounding connection, a current sensing resistor, and two pairs of series electrical switches connected in parallel between the voltage supply connection and the grounding connection, the sequence of tests selected to test each switch connection of the electrical switches in the H-bridge circuit;   during each test, monitoring a current flow through a current sensing resistor electrically connected between the current source of the H-bridge circuit and a ground, wherein current flow through the current sensing resistor indicates that both series electrical switches within at least one of the two pairs of series electrical switches are active during that test.   
     
     
         2 . The method according to  claim 1 , wherein monitoring the current flow further comprises:
 receiving signals indicative of a voltage drop across the current sensing resistor by the microprocessor, the microprocessor electrically coupled to the H-bridge circuit;   processing the signals to determine the current flow through the sensing resistor by the microprocessor; and sending a resulting signal to the FPGA to continue therapy or to abort therapy, the FPGA electrically coupled to the microprocessor.   
     
     
         3 . A method according to  claim 2 , further comprising controlling the voltage of the gate inputs of the H-bridge circuit by the FPGA based upon the resulting signal. 
     
     
         4 . A method according to  claim 2 , wherein receiving signals further comprises:
 receiving a signal from an analog-to-digital converter indicative of a voltage drops across the sensing resister, wherein the analog-to-digital converter is electrically connected to the microprocessor, and electrically connected to the current sensing resister.   
     
     
         5 . The method according to  claim 1 , further comprising performing the sequence of tests periodically during use of the implantable medical device but not while therapy is being delivered. 
     
     
         6 . The method according to  claim 1 , further comprising aborting use of the implantable medical device if the current flow is above or below predetermined limits. 
     
     
         7 . The method according to  claim 1 , wherein the two pairs of series electrical switches includes first, second, third, and fourth electrical switches, the first and second electrical switches connected in series to form a first pair and the third and fourth electrical switches connected in series to form a second pair, the first and second pairs connected in parallel with each other between the voltage supply connection and the current source. 
     
     
         8 . The method according to  claim 7 , wherein the sequence of tests includes:
 activating the first and second electrical switches, while deactivating at least the third and fourth electrical switches;   activating the third and fourth electrical switches while deactivating at least the first and second electrical switches;   activating the first and third electrical switches while deactivating at least the second and fourth electrical switches; and   activating the second and fourth electrical switches, while deactivating at least the first and third electrical switches.   
     
     
         9 . The method according to  claim 7 , wherein the implantable device comprises a second H bridge circuit, the second H-bridge circuit including a second two pairs of series electrical switches connected in parallel between the voltage supply connection and a second current source, the second two pairs of series electrical switches including fifth, sixth, seventh, and eighth electrical switches, the fifth and sixth electrical switches connected in series to form a third pair and the seventh and eighth electrical switches connected in series to form a fourth pair, the third and fourth pairs connected in parallel with each other between the voltage supply connection and the current source. 
     
     
         10 . The method according to  claim 9 , wherein the sequence of tests includes:
 activating the fifth and sixth electrical switches, while deactivating the first, second, third, fourth, seventh, and eighth electrical switches;   activating the seventh and eighth electrical switches while deactivating the first, second, third, fourth, fifth, and sixth electrical switches;   activating the fifth and seventh electrical switches while deactivating the first, second, third, fourth, sixth, and eighth electrical switches; and   activating the sixth and eighth electrical switches while deactivating the first, second, third, fourth, fifth, and seventh electrical switches.   
     
     
         11 . A medical device configured to apply an electrical stimulus to tissue of a patient, the medical device comprising:
 a first electrical lead, including a first tip connection and a first ring connection;   a second electrical lead, including a second tip connection and second ring connection;   a voltage supply connection;   a field programmable gate array;   a microprocessor electrically connected to the field programmable gate array;   a first current source;   a first grounding connection;   a first sensing resistor electrically connected to the first current source and the first grounding connection;   a digital to analog convertor electrically connected to the microprocessor and the first current source;   an analog to digital convertor electrically connected to first sensing resistor and the microprocessor; and   a first H-bridge circuit electrically connected to the field programmable gate array, the voltage supply connection, and the first current source and including first and second pairs of series electrical switches connected in parallel, and wherein:   the first tip connection is electrically connected between the first pair of series electrical switches of the first H-bridge circuit;   the first ring connection is electrically connected between the second pairs of series electrical switches of the first H-bridge circuit.   
     
     
         12 . The medical device of  claim 11  further comprising
 a second current source electrically connected to the digital to analog converter; 
 a second grounding connection, 
 a second sensing resistor electrically connected to the second current source and the second grounding resistor; 
 a second analog to digital converter electrically connected between the second current source and the second sensing resister; 
 a second digital to analog converter electrically connected to the microprocessor and the second current source; 
 a second H-bridge circuit electrically connected to the field programmable gate array, the voltage supply connection, and the second current source and including first and second pairs of series electrical switches connected in parallel, and wherein: 
 the second tip connection is electrically connected between the first of series electrical switches of the second H-bridge circuit; 
 the second ring connection is electrically connected between the second pair of series electrical switches of the second H-bridge circuit. 
 
     
     
         13 . The medical device according to  claim 12 , wherein the microprocessor is electrically coupled to each H-bridge circuit through an analog-to-digital converter and a digital-to-analog converter, wherein:
 the first digital-to-analog converter is electrically connected to the first current source between the first H-bridge circuit and the first current sensing resistor and the second digital-to-analog converter is electrically connected to the second current source between the second H-bridge circuit and the second current sensing resistor;   the first analog-to-digital converter is electrically connected to the first sensing resistor;   the second analog to digital converter is electrically connected to the second sensing resistor; and   wherein the first and second analog-to-digital converter send signals to the microprocessor indicative of voltage drops across each of the first and second current sensing resistors and the first and second digital to analog convertors control the first and second current sources.   
     
     
         14 . The medical device according to  claim 11 , wherein at least one of the microprocessor and the FPGA is configured to periodically perform a sequence of tests on the H-bridge circuit during operation of the medical device but not while therapy is being delivered to ensure proper operation of the device; wherein if the sequence of tests indicate an abnormality in operation of the H-bridge circuit, the microprocessor aborts use of the medical device. 
     
     
         15 . The medical device according to  claim 14 , wherein at least one of the microprocessor and the FPGA is configured to perform the sequence of tests every four seconds. 
     
     
         16 . The medical device according to  claim 11 , wherein the medical device is used for treating at least one of a plurality of disorders of the patient selected from the group consisting of obesity, pancreatitis, irritable bowel syndrome, diabetes, hypertension, metabolic disease, inflammatory disorders and combinations thereof. 
     
     
         17 . A method of calibrating a medical device configured to deliver electrical signals to a patient as at least a portion of a therapy, the method comprising:
 detecting a positive voltage peak output between two electrical contacts of the medical device;   detecting a negative voltage peak output by the two electrical contacts of the medical device;   comparing the positive voltage peak and the negative voltage peak to determine at least a portion of an impedance between the two electrical contacts; and   upon detecting that the impedance is outside a predetermined range, generating an alarm indicating the presence of a direct current signal applied to the tissue of the patient.   
     
     
         18 . The method according to  claim 17 , further comprising:
 detecting a second positive voltage output between two electrical contacts of the medical device;   detecting a second negative voltage output by the two electrical contacts of the medical device;   comparing the second positive voltage and the second negative voltage to determine a second portion of the impedance between the two electrical contacts.   
     
     
         19 . The method according to  claim 18 , wherein the portion of the impedance is a capacitive portion of the impedance. 
     
     
         20 . The method according to  claim 17 , wherein the predetermined range is determined based at least on a difference between magnitudes of the positive voltage peak and the negative voltage peak, wherein the predetermined range is exceeded if the difference would result in a current which exceeds about one microamp. 
     
     
         21 . The method according to  claim 17 , further comprising, upon detecting that the impedance is outside a predetermined range, and prior to generating the alarm:
 decreasing an operational voltage of the medical device;   detecting a second positive voltage peak output by the two electrical contacts of the medical device operating at the decreased operational voltage;   detecting a second negative voltage peak output by the two electrical contacts of the medical device operating at the decreased operational voltage;   comparing the second positive voltage peak and the second negative voltage peak to determine a second impedance between the two electrical contacts;   upon detecting that the second impedance is outside a second predetermined range, generating an alarm indicating the presence of a direct current signal applied to the tissue of the patient and halting use of the medical device.   
     
     
         22 . The method according to  claim 17 , further comprising:
 halting use of the medical device upon detection of the direct current signal.   
     
     
         23 . A medical device configured to apply an electrical stimulus to tissue of a patient, the medical device comprising:
 a first electrical lead configured to be implanted in a patient and to introduce electrical signals at a nerve, the first electrical lead having electrode connections including a first tip connection and a first ring connection;   a second electrical lead configured to be implanted in a patient and to introduce electrical signals at a nerve, the second electrical lead having electrode connections including a second tip connection and a second ring connection;   a voltage source;   at each of the first and second tip connections and first and second ring connections, a first capacitor and a second capacitor connected in series between the respective electrode connection and a ground;   a programmable circuit electrically connected to locations between each of the first and second capacitors, the programmable circuit configured to execute program instructions which, when executed, cause the programmable circuit to:   calculate initial capacitive ratios between the first capacitor and the second capacitor for each of the first and second tip connections and first and second ring connections;   store each of the initial capacitive ratios in a memory associated with the programmable circuit;   prior to initiating delivery of an electrical therapy to a patient via the first and second electrical leads, calculating second capacitive ratios between the first capacitor and the second capacitor for each of the first and second tip connections and first and second ring connections;   compare each of the second capacitive ratios to the respective initial capacitive ratios to validate the integrity of the capacitive divider network.   
     
     
         24 . The medical device of  claim 23 , wherein the programmable circuit comprises a microprocessor, and wherein the locations between each of the first and second capacitors are electrically connected to general purpose I/O pins of the microprocessor. 
     
     
         25 . The medical device of  claim 24 , wherein the programmable circuit is further configured to calculate an average initial capacitive ratio based on the initial capacitive ratios associated with each of the first and second tip connections and first and second ring connections. 
     
     
         26 . The medical device of  claim 24 , wherein the programmable circuit is further configured to determine if any of the initial capacitive ratios varies from the average initial capacitive ratio by more than a predetermined amount, and if so, halt calibration of the output current and suspend delivery of the electrical therapy. 
     
     
         27 . The medical device of  claim 26 , wherein the predetermined amount comprises about 10% variation from the average initial capacitive ratio. 
     
     
         28 . The medical device of  claim 23 , wherein the programmable circuit is further configured to determine if any of the second capacitive ratios varies from the average initial capacitive ratio by more than a predetermined amount, and if so, halt calibration of the output current and suspend delivery of the electrical therapy. 
     
     
         29 . A method of calibrating an output measurement circuit on one or more electrical leads of an implantable medical device, each of the one or more electrical leads having one or more electrode connections positioned to introduce electrical signals at a portion of the vagal nerve, the method comprising:
 calculating, using a programmable circuit, initial capacitive ratios between a first capacitor and a second capacitor connected in series between each respective electrode connection and a ground for each of the first and second tip connections and first and second ring connections;   storing each of the initial capacitive ratios in a memory associated with a programmable circuit;   prior to initiating delivery of an electrical therapy to a patient via the one or more electrical leads, calculating second capacitive ratios between the first capacitor and the second capacitor for each of the electrode connections;   comparing each of the second capacitive ratios to the respective initial capacitive ratios.   
     
     
         30 . The method of  claim 29 , further comprising determining if any of the initial capacitive ratios varies from the average initial capacitive ratio by more than a predetermined amount, and if so, halt calibration of the output current and suspend delivery of the electrical therapy. 
     
     
         31 . The method of  claim 30 , wherein the predetermined amount comprises about 10% variation from the initial capacitive ratio for each capacitive divider.