Abstract:
A method and system for monitoring phrenic nerve function and preventing phrenic nerve injury during cardiac ablation. The system includes a pacing device operable to transmit stimulation energy to the phrenic nerve through target tissue proximate the phrenic nerve, a plurality of assessment electrodes operable to make comparisons between baseline, real-time, and predetermined threshold values for CMAP signal amplitude and amplitude over time. The processing device may be connected to an ablation console, and the processing device may interrupt or adjust an ablation procedure controlled by the ablation console and/or generate a system alert in response to one of these comparisons, if the comparison indicates phrenic nerve injury. The method includes applying stimulation energy to the phrenic nerve, recording diaphragmatic CMAP signals in response to the stimulation energy, and adjusting an ablation procedure and/or automatically generating a system alert in response to comparisons performed by the processing device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    n/a 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    n/a 
       FIELD OF THE INVENTION 
       [0003]    The present invention relates to a method and system for monitoring phrenic nerve function and preventing phrenic nerve injury during an ablation procedure. 
       BACKGROUND OF THE INVENTION 
       [0004]    Cryoablation techniques eradicate arrhythmogenic tissue by inducing hypothermia, necrosis, and apoptosis through the application of freezing temperatures to the target cardiac tissue. An unintended consequence of cryoablation, particularly when ablating tissue near the right-sided pulmonary veins in the left atrium, is the attenuation of phrenic nerve function due to the freezing temperatures permeating through the cardiac tissue and into the phrenic nerve. This phenomenon is also a concern when ablating nearby tissue using non-cryogenic ablation modalities, such as radiofrequency ablation. The phrenic nerve is made up mostly of motor nerve fibers that produce contractions of the diaphragm and thus affect breathing and respiration patterns and conditions. In addition, the phrenic nerve provides sensory innervation for many components of the mediastinum and pleura, as well as the upper abdomen, especially the liver, and the gall bladder. Injury to the phrenic nerve may severely impact normal respiratory function and can require many weeks or months to resolve. In the worst cases, this reduced function requires mechanical ventilation assistance to maintain respiration. This side effect of cryoablation can manifest as a transient phrenic functional block, transient phrenic nerve palsy (PNP), or longer-term phrenic nerve injury. 
         [0005]    The potential for phrenic nerve injury can be significantly reduced through the use of phrenic nerve monitoring during ablation procedures. By pacing the phrenic nerve superior to the ablation site and monitoring the amplitude of compound motor action potentials (CMAP), the integrity of the phrenic nerve can be continuously assessed during ablation. If cryoablation energy is removed at the first sign of phrenic impairment, the injury is in almost all cases transient, with normal phrenic function returning within minutes. 
         [0006]    Currently, phrenic monitoring is typically done using one or more of several possible methods, such as pacing the phrenic nerve and using continuous fluoroscopy during the ablation to visualize a consistent diaphragmatic response, palpitating the abdomen to confirm diaphragmatic movement, intercardiac echocardiography (ICE imaging), or fetal heart monitoring. However, these methods all require vigilance on the part of the operator, and can distract the physician from the main focus of ablating tissue. Additionally, in the case of fluoroscopic monitoring, the patient is exposed to increased x-ray radiation. 
         [0007]    It is therefore desirable to provide an automated method of monitoring phrenic nerve function. This automated method would reduce physician distraction, reduce procedure fluoroscopy time, and ensure timely identification of transient injury, leading to prevention of long-term phrenic injury. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention advantageously provides a system and method for monitoring phrenic nerve function and preventing phrenic nerve injury during an ablation procedure. In one embodiment, the system may generally include a pacing device operable to transmit a stimulation energy to the phrenic nerve through a target tissue proximate the phrenic nerve, a plurality of assessment electrodes operable to detect diaphragmatic compound motor action potential (CMAP) signals in response to the stimulation energy, and a processing device. The processing device may be programmable to determine, based on the CMAP signals detected by the plurality of assessment electrodes, at least one of a baseline amplitude value for the diaphragmatic CMAP signals and a baseline amplitude over time value for the diaphragmatic CMAP signals, the processing device also being programmable to assess in real time at least one of a treatment amplitude value for the diaphragmatic CMAP signals and a treatment amplitude over time value for the diaphragmatic CMAP. Further, the processing device may be programmable to average the diaphragmatic CMAP signals. Still further, the processing device may be programmable to perform several comparisons between signals. For example, the processing device may compare the baseline amplitude value and the treatment amplitude value for the diaphragmatic CMAP signals, compare the treatment amplitude value to a predetermined threshold amplitude value for the diaphragmatic CMAP signals, compare the baseline amplitude over time value and the treatment amplitude over time value for the diaphragmatic CMAP, and compare the treatment amplitude over time value to a predetermined threshold amplitude over time value for the diaphragmatic CMAP. Additionally, the processing device may be programmable to automatically generate a system alert when at least one alert criterion occurs. Alert criteria may include the CMAP signals being out-of-phase between assessment electrodes, the baseline CMAP signal amplitude being low relative to a running average of CMAP signal amplitudes, and stimulation energy failing to be detected by the processing device. The alert criteria may also include the treatment amplitude value exceeding the threshold amplitude value and the treatment amplitude over time value exceeding the threshold amplitude over time value. These alert criteria may indicate phrenic nerve injury. The system may be in communication with a treatment system, such as a treatment system including a console and a treatment device capable of ablating tissue when in use. As a non-limiting example, the treatment system may be a cryoablation system, a radiofrequency ablation system, or combination thereof. Other energy modalities may also be used in the treatment system. The processing device may be programmable to interact with the treatment system to automatically interrupt or adjust the treatment procedure. For example, the processing device may interact with the treatment console, either directly or indirectly (such as when the processing device communicates with a foot switch of the treatment system) to interrupt or adjust the circulation of cryogenic fluid through a treatment element of the treatment device. The processing device may interrupt or adjust the treatment procedure in response to the satisfaction of one or more alert criteria indicating phrenic nerve injury and/or impairment. 
         [0009]    In another embodiment, the system may generally include a pacing device operable to transmit stimulation energy to the phrenic nerve through a target tissue structure proximate the phrenic nerve, a plurality of assessment electrodes operable to detect diaphragmatic compound motor action potential (CMAP) signals in response to the stimulation energy, and a processing device programmable to make at least one comparison based on the CMAP signals detected by the plurality of assessment electrodes. The comparison may be, for example, between a baseline amplitude value and a real-time treatment amplitude value for the diaphragmatic CMAP signals, a real-time treatment amplitude value and a predetermined threshold amplitude value for the diaphragmatic CMAP signals, a baseline amplitude over time value and a real-time treatment amplitude over time value for the diaphragmatic CMAP signals, and a real-time treatment amplitude over time value and a predetermined threshold amplitude over time value for the diaphragmatic CMAP signals. The processing device may further be programmable to average the diaphragmatic CMAP signals. The processing device may be in electrical communication with an ablation console, and the ablation console may be in electrical and/or fluid communication with an ablation device. Further, the processing device may be programmable to automatically adjust the operation of the ablation device by the console based on one or more of the comparisons. 
         [0010]    A method for monitoring patient phrenic nerve function in response to the transmission of a stimulation energy to the phrenic nerve may generally include applying a stimulation energy from a pacing device to the phrenic nerve, recording diaphragmatic compound motor action potential (CMAP) signals using a plurality of assessment electrodes attached to the patient proximate the xiphoid process and the right costal margin of the patient&#39;s ribcage, and transmitting the CMAP signals from the plurality of assessment electrodes to a processing device having a microcontroller. The microcontroller may be programmable to average the CMAP signals, determine a baseline CMAP signal amplitude and a baseline CMAP signal amplitude over time, receive from the plurality of assessment electrodes record real-time CMAP signal amplitudes and CMAP signal amplitudes over time, the real-time CMAP signal amplitudes and amplitudes over time being received during an ablation procedure within the patient&#39;s heart, receive from a user a predetermined threshold CMAP signal amplitude value and a predetermined threshold CMAP signal amplitude over time value, compare the baseline CMAP signal amplitude and the real-time CMAP signal amplitudes, compare the baseline CMAP signal amplitude over time and the real-time CMAP signal amplitudes over time, compare the real-time CMAP signal amplitudes and the predetermined threshold CMAP signal amplitude, and compare the real-time CMAP signal amplitudes over time and the predetermined threshold CMAP signal amplitudes over time. Finally, the processing device may be in electrical communication with a cryoablation console having an ablation device positioned in contact with an area of tissue within the patient&#39;s heart. The processing device may automatically adjust the amount of heat removed from the area of tissue by the ablation device (for example, interrupting or adjusting the amount of cryogenic fluid circulated within the ablation device and thereby interrupting the ablation of tissue) in response to the comparisons performed by the microcontroller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
           [0012]      FIG. 1  shows an illustration of a human heart and related anatomy; 
           [0013]      FIG. 2  shows an additional illustration of a human heart and related anatomy; 
           [0014]      FIG. 3  shows a medical device system constructed in accordance with the principles of the present invention, the medical device system including a pacing device; 
           [0015]      FIG. 4  shows the distal portion of a pacing device; 
           [0016]      FIG. 5  shows electrode lead positioning on a patient; 
           [0017]      FIG. 6  shows a schematic diagram of the input and output ports of an exemplary processing device; 
           [0018]      FIG. 7  shows an exemplary treatment device usable with the system of  FIG. 4 ; 
           [0019]      FIG. 8  shows an exemplary placement of a pacing device and a treatment device within a heart; 
           [0020]      FIG. 9  shows exemplary CMAP signals in response to pacing; 
           [0021]      FIG. 10  shows exemplary CMAP signal indicating phrenic nerve injury; 
           [0022]      FIG. 11  shows a flowchart of initial processing device operation; 
           [0023]      FIG. 12  shows a flowchart of processing device operation in steady state mode; 
           [0024]      FIG. 13  shows a schematic diagram of signal acquisition, display, and storage; and 
           [0025]      FIGS. 14A and 14B  show a circuit diagram of the medical device system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring to  FIGS. 1 and 2 , the close proximity of the phrenic nerve segments to the right atrium and left ventricle is illustrated. These cardiac regions may be the location or origin of heart arrhythmias or other physiological maladies and thus targeted for tissue ablation in order to remove or otherwise remedy the abnormal electrophysiological occurrence. In thermally treating or ablating select cardiac regions, the phrenic nerve may be at risk of being similarly, although unintentionally, ablated. This could severely impact the normal respiratory functioning of the patient. The risk of such unintentional and undesirable destruction or application of cryogenic treatment or thermal energy to this and other cursory structures compels a desire to monitor or otherwise detect potentially-damaging consequences during treatment. 
         [0027]    The phrenic nerve generally includes two segments: the right and left phrenic nerves. Both phrenic nerves run from C3, C4 and C5 vertebrae along the anterior scalene muscle deep to the carotid sheath. The right phrenic nerve passes over the brachlocephalic artery, posterior to the subclavian vein, and then crosses the root of the right lung anteriorly and then leaves the thorax by passing through the vena cava hiatus opening in the diaphragm at the level of T8. The right phrenic nerve passes over the right atrium. The left phrenic nerve passes over the pericardium of the left ventricle and pierces the diaphragm separately. 
         [0028]    Referring now to  FIGS. 3-8 , a medical device system is shown. The system  20  may generally include a pacing device  22 , one or assessment electrodes  24 , and a processing device  26  (as shown in  FIG. 3 ). The pacing device  22 , assessment electrodes  24 , and processing device  26  may be usable with existing treatment systems, such as cryoablation consoles (for example, the CryoConsole Cardiac CryoAblation System (Medtronic, Inc., Minneapolis, Minn.). Thus, existing systems may be retrofitted for assessment functionality, thereby providing the physician with an automated method of monitoring phrenic nerve function during cardiac treatment procedures. The system  20  may be operated in conjunction with a treatment device  28  (as shown in  FIG. 7 ) and a console  30 , which may be the same console  30  as shown in  FIGS. 3 and 7 . The pacing device  22  may include an elongate body  32  sized to be passable through a patient&#39;s vasculature. The elongate body  32  may define a proximal portion  34  and a distal portion  36 , and may further include one or more lumens disposed within the elongate body  32  to provide mechanical and/or electrical communication between the proximal portion  34  of the elongate body  32  and the distal portion  36  of the elongate body  32 . The distal portion  36  of the pacing device  22  may further include one or more pacing electrodes  38 . The pacing electrodes  38  may be used to apply electrical and/or magnetic impulses to the phrenic nerve, such indirectly through adjacent tissue like the SVC. As a non-limiting example, the pacing electrodes  38  may be band electrodes that at least partially encircle the device (as shown in  FIGS. 3 and 4 ). However, it will be understood that any size, number, and configuration of pacing electrodes  38  may be used to effectively pace the phrenic nerve. The excitation (i.e. pacing) energy source  39  may be housed within or otherwise provided as part of the console  30 , with the pacing device  22  being releasably coupled to the console  30  during operation thereof. 
         [0029]    The pacing device  22  may be steerable, or at least transitionable from a substantially linear configuration  40  to an arcuate configuration  42 , such as the lasso-type shape shown in  FIG. 4 . That is, the distal portion  36  of the elongate body  32  may be flexible enough to be deflected in any of a variety of directions and into a circular or semi-circular shape of the lasso-configuration  42 . The pacing device  22  may be coupled to an energy source for stimulating tissue. For example, the energy source may be integrated within the processing device  26 . The pacing device  22  may include a handle element  46  coupled to the proximal portion  34  of the elongate body  32 , where the handle  46  may include an element such as a lever or knob  47  for manipulating the elongate body  32  of the pacing device  22 , for example, to transition the distal portion  36  of the elongate body  32  from a substantially linear configuration  40  to a lasso-type configuration  42 , or to steer the pacing device  22  through the patient&#39;s vasculature to the treatment site. The handle  46  may include an electrical connector  48  for connecting the pacing device  22  to the console  30 . 
         [0030]    Continuing to refer to  FIGS. 3-8 , the system  20  may include one or more assessment electrodes  24  in electrical communication with a processing device  26 . As a non-limiting example, the system  20  may include a standard set of ten electrodes, signals from which the processing device  26  may use to generate typical 12-lead electrocardiogram (EKG) signal interpretation results. Alternatively, only two or three assessment electrodes  24  may be used (as shown in  FIG. 3 ). The assessment electrodes  24  may be operable to detect compound motor action potential (CMAP), which represents a summation of substantially simultaneous action potentials from several muscle fibers in the same anatomical area. For example, the assessment electrodes  24  may be operable to detect CMAP signals from the diaphragm. Each of the assessment electrodes  24  may be connected to the processing device  26  by a lead  49 . The processing device  26  may generally include a housing  50 , at least one microcontroller  52 , a display  54 , one or more user control devices (such as buttons, knobs, and the like), input ports, and output ports (for example, as shown in  FIG. 6 ). It will be understood that as used herein to refer to processing steps, the term “processing device  26 ” includes the microcontroller  52  and any additional circuitry necessary to perform one or more algorithms required by the processing step. In the non-limiting example shown in  FIG. 6 , the processing device  26  may include a plurality of input and output ports, including USB ports, serial ports, and user input devices. As shown in  FIG. 10 , the processing device  26  may include such input ports as a power input port  56 , ECG lead input ports  58 , an on/off button  60 , and user input buttons such as threshold percentage set  62 , monitor activation  64 , and reset  66 . Additionally, the processing device  26  may include such output ports as audio alerts  68  (for example, a connection for a piezoelectric buzzer), visual alerts  70  (for example, one or more LED lights), low amp alerts  71 , an output port  72  for a visual display (for example, a parallel or HDMI port connectable to a computer screen), an output port  73  for connection between the processing device  26  and a console  30  (for example, a cryoconsole), and an output port  74  for exporting and storing data, such as a USB or serial port. 
         [0031]    The processing device  26  may be in electrical communication with the console  30 . For example, when the assessment electrodes  24  transmit signals to the processing device  26  that the processing device  26  interprets as indicating impaired phrenic nerve function or phrenic nerve injury, the processing device  26  may automatically adjust operation of the console  30 , such as increasing the temperature of a cryoballoon, stopping the application of radiofrequency energy by the treatment device, or the like. Output port  73  may transmit signals to the console  30  to directly or indirectly adjust treatment characteristics, such as the temperature of the treatment element, phase of ablation energy, pulsing sequence of ablation energy, or the like. The console  30  may include a foot switch  75 , and the processing device  26  may communicate with the console  30  to open or close the foot switch in order to stop, start, or otherwise control the administration of ablation energy. In the case of cryoablation using a cryogenic fluid, the processing device  26  may communicate with the console  30  to operate the foot switch  75  to control the flow of cryogenic fluid within, and thus the temperature of, the treatment element. Alternatively, the processing device  26  may be in direct communication with the foot switch  75 . It will be understood that the processing device  26  may include additional or different input and output ports than those shown and described herein. 
         [0032]    Referring to  FIG. 7 , the system  20  may be used in conjunction with a treatment device  28  having an elongate body  76  with a proximal portion  78  and a distal portion  80 , and a treatment element  82  at the distal portion  80 . For example, the treatment element  82  may be a cryoballoon (as shown in  FIG. 7 . Although not shown, the elongate body  76  may include one or more lumens within the elongate body  76 . For example, if the treatment device  28  is configured for use with cryotreatment procedures, the elongate body  76  may include a fluid injection lumen and a fluid exhaust lumen. Additionally, the elongate body  76  may include a guidewire lumen  84  that may be movably disposed within and/or extending along at least a portion of the length of the elongate body  76  for over-the-wire applications. The guidewire lumen  84  may define a proximal end and a distal end, and the guidewire lumen  16  may be movably disposed within the elongate body  76  such that the distal end of the guidewire lumen  84  extends beyond and out of the distal portion of the elongate body  12 . Further, the cryoballoon  82  may be affixed to the distal portion of the guidewire lumen  84  at one end and affixed to the distal portion  80  of the treatment device elongate body  76 , such that longitudinal movement of the guidewire lumen  84  within the treatment device  28  changes the shape of (such as by compressing or extending) the cryoballoon  84 . However, it will be understood that the treatment device  28  can have any type or configuration of one or more treatment elements. As an alternative example, the treatment device  28  may be a focal catheter having one or more radiofrequency electrodes, a basket catheter having a plurality of electrodes affixed to a plurality of splines, or may be any other device capable of ablating cardiac tissue, including, for example, by cryoablation, radiofrequency ablation, ultrasound ablation, laser ablation, hot balloon ablation, or combinations thereof. 
         [0033]    Like the pacing device  22 , the treatment device  28  may include a handle element  86  coupled to the proximal portion of the elongate body  76 , where the handle  86  may include an element such as a lever or knob  88  for manipulating the elongate body  76  of the treatment device  28 , for example, to extend or retract the guidewire lumen  84  to adjust the shape of the cryoballoon  82  (or to expand or adjust the size, shape, or configuration of a treatment element other than a cryoballoon), or to steer the treatment device  28  through the patient&#39;s vasculature to the treatment site. The handle  86  may further include circuitry  89  for identification and/or use in controlling of the treatment device  28  or another component of the system  20  or device used therewith. For example, the handle  86  may include one or more pressure sensors to monitor the fluid pressure within the treatment device  28 . Additionally, the handle  86  may be provided with a fitting  90  for receiving a guidewire that may be passed into the guidewire lumen  84 . 
         [0034]    The handle  86  may also include connectors  91  that are matable directly to the console  30  by way of one or more umbilicals for providing fluid communication between, for example, a cryogenic fluid source  92  and a cryogenic fluid recovery container  94 . Additionally or alternatively, the handle  86  may also include connectors that are matable to the console  30  to provide energy to one or more treatment elements. For example, the handle  86  may be connectable to a power generator  95  (such as a radiofrequency generator, ultrasound generator, light source, or the like) that is located within the console or at an external location. The console  30  may also include a vacuum pump  96  to facilitate removal of expanded cryogenic fluid from the treatment element. The console  30  may further include one or more computers  98 , displays  100 , user input devices, and the like for controlling the system  20 , treatment device  28 , and the treatment procedure. 
         [0035]    When the system  20  is in use, each of the assessment electrodes  24  may be attached to an external surface of the patient, such as in the typical 12-lead EKG placement. However, the right arm  24 A and left arm  24 B electrodes may be placed on the patient as shown in  FIG. 5 , above the xiphoid process and along the right costal margin to receive CMAP signals which are monitored on lead I during the ablation procedure. Lead I may be the voltage difference between the right arm  24 A and left arm  24 B electrodes, and thus the positioning shown in  FIG. 5  may allow the processing device  26  to monitor signals from the diaphragm. As shown in  FIG. 8 , for example, the distal portion  36  of the pacing device  22  may be positioned within the superior vena cava (SVC) of the patient&#39;s heart and used to transmit electrical impulses to the phrenic nerve through the SVC tissue. The phrenic nerve is within the range of the pacing impulses, shown as area  101 . The assessment electrodes  24  then detect and record electrical signals from the diaphragm to monitor phrenic nerve function. 
         [0036]    Referring now to  FIGS. 9 and 10 , exemplary CMAP signals are shown, which may be the type of output generated by the one or more assessment electrodes  24 . As shown and described in  FIG. 12 , the processing device  26  monitors CMAP signal amplitude A during the treatment procedure.  FIG. 9  shows CMAP signals in response to pacing, such as by the pacing device  22  positioned within the SVC. D m  is the detection window, the time after the pacing spike during which the processing device  26  may monitor the CMAP signal.  FIG. 10  shows a first CMAP signal of which the maximum amplitude (A maxB ) is determined. The amplitude A of the second CMAP signal is less than A maxB  by amount delta Δ. If phrenic nerve impairment occurs as the result of treatment by the treatment device  28 , the subsequent CMAP signals may display attenuated amplitude (for example, as shown in the second CMAP signal of  FIG. 9 ). If Δ is greater than the predetermined threshold percentage, the processing device  26  will generate an alert for the operator. The operator may manually enter a desired threshold percentage for an alert to be generated. The threshold may be correlated to A; that is, the threshold percentage in CMAP amplitude decline may be set based on a target A. For example, the operator may request an alert if the CMAP amplitude is less than approximately 35% of the baseline amplitude. 
         [0037]    Referring now to  FIGS. 11 and 12 , flowcharts of processing device operation are shown.  FIG. 11  shows the initial processing device operation, from initialization to steady state mode. As shown, once the physician begins pacing the phrenic nerve using the pacing device  22 , the processing device  26  is turned on by the user (although it will be understood that the processing device  26  may be turned on before pacing begins), at which point the processing device  26  identifies the baseline amplitude of CMAP signals obtained from the one or more assessment electrodes  24  affixed to the patient&#39;s body and perform a standard analog-to-digital (“A-to-D”) conversion on the signals. For example, the processing device  26  may include an algorithm for automatically determining a baseline amplitude value and for identifying substantial variation between heartbeats. For example, a baseline amplitude may be between approximately 1.5 mV and approximately 6 mV. A substantial variation between heartbeats may be indicated if the baseline amplitude changes by more than 0.275 V. The signals may also undergo one or more filtering processes before A-to-D conversion (for example, as shown in  FIG. 13 ). Once a baseline CMAP amplitude is determined, the processing device  26  may generate one or more visual and/or audible alerts that indicate the system  20  is ready for the user to begin ablation. Amplitude data can be stored, exported (for example, using a USB port and device), and/or displayed. If the processing device  26  identifies a low initial amplitude, the processing device  26  may alert the operator that, for example, connections between the processing device  26  and the assessment electrodes  24  and/or between the assessment electrodes  24  and the patient are poor, or that the patient may be more susceptible to phrenic nerve palsy (PNP). The user sets the baseline amplitude (A maxB ) in the processing device  26 . The user then enters into the processing device  26  a threshold percentage of CMAP amplitude decline that will cause the processing device  26  to generate an alert or alarm signal (also referred to as “trigger threshold,” T t ). In determining this threshold, the user may consider the site of ablation, the duration of treatment, and/or other factors. Further, research suggests that a 30% to 35% reduction in CMAP amplitude during a treatment (i.e. ablation) period is indicative of phrenic nerve impairment. Slope information (i.e. amplitude over time) is also measured. Once these steps are performed, the processing device  26  may be said to be operating in steady state mode. 
         [0038]      FIG. 12  shows a flowchart of the processing device operating in steady state mode. Generally, the processing device  26  monitors the amplitude of the CMAP signals received from the assessment electrodes  24  and compares them to the A maxB . The processing device  26  also may assess slope information (i.e. amplitude over time). Further, the processing device  26  may include an algorithm that uses pattern and rate analysis to assess complex CMAP waveforms. If the CMAP signal slope and/or amplitude crosses the preset threshold, the processing device  26  identifies this change as impairment of phrenic nerve function and generates and alert signal for the operator and/or communicates with the console  30  to terminate ablation energy to prevent further, and potentially long-lasting, impairment. By alerting the operator to possible phrenic nerve impairment, manifested by a significant measurable decrease in CMAP amplitude during the treatment procedure, the processing device  26  may allow the physician to intervene before phrenic nerve injury occurs, reducing procedural risk and potential patient sequelae. 
         [0039]    As shown in the flow chart of  FIG. 12 , the processing device  26  may first detect the pacing spikes. For example, the device  26  may include a counter with a pacing delay. As a non-limiting example, the pacing delay may be approximately 2500 ms to approximately 3000 ms. If this window of time expires without the processing device  26  detecting any pacing spikes, an indication light may show the user that the processing device  26  has failed to detect any pacing spikes (for example, an LED visible on the processing device  26  may display a red light), and the processing device  26  may emit an audible alert signal. The processing device  26  may then go through a loop and repeat the detection sequence until either pacing spikes are detected or the device  26  is reset. If, on the other hand, the processing device  26  does detect pacing spikes within this window of time, the indication light may display a green light, alerting the user that the pacing spikes have successfully been detected and the remaining steps in the flowchart may be performed. If the processing device  26  is able to detect the pacing spikes, the processing device  26  then starts a counter that records the duration of the measurement cycle, also referred to as the detection window (D m ). The counter may be integrated with the microcontroller  52 . Next, the processing device  26  may measure the amplitude of the lead I CMAP signals (A m ). To do this, the processing device  26  may, for example, employ auxiliary circuitry including an on-board, high-grain instrumentation amplifier with a high common-mode rejection ratio (CMRR) as well as a band-pass filter to detect the signal and reduce extraneous noise artifact. An analog-to-digital circuit can then be used to convert the signal to the digital form used by the device  26  for processing. Next, the processing device  26  may determine whether the amplitude of the lead I CMAP signals (A m ) is less than the baseline amplitude (A maxB ) multiplied by the trigger threshold (T t ) (A maxB ×T t  may be referred to as the threshold amplitude value). In other words, the processing device  26  asks the question: A m &lt;A maxB ×T t ? If the answer is no, the counter may reset the measurement cycle duration (D m ) to a zero value and start the process from the beginning (i.e. determine whether a pacing spike may be detected). If, on the other hand, the answer is yes, the processing device  26  may generate an alert, for example, an audible alarm, and the processing device  26  may communicate with the console  30  to open (that is, turn off) the foot switch  75 . Alternatively, the system  20  may be configured such that the processing device  26  communicates directly with the foot switch  75 , and the foot switch communicates to the console  30 . In either configuration, the flow of cryogenic fluid, for example, may be reduced or stopped to prevent injury to the phrenic nerve by the treatment device  28 . If the foot switch  75  is not reset, the operator alerts may continue and treatment may continue to be stopped or paused. If the foot switch  75  is reset, the processing device  26  may repeat the steady state mode operating cycle by resetting the D m  to a zero value and once again assessing whether pacing spikes are detectible. 
         [0040]    Referring now to  FIG. 13 , a schematic diagram of signal acquisition, display, and storage is shown. The assessment electrodes  24  transmit signals through the leads  49 . The signals may then be pre-filtered  102 , filtered  104 , and may undergo an A-to-D conversion  106 . For example, at either the pre-filtering  102  and/or filtering  104  stages, the signals may be averaged to remove the peaks and valleys that may represent noise artifacts. Such filtering may make the signal analysis process less sensitive to sudden shifts and to false positives that may lead to prematurely stopping ablation. The digitalized signals and the operator inputs  108 , such as threshold percentage setting, may then enter the microcontroller  52 . Data may then be output by the microcontroller  52  to a display  110 , to an alert device  112  (for example, one or more LEDs of a visual alert device or a speaker of an audible alert device), to the console  30 , or to a data storage device  114 . The data storage device  114  may be located within the processing device  26 , or the data may be exported from the microcontroller  52  to an external storage device  116 , which may be, for example, connected to the processing device  26  via a USB connection. 
         [0041]    Referring now to  FIGS. 14A and 14B , a circuit diagram of the medical device system is shown. For the sake of clarity, the circuit diagram is shown in the separate but connected images of  FIGS. 14A and 14B . The circuit may capture the lead I signal, and filtering and gain may be adjusted to optimize CMAP signals. As a non-limiting example, the system  20  may include a parallax stamp microcontroller running a PBASIC interpreter/compiler. The microcontroller  52  may be programmed to run one or more algorithms designed to emulate the operational flow chart shown in  FIGS. 11 and 12 . The device  26  may use standard ECG acquisition circuitry. Assessment electrodes  24  may be attached to the patient. For example, a right arm  24 A electrode and left arm  24 B electrode may be used, and an additional right leg electrode  24 C. The circuit may include a right-leg drive loop  130  to reduce signal noise, and a capacitor  132  to stabilize the right leg drive loop  130 . The circuit may also include an input amplifier  134 . The CMAP signals may then undergo filtering, such as by a high-pass filter  136 , and the signals may then be amplified by an additional amplifier  138 . From the amplifier  138 , the signal may then enter the microcontroller  52 , at which point the microcontroller  52  may run one or more algorithms to evaluate the signals received and generate an output signal. For example, the microcontroller  52  may assess whether there is a good connection between the assessment electrodes  24  and the patient (that is, the signal is in-range between electrodes  24 ), whether the CMAP baseline is too low (that is, whether the baseline CMAP amplitude is low relative to a running average; for example, if the CMAP amplitude is less than approximately 1.5 mV), whether pacing spikes are detected, and/or whether the CMAP amplitude is greater or lesser than the threshold baseline (A maxB ). As shown and described in  FIGS. 11-13 , the microcontroller  52  may output signals and/or data to one or more alert devices  106 , memory devices  108 , displays  104 , and to the console  30  and/or foot switch  75  to adjust treatment to prevent phrenic nerve injury. 
         [0042]    The system  20  automatically monitors phrenic nerve function during ablation of, for example, the right-sided pulmonary veins. By providing a system  20  that automatically detects warning signs for the development of phrenic attenuation or palsy, and that audibly and/or visually alerts the user to these potential issues, patient safety is maximized and physician concerns are alleviated. Further, the system  20  provides an additional level of patient safety when in communication with an ablation console, thereby enabling automatic shut-off if phrenic nerve compromise is detected by the system  20 . 
         [0043]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.