Abstract:
A method and apparatus for continuously measuring the absolute intracranial pressure in a non-invasive manner is described by using an ultrasonic Doppler device which detects the pulsatility indexes of the blood flow inside the eye artery for both intracranial and extracranial eye artery portions. The eye in which the blood flow is monitored is subjected to a small pressure, sufficient to equalize the pulsatility index measurements of the internal and external portions of the eye artery. The pressure at which such equalization occurs is used as a reference for autocalibration of the apparatus so that continuous absolute intracranial pressure measurements may be taken over a particular sampling period.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a divisional of currently pending U.S. patent application Ser. No. 12/121,161, filed May 15, 2008, the content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to a method and apparatus for continuously and non-invasively monitoring intracranial pressure, and more specifically relates to an improved method and apparatus for continuously determining intracranial pressure using ultrasonic measurements of the velocity of blood flow through an eye artery. 
       BACKGROUND OF THE INVENTION 
       [0003]    This invention is an extension and improvement of our previously invented method and apparatus U.S. Pat. No. 5,951,477 (the &#39;477 patent) for single or single repeatable absolute intracranial pressure (ICP) value measurement and diagnosing of brain pathologies based on such measurements. This document is incorporated by reference in the present application. 
         [0004]    The &#39;477 patent teaches an apparatus and method for deriving an indication of intracranial pressure in a non-invasive manner using an ultrasonic Doppler measuring technique that is applied to the eye artery. In one aspect, this is achieved by a chamber which can apply a slight pressure to the eye and an ultrasonic apparatus which can simultaneously measure the internal and external blood flows in the eye artery. Signals representative of these velocity measurements, V I  and V E  are then compared and their difference, ΔV, is used to control the pressure in the chamber. When the pressure in the chamber causes ΔV to approach a desired minimum value, that pressure becomes an indication of the intracranial pressure. 
         [0005]    One disadvantage of the method and apparatus taught in the 477&#39; patent is that it is impossible to continuously and non-invasively monitor the absolute ICP value. Continuous monitoring of absolute ICP value is one of the aims of the US and EU traumatic brain injury management guidelines. 
         [0006]    Therefore, one objective of the present invention is the continuous non-invasive monitoring of absolute ICP value. To achieve this objective, we non-invasively determine an absolute intracranial pressure value Po i  in i-th measurement cycle using the method and apparatus taught in the &#39;477 patent. This non-invasive measurement of Po i  is then used as a single autocalibration procedure for the non-invasive ICP monitor during i-th time interval of ICP monitoring, and becomes the initial value of the absolute ICP scale for the next continuous absolute ICP monitoring cycle, Po (i+1) . After the time of continuous ICP monitoring during (i+1)-th time interval, the next single autocalibration procedure is performed and new value Po (i+2)  is identified. That value is used as the initial value of the absolute ICP scale for the next continuous absolute ICP monitoring cycle. This process is repeated for the desired number of monitoring cycles. 
         [0007]    When several Po i  data points are collected, a conversion factor can be determined as a function of pulsatility indexes for a wider interval of absolute ICP values. Stability of the conversion factor dictates whether the time interval of continuous ICP monitoring should be decreased, or increased. If the conversion factor Ω is stable, the continuous absolute ICP monitoring time interval can be increased. If the conversion factor and the pathophysiological conditions of the patient are changing, the continuous absolute ICP monitoring time interval must be decreased. 
         [0008]    Advantages of the present invention are that the absolute ICP monitoring is continuous and the external pressure Pe is used only for autocalibration of the system “individual patient—non-invasive ICP meter”. In the &#39;477 patent, it was necessary to apply external pressure Pe to the eye for the entire sampling period of discrete absolute ICP monitoring. Thus, the added value of the invention is the possibility to obtain important information about absolute ICP value non-invasively and continuously between two single absolute ICP measurements which are used for the system&#39;s autocalibration. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, it is an object of the present invention to provide a non-invasive method and apparatus for continuous absolute ICP monitoring. 
         [0010]    In order to overcome the deficiencies of the prior art and to achieve at least some of the objects and advantages listed, an embodiment of the apparatus for continuously obtaining an indication of intracranial pressure comprises a device for measuring the pulsatility index of blood flow in the intracranial portion of an eye artery (P.I.(int)) and generating an internal pulsatility index signal representative thereof, and measuring the pulsatility index of blood flow in the extracranial portion of an eye artery (P.I.(ext)) and generating an external pulsatility index signal representative thereof; a device for applying an external pressure against an eye, measuring the external pressure applied against the eye, and generating an external pressure signal representative of the measured external pressure; and a processor for receiving the external pressure signal, the internal pulsatility index signal, and the external pulsatility index signal and calculating a conversion factor (Ω) therefrom for converting said internal pulsatility index signal and said external pulsatility index signal into an indication of continuous absolute intracranial pressure. 
         [0011]    The device for continuously measuring the pulsatility index of blood flow in the intracranial portion of an eye artery and measuring the pulsatility index of blood flow in the extracranial portion of an eye artery may be provided as an ultrasonic Doppler device. 
         [0012]    Conversion factor (Ω) may be calculated as the value of the measured external pressure which causes the ratio of the external pulsatility index (P.I.(ext)) to the internal pulsatility index (P.I.(int)) to become equal to one (1). Additionally, the indication of continuous absolute intracranial pressure (ICP) may be calculated from the formula ICP=Ω (P.I.(ext)/P.I.(int)) and it may be calculated for at least on sampling period. The processor may also calculate a value of said conversion factor (Ω) for each of the at least one sampling periods. 
         [0013]    The processor provided in one embodiment may also determine whether the conversion factor is stable by comparing the value of the conversion factor for each of the at least one sampling periods. The conversion factor is stable if there is an insubstantial change in the value of the conversion factor for each of the at least one sampling periods. If the conversion factor is stable, the length of each of the at least one sampling periods may be increased. If the conversion factor is not stable, the length of each of the at least one sampling periods may be decreased. 
         [0014]    An embodiment of a method for continuously obtaining an indication of absolute intracranial pressure is also provided. The method may comprise the steps of: A) measure the pulsatility index of blood flow in the intracranial portion of an eye artery (P.I.(int)); B) measure the pulsatility index of blood flow in the extracranial portion of an eye artery (P.I.(ext)); C) apply an external pressure against an eye and measure said external pressure applied against the eye; D) calculate a ratio of P.I.(ext) to P.I.(int); E) calculate a conversion factor to convert the ratio of P.I.(ext) to P.I.(int) into intracranial pressure, wherein said conversion factor is equal to the external pressure measured which causes said ratio of P.I.(ext) to P.I.(int) to become equal one (1); and F) repeat steps A, B, and D and determine intracranial pressure (ICP) by applying said conversion factor to the calculated ratio of P.I.(ext) to P.I.(int). 
         [0015]    The method may also comprise the steps of: G) periodically repeat steps A-E; and I) repeat step F. In another embodiment, the method of claim further comprises the step of: J) determine whether the conversion factor is stable by comparing the value of the conversion factor calculated in steps E and G. In yet another embodiment, the method further comprises the step of: K) determine that the conversion factor is stable if there is an insubstantial change in the value of the conversion factor calculated in steps E and G. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a time chart of non-invasive absolute ICP continuous monitoring device. 
           [0017]      FIG. 2  is a structural diagram of the apparatus for non-invasive absolute ICP continuous monitoring. 
           [0018]      FIG. 3  is an algorithm of the apparatus for non-invasive absolute ICP continuous monitoring. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    We have found that with the apparatus in accordance with the present invention, the absolute value of intracranial pressure can be monitored continuously and non-invasively. 
         [0020]    As shown in  FIG. 1 , the present invention involves an apparatus for non-invasively determining an absolute intracranial pressure value Po i  in i-th measurement cycle. Using the method and apparatus taught in the &#39;477 patent, the intracranial pressure value Po i  is determined by applying an extracranial pressure P e  to the eye over some time interval, for example 200 seconds. During that interval, P e  is increased step-by-step from 0 mmHg to 25.0 mmHg, at 5.0 mmHg increments, until the ratio of extracranial to intracranial pulsatility indexes P.I.(ext)/P.I.(int) of blood flow in the intracranial and extracranial segments of the eye artery (ophthalmic artery) becomes equal to 1.0. 
         [0021]    Pulsatility index PI is defined by the ratio (Vsyst−Vdiast)/Vmean, where Vsyst the systolic blood flow velocity value, Vdiast is the diastolic blood flow velocity value, and Vmean is the mean value of blood flow. Blood flow velocities are measured by a two-depth transcranial Doppler (TCD) device in both the intracranial and extracranial segment of the eye artery. Thus, both pulsatility indexes P.I.(int) in the intracranial segment and P.I.(ext) in the extracranial segment of eye artery are defined using results of Vsyst, Vdiast and Vmean measurement. 
         [0022]    The non-invasive measurement of Po i  is used as a single autocalibration procedure for the non-invasive ICP monitor during i-th time interval of ICP monitoring. The value of Po i  also becomes the initial value of the absolute ICP scale for the first continuous absolute ICP monitoring cycle, Po (i+1) . 
         [0023]    Continuous monitoring of absolute ICP data begins from Po (i+1)  (e.g., Po i =10 mmHg in  FIG. 1 ). ICP is continuously and non-invasively monitored using the ratio of the pulsatility indexes, i.e., P.I.(ext)/P.I.(int). Once the ratio is known, we can calculate ICP=Ω(P.I.(ext)/P.I.(int)), where Ω is a conversion factor for converting pulsatility indexes into absolute non-invasive ICP values. Conversion factor Ω is individual to each patient and depends on the physiological state of the patient, i.e., arterial blood pressure, cerebrospinal compliance, etc. 
         [0024]    After the time of continuous ICP monitoring during (i+1)-th measurement cycle (e.g., 1 hour in  FIG. 1 ), the next single autocalibration procedure—non-invasive measurement of absolute ICP Po (i+2) —is performed and new value Po (i+2)  is identified. That value is used as the initial value of the absolute ICP scale for the next continuous absolute ICP monitoring cycle. This process is repeated for the desired number of monitoring cycles. 
         [0025]    When several Po i  data points are collected, values of conversion factor Ω can be determined as a function of ICP for a wider interval of absolute ICP values. Stability of the conversion factor as a function of time can also be checked. The conversion factor is stable if there is in an insubstantial change in its value over time. If the conversion factor Ω is stable, the continuous absolute ICP monitoring time interval can be increased. If the conversion factor and the pathophysiological conditions of the patient are changing, the continuous absolute ICP monitoring time interval must be decreased. In practice, a single absolute ICP non-invasive measurement time for the system&#39;s calibration is much shorter than the fifteen (15) minute continuous non-invasive absolute ICP monitoring time. 
         [0026]    With reference to  FIG. 2 , an apparatus  20  is shown to practice the continuous measurement of the intracranial pressure as described above. The apparatus is mountable to the head of a person so that an eye engaging inflatable device  22  can apply a slight pressure against the eye lid  23 . Suitable braces and positioning bands  24 ,  26  are used to hold the device  22  in place. The device  22  is formed of a suitable soft material such as rubber or other polymer film to form an inflatable chamber  28 . Chamber  28  is approximately annular in shape so as to enable an ultrasonic transducer  30  to be mounted against an inner flexible membrane  32  and enable a pressurization of the chamber by a pump  34 . 
         [0027]    The inner membrane conforms to the shape of the eye  35  as illustrated and in such manner enables the pressure from the inflation of chamber  28  to provide a slight pressurization of the tissues surrounding the eye and thus the eye socket. This results in a pressurization of the eye artery  36 , which originates from inside the cranium  40  and passes through the optic nerve canal  42  to the eye  35 . 
         [0028]    The ultrasonic transducer  30  has a central axis  44 , which can be aligned by adjusting the position of the transducer inside its mounting to device  22 . This alignment allows one to adjust the angle of axis  44  so as to direct its ultrasonic acoustic pulses at both interior and exterior portions  46 ,  48  of the eye artery  36  at the same angle. With such alignment, Doppler measurements of the blood flow velocities in these different portions  46 ,  48  can be made without the introduction of errors from the use of different angles of axis  44  with respect to portions  46  and  48 . Hence, a reliable measurement of the intracranial and extracranial pulsatility indexes, P.I.(int) and P.I.(ext) respectively, can be determined. 
         [0029]    The ultrasonic transducer  30  has its input line  50  coupled to an acoustic pulse transmitter  52 . The transducer  30  also acts as a sonic receiver so that its input line  50  is connected to a gate  54 . A gate input  56  is connected to the transmitter  52  to protect a receiver  58  from the high transmitter output pulses during pulsing of the transducer  30 . The receiver  58  produces an output signal on line  60  representative of the acoustic echoes from the blood flow in the eye artery OA and caused by the ultrasonic pulses from the transmitter  52 . 
         [0030]    A depth control network  62  is provided to enable the apparatus  20  to select that portion of received echoes representative of either the internal or external, cranium, eye artery, blood velocities. The network  62  produces an internal selection signal on line  64  and an external selection signal on line  66 . The internal selection signal is applied to an AND gate  67  to enable the echoes related to the blood flow inside the cranium to be selected for further processing. Similarly, the external signal is applied to an AND gate  68  to select the echoes related to the blood flow in the eye artery external of the cranium. The network  62  operates as a range gating system with which acoustic returns of different depths can be selected and analyzed for their Doppler frequency shift relative to the transmitter frequency f c . 
         [0031]    The internal and external selection signals are generated in sequence in a manner as is well known by a control  70  activated after each transmitter pulse by the signal on line  56 . The outputs from AND gates  67 ,  68  are coupled through an OR gate  72  to a sampler frequency counter  74 . This samples the received echo signals and produces sample signals, such as the signal frequency, f I , in the pulse representative of blood velocity inside the cranium, the signal frequency, f E , inside the echo pulse from the eye artery external of the cranium, and the frequency, f C , in the transmitted pulse. The sampled frequency signals are stored at  76  in a suitable memory and at  78  the shifts in the frequencies from the frequency of the transmitted pulse, such as f C −f I  and f C −f E , are determined. A suitable microprocessor can be used to implement these functions. 
         [0032]    The frequency shifts can be determined for each transmitter pulse and resulting echo. Each frequency shift is representative of the blood velocity in the eye artery and the values can be so stored to provide an indication of the internal blood velocity, V I , and external blood velocity, V E , at  78 . The velocity difference value ΔV at  80  can then be displayed and the display is used to determine the intracranial pressure. 
         [0033]    The difference values ΔV are used to determine the intracranial pressure Po i . This is done by increasing the pressure inside the inflatable device  22  to a level where ratio of the intracranial and extracranial pulsatility indexes (which depend on the value of ΔV) becomes equal to one. The Po i  measurement can be made by manually increasing the pressure inside the device  22  until the visual indications of the measured pulsatility indexes P.I.(int) and P.I.(ext), or the frequency shifts, appear the same or with an automatic control such as  82 . 
         [0034]    Alternatively an automatic control  82  can be implemented, for example, by first testing at  84  whether the value of ΔV is below a minimum value such as ΔV min . If not, then at  86  a value for the pump pressure is incremented and its value applied to pump  34  to cause it to increase the pressure P e  inside the inflatable device  22 . A pressure transducer  90  senses the pressure inside the chamber  28 . 
         [0035]    When the test at  84  shows positive, the value P e  is stored at  92  as an indication of the intracranial pressure, Po i . This can be displayed at  94  and suitably recorded. 
         [0036]    In the operation of apparatus  20 , it desirable that an initial alignment mode be undertaken to assure that the transmitter pulses from the transducer  30  are properly directed at both the internal and external portions  46  and  48  of the eye artery  36 . This involves adjustments in the angle φ between the axis  44  of the ultrasonic transducer  30  and the alignment axis  96  of the eye artery passage  42 . 
         [0037]    A controller  97  of continuous ICP monitoring is added to the structural diagram of previously invented method and apparatus of the &#39;477 patent. The additional controller  97  is connected with clock  98  and increment pump  86 . The controller  97  manages i-th measurement cycles, performs pressure P e  increments of the pump  34 , calculates the ratio of pulsatility indexes P.I.(ext)/P.I. (int), compares it with the predetermined 1.0 value, and calibrates the non-invasive ICP monitoring display  94  by the output signal  99 . 
         [0038]    With reference to  FIG. 3 , a routine  100  for making such alignment is illustrated. Thus at  102  the apparatus  20  is initialized and at  104  operative contact between the acoustic transmitter  30  and the eye cavity is established by observing return echoes on a display. At  106  the depth of the operative probe depth R E , see  FIG. 1 , is entered by the probe depth control block  70  (See  FIG. 1 ). Typical initial values of R E  are approximately between 40 mm and 50 mm. 
         [0039]    At  108  the spatial angle φ of the transducer axis  44  is changed to find the velocity signal associated with the extracranial eye artery portion  48 . This is found by observing the shape of the blood velocity pulsation curve of the extracranial part  48  of the eye artery  44 , (see  FIG. 5 ). The spatial angle, φ 1 , which yields the maximum Doppler signal level, is selected at  110  and noted. 
         [0040]    At  112 , the initial value of the internal probe depth R I  is entered by the control block  70 . The typical values of R I  are approximately between 52 mm and 65 mm. 
         [0041]    At  114  the spatial angle φ 2  is determined for the alignment of the transducer  30  yielding the maximum Doppler signal pulsation from the internal portion  46  of the eye artery  36 . The operating orientation of the transducer  30  is the selected at  116  by aligning the axis  44  of the transducer  30  along the middle between the angles φ 1  and φ 2 . 
         [0042]    Then at  118  the probe depth control  70  is actuated so that the blood velocities, within the internal and external eye artery portions  46 ,  48 , are sequentially measured. The depths of external and internal optic nerve canal&#39;s entrances are determined by increasing R E  from the values between those selected at  106  and the values selected at  112  while observing the blood velocity pulsation of  FIG. 5 . The blood velocity pulses have smaller amplitudes inside the optic nerve canal. 
         [0043]    Then at step  120  the depths R 1  and R 2  of respectively the external and internal optic nerve canal entrances are determined. This is done by observing a decrease in the amplitudes of the blood velocity pulses, as shown in  FIG. 5 , and typical for measurements made inside the optic nerve canal in comparison with the amplitudes of blood velocity pulses from outside the optic nerve canal. 
         [0044]    After that, at step  122  the final value of R E  and R I  are set using the criteria R E &lt;R 1  and R I &gt;R 2 . Once the position of the ultrasonic transducer is determined and set a measurement of the internal and external blood velocities can be made as described above. A determination of the intracranial pressure Po i  is obtained when the pulsatility index measurements are the same. 
         [0045]    As shown in  FIG. 3 , periodical autocalibration and continuous ICP monitoring procedures  125  are also added to the algorithm of non-invasive absolute ICP continuous monitoring apparatus. 
         [0046]    It should be understood that the foregoing is illustrative and not limiting, and that obvious modifications may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, reference should be made primarily to the accompanying claims, rather than the foregoing specification, to determine the scope of the invention.