Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/02—Detecting, measuring or recording for evaluating the cardiovascular system, e.g. pulse, heart rate, blood pressure or blood flow
  • A61B5/021—Measuring pressure in heart or blood vessels
  • A61B5/022—Measuring pressure in heart or blood vessels by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers
  • A61B5/02225—Measuring pressure in heart or blood vessels by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers using the oscillometric method
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/02—Detecting, measuring or recording for evaluating the cardiovascular system, e.g. pulse, heart rate, blood pressure or blood flow
  • A61B5/021—Measuring pressure in heart or blood vessels
  • A61B5/022—Measuring pressure in heart or blood vessels by applying pressure to close blood vessels, e.g. against the skin; Ophthalmodynamometers
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B2560/00—Constructional details of operational features of apparatus; Accessories for medical measuring apparatus
  • A61B2560/02—Operational features
  • A61B2560/0223—Operational features of calibration, e.g. protocols for calibrating sensors
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B2562/00—Details of sensors; Constructional details of sensor housings or probes; Accessories for sensors
  • A61B2562/02—Details of sensors specially adapted for in-vivo measurements
  • A61B2562/0247—Pressure sensors

Definitions

• NIBP non- invasive blood pressure
• the invention relates to a method for use in cuff-based oscillatory noninvasive blood pressure (NIBP) measurements and a control unit for an NIBP apparatus, and in particular relates to a method for acquiring oscillatory NIBP measurements with a minimal error and a control unit which enables an NIBP apparatus to implement the method.
• NIBP noninvasive blood pressure
• BP Arterial blood pressure
• NIBP Non- invasive arterial blood pressure
• the NIBP is determined either by measuring sound distal from the cuff (the auscultatory method, based on Korotkoff sounds) or by measuring pressure pulsations in the cuff caused by volume pulsations of the arm and brachial artery and extracting features from the envelope of these pressure pulses (the oscillo metric method).
• the oscillo metric method is easily automated and is widely used.
• the auscultatory method is the "gold standard" for cuff based NIBP measurements.
• Figure 1 shows a graph of cuff pressure 10, and a processed high pass filtered trace 12 of this cuff pressure, versus time.
• the left-hand y-axis shows pulse amplitude
• the right-hand y-axis shows cuff pressure
• the x-axis shows time.
• the cuff pressure 10 is ramped up until it is sufficiently larger than systolic blood pressure. After ramp up, the cuff is deflated (in Figure 1 the deflation is done gradually, but step wise deflation is also possible).
• the measured cuff pressure 10 is high pass filtered, and the resulting trace 12 shows the cuff pressure oscillations due to volume changes in the brachial artery.
• An envelope 14 of the oscillation amplitudes is determined.
• the maximum A max of this pulse envelope 14 is taken as a reference point for determining the systolic 16 and diastolic pressure 15.
• the systolic pressure 16 is determined as the cuff pressure where the pressure oscillation is approximately 0.8 times the maximum amplitude A max at a pressure higher than the pressure at the reference point.
• the diastolic pressure 15 is determined as the cuff pressure where the pressure oscillation is approximately 0.55 times the maximum amplitude A max at a pressure lower than the pressure at the reference point. These ratios are based on empirical values (see, e.g., LA Geddes et. al., Annals of Biomedical Engineering 10 pp 271-280, 1982). The exact algorithms that are employed by manufacturers of blood pressure devices to determine systolic and diastolic pressures are usually trade secrets.
• the typical apparatus 20 used for acquiring oscillometric NIBP measurements is illustrated in Figure 2.
• a pump 21, first and second pressure sensors 22, 23, and first and second valves 24, 25 are connected to a cuff 26 by tubing 27.
• the first and second pressure sensors 22, 23 measure the pressure in the system (and therefore the pressure in the cuff 26).
• the pump 21 is disabled, the first valve 24 is opened and slow (or step wise) deflation occurs, during which the cuff pressure is continuously measured and the measurements stored.
• the pump and valves are controlled by a control unit (not shown), which also receives the cuff pressure measurements and calculates the pulse envelope and the systolic and diastolic pressure using these measurements. Multiple sensors and valves are used for safety reasons.
• Oscillatory blood pressure measurements can have large errors (10s of mmHg, corresponding to 10s of percent), both for subjects with low blood pressure and for subjects with high blood pressure (see, e.g., Wax DB et. al, Anaesthesiology 115 pp 973-978, 2011).
• the errors are due to systematic flaws associated with using a cuff. Error sources include, for example:
• Cuff compliance Cc is the value that relates the pressure change in the cuff due to a volume change of the cuff when the number of air particles in the cuff is constant and elasticity of the cuff wall is negligible, it is represented by the function
• Vc is the volume of the cuff and Pc is the pressure in the cuff.
• Pc is the pressure in the cuff.
• the compliance function varies depending on the pressure in the cuff; on how exactly the cuff is wrapped around a subject's arm and also with the size and mechanical properties of the arm. The pressure in the cuff has a significant influence on the cuff compliance.
• Figure 4 shows a plot of a measured cuff compliance versus cuff pressure for a particular adult cuff.
• cuff compliance At high cuff pressure (> 100 mmHg) cuff compliance is nearly constant but at low pressure the compliance is strongly dependent on pressure. This gives rise to errors in the pressure oscillation-amplitude measurement, as for a given volume change in the cuff, the pressure change depends on cuff compliance.
• the cuff compliance should be constant for all cuff pressures, which is clearly not the case, especially at low cuff pressures.
• Figure 3a, 3b and 3c present a model of the cuff measurement principle.
• Figure 3a shows an electrical model of the cuff around the arm (it is well known in the art that the electrical and mechanical domain are equivalent, and in practice it is often easier to analyse mechanical systems in the electrical domain).
• the arm-plus-artery system is well known in the art that the electrical and mechanical domain are equivalent, and in practice it is often easier to analyse mechanical systems in the electrical domain.
• Figure 3b shows a typical volume-transmural pressure curve of the arm-plus- artery
• Figure 3c shows a typical volume-pressure relation of the cuff. It can clearly be seen from Figures 3b and 3 c that the cuff compliance is much bigger than the arm-plus-artery compliance (i.e. the cuff experiences a much greater volume change for a similar pressure change).
• the volume of the arm-plus-artery changes in dependence on the transmural pressure over the arm (where the transmural pressure is given by PBioodPressure - Pskin, the internal pressure blood pressure as modelled by the voltage source 30 minus the external skin pressure).
• Figure 3b shows that, in the illustrated example, a typical oscillation amplitude is about 0.1 ml (at a blood pressure of 120/80, when the external skin pressure is zero).
• the measurement cuff is modelled as another (variable) compliance in series with the arm-plus- artery.
• the cuff compliance can be modelled as a parallel combination of three compliances: (1) the compliance due to the air in the cuff (C a i r ), (2) the compliance due to the cuff elasticity (Ccei), and (3) the compliance due to the elasticity of the arm tissue (C arm ).
• the pump 21 (represented by current source 31 in Figure 3a) causes air to be pumped into the cuff.
• the volume of the cuff is increased and the volume of the arm-plus-artery is decreased.
• the effect on the volume of the cuff is significantly greater than the effect on the volume of the arm-plus-artery, because of the significantly greater compliance of the cuff.
• cuff inflation the pressure in the cuff increases, while the transmural pressure over the arm-plus-artery decreases. The change in transmural pressure results in a change of volume in the arm-plus-artery.
• a distortion of the shape of the envelope of the high-pass filtered cuff pressure oscillation amplitude will cause systematic errors in estimated blood pressures, because the pressures corresponding with the required amplitude points for systole and diastole will be altered due to the distortion.
• the cuff pressure and volume changes are related by:
• V A is the change in arm volume due to artery volume pulsations (in units of ml)
• C QS is the pressure-dependent QS cuff compliance (in units of ml/mmHg)
• Pc is the measured cuff pressure.
• V a is time dependent due to the varying artery-cuff transmural pressure.
• Compliance data for a given cuff which has been obtained under controlled conditions cannot be used in a lookup table or in a feed forward mode to correct oscillatory NIBP measurements because the cuff compliance is affected by the tightness of the wrapping of the cuff, the arm diameter, and the mechanical properties of the arm (e.g. the amount of soft tissue, the soft tissue pressure dependent compliance, changes to soft tissue properties due to hysteresis and/or previous
• Cuff compliance must therefore be measured during the actual NIBP measurement.
• the flow resistance of the tubing 27 can also give rise to errors. This can be due to Ohmic pressure drops during ramps, or due to RC filtering effects on the rapidly changing flows and pressures.
• a method for use in cuff-based oscillatory non-invasive blood pressure, NIBP, measurement comprising:
• determining a relationship between quasi-static cuff compliance and cuff pressure by calculating the quasi-static cuff compliance at a plurality of instances during the measurement period, based on the flow rate measurements and the air pressure measurements obtained during the measurement period.
• Embodiments of the invention permit reduction or elimination of errors in the blood pressure estimation which are due to distortion of the pulse pressure envelope resulting from non-constant QS cuff compliance.
• embodiments of the invention enable pressure dependent cuff compliance to be determined during a normal NIBP measurement using a conventional single lumen cuff. Some embodiments also enable tube resistance to be determined during a normal NIBP
• Some advantageous embodiments permit the measurement time to be shortened. For example, by combining measurements corrected for cuff compliance and tube flow resistance acquired during both ramp up and ramp down of cuff pressure, a faster ramp rate can be used and the overall measurement time can be reduced.
• the measurement period comprises an inflation period during which the volume of air in the cuff is progressively increased and a deflation period during which the volume of air in the cuff is progressively decreased.
• the rate at which the volume of air in the cuff is altered during the inflation period is different to the rate at which the volume of air in the cuff is altered during the deflation period.
• the volume of air in the cuff is altered during the deflation period is non-constant.
• the volume of air in the cuff is altered in a step-wise manner during the deflation period.
• the method further comprises using the obtained flow rate measurements to determine the resistance of a tube passed through by air flowing into or out of the cuff.
• progressively altering the volume of air in the cuff during the measurement period comprises controlling a flow of air into the cuff such that the pressure in the cuff increases at a predetermined rate during the inflation period and subsequently controlling a flow of air out of the cuff such that the pressure in the cuff decreases at a predetermined rate during the deflation period.
• determining the tube resistance comprises:
• the rate at which the volume of air in the cuff is altered during the measurement period is selected such that the measurement period includes at least a predefined minimum number of heartbeats of the subject.
• the predefined minimum number of heartbeats is ten heartbeats.
• embodiments which define a minimum number of heartbeats ensure that an accurate blood pressure value can be obtained whilst minimizing the measurement time as far as possible.
• the NIBP measurement apparatus is arranged to acquire a measurement of the blood pressure of a subject.
• the method further comprises calculating one or more of: a systolic blood pressure of the subject, a diastolic blood pressure of the subject and a mean blood pressure of the subject, based on the air pressure measurements obtained during the measurement period and on the determined relationship between quasi-static cuff compliance and cuff pressure.
• the calculating is additionally based on the determined tube resistance.
• the rate at which the pressure in the cuff is altered during the measurement period is greater than 10 mmHg/s.
• embodiments of the invention can compensate for the tube resistance errors incurred at higher ramp rates, enabling measurement time to be reduced without lowering measurement accuracy.
• control unit for a NIBP measurement apparatus having an inflatable cuff for wrapping around a body part of a subject.
• the control unit comprises:
• At least one output for sending control signals to the NIBP measurement apparatus and to a flow meter
• a processing unit configured to:
• control the NIBP measurement apparatus to progressively alter the volume of air in a cuff during a measurement period and to obtain a plurality of measurements of the air pressure in the cuff during the measurement period;
• control the flow meter to obtain a plurality of measurements of the flow rate of the air into/out of the cuff during the measurement period;
• the processing unit is configured to control the NIBP measurement apparatus to progressively alter the volume of air in a cuff during a measurement period at a first rate during a first part of the measurement period and at a second, different, rate during a second part of the measurement period.
• a system for use in oscillatory non-invasive blood pressure, NIBP, measurement comprises:
• a NIBP measurement apparatus having an inflatable cuff for wrapping around a body part of a subject
• a flow meter configured to measure the flow rate of air into/out of the cuff
• the flow meter comprises at least one pressure sensor and the NIBP apparatus comprises at least one pressure sensor, and at least one pressure sensor comprised in the flow meter is also comprised in the NIBP measurement apparatus.
• the flow meter comprises two pressure sensors and the NIBP measurement apparatus comprises two pressure sensors, and the two pressure sensors of the flow meter are the same as the two pressure sensors of the NIBP measurement apparatus.
• a computer program product comprising computer readable code embodied therein, the computer readable code being configured such that, on execution by a suitable computer or processor, the computer or processor operates as a control unit according to the second aspect of the invention.
• Figure 1 is a graph of cuff pressure versus time measured using a conventional oscillometric method and apparatus
• Figure 2 shows a graphical overview of the main elements in a conventional oscillatory NIBP measurement apparatus
• Figure 3 a is a circuit diagram relating to a conventional oscillatory NIBP measurement apparatus
• Figure 3b is a graph showing the volume-transmural pressure relation of an exemplary arm-plus-artery system
• Figure 3c is a graph showing the volume-cuff pressure relation of an exemplary cuff
• Figure 4 is a graph of cuff compliance versus cuff pressure for an exemplary cuff
• Figure 5 shows a graphical overview of the main elements of the NIBP apparatus according to an embodiment of the invention
• Figure 6 shows a method for use in oscillatory NIBP measurement according to a first embodiment of the invention
• Figure 7 is a graph of cuff volume versus cuff pressure for an exemplary cuff
• Figure 8 is a graph of cuff compliance versus cuff pressure for the exemplary cuff of Figure 7 obtained using two different methods;
• Figure 9a is a graph showing uncorrected and corrected normalized volume envelopes for a first subject
• Figure 9b is a graph showing uncorrected and corrected normalized volume envelopes for a second subject
• Figure 10 shows a method for use in oscillatory NIBP measurement according to a second embodiment of the invention.
• Figure 11 shows a hysteresis loop used to extract tube resistance in certain specific embodiments of the invention.
• Embodiments of the invention use a quasi-static method to measure cuff compliance during an NIBP measurement.
• a cuff compliance curve specific to that measurement is thereby generated, and is used to correct the pressure envelope.
• the large relative error in blood pressure measurement due to non-constant cuff compliance and changing cuff compliance between measurements is thereby reduced or eliminated.
• Figure 5 shows an apparatus 50 for use in oscillatory NIBP measurement that is suitable for implementing the method according to the invention.
• the apparatus 50 comprises the same components as a
• NIBP measurement device namely a pump 51 , first and second pressure sensors 52, 53, and first and second valves 54, 55, connected to a cuff 56 by tubing 57.
• the apparatus 50 is configured such that the air volume flow between the pump
• the layout of the tubing 57 has been modified from the conventional layout shown in Figure 2, such that the first pressure sensor
• the pressure sensors 52, 53 have a dual function during measurement - they are used both for sensing cuff pressure and for measuring air volume flow. It will be appreciated that this arrangement allows a flow sensor to be realized with minimum changes to the hardware of a conventional NIBP device.
• the first and second pressure sensors 52, 53 are replaced by a differential pressure sensor.
• the method of the claimed invention can only be performed during inflation of the cuff (by contrast, the apparatus shown in figure 5 enables measurement of the cuff compliance during both inflation and deflation of the cuff).
• Figure 6 shows a method for use in oscillatory NIBP measurement according to a first embodiment of the invention.
• a pressure ramp is applied to the cuff, to reach a cuff pressure above systolic blood pressure.
• the pressure ramp is sufficiently slow ( ⁇ 5 mmHg/s) for the method to be quasi-static.
• the ramp is upwards (i.e. the cuff pressure increases over the course of the ramp). In alternative embodiments the ramp is downwards (i.e. the cuff pressure decreases, starting from above systolic, over the course of the ramp). In some embodiments two pressure ramps are applied (e.g. an upwards ramp corresponding to inflation of the cuff by the pump followed by a downwards ramp corresponding to deflation of the cuff through one or more of the valves).
• step 602 cuff pressure measurements are obtained periodically during the pressure ramp, in the conventional manner. In some embodiments in which two pressure ramps are applied, cuff pressure measurements are obtained periodically during both pressure ramps.
• step 603 the air volume flow into the cuff during the pressure ramp is measured. This airflow is measured by measuring the pressure drop (with the second pressure sensor 53) over the flow limiting element 58:
• V s is the air volume flow rate under standard conditions (i.e. atmospheric pressure and ambient temperature)
• P s is standard (i.e. atmospheric) pressure
• P am bient is ambient pressure
• R is the resistance of the flow limiting element.
• V c is the air volume flow rate into the cuff
• Pc is the cuff pressure
• ⁇ is a constant which takes a value of 1 for an isothermal process and a value of 1.4 for an adiabatic process. In embodiments in which the method is quasi-static (i.e. for typical use in methods according to the invention), ⁇ is approximately equal to 1.
• V and P are alternative notations for the time derivative of the air volume dV/dt and the time derivative of the pressure dP/dt respectively.
• the flow pressure sensor should measure absolute pressure, because the pressures in equation 4 are absolute pressures.
• step 604 the cuff compliance is evaluated using the following procedure.
• the complete pressure change over time dP/dt during the ramp is known (for example, because the pressure is measured and converted into the digital domain by an analogue-to- digital converter so that the time series of pressure -time is automatically available, and numerical differentiation methods can then be applied to obtain dP/dt), and the air volume flow into the cuff V c is known from step 603. It is assumed that the cuff bladder volume can be neglected at the start of the pressure ramp up (alternatively, this volume is known). The cuff volume at time t is obtained by integration of the air volume flow V c (this includes the air volume in the tube).
• the cuff volume is calculated using the low pass filtered data. If artefacts (due to, e.g., outliers, missing beats, arrhythmias, etc.) are present in the measured data, appropriate corrections can be applied.
• C QS P C I V C (5)
• C QS is the QS cuff compliance and P c is the time derivative of cuff pressure.
• the QS cuff compliance at pressure Pc can be estimated from the known cuff volume-pressure relation using:
• the output of step 604 is a data set relating cuff compliance to pressure across the whole pressure range of the ramp up. This data set can then be used to determine a relationship between quasi-static cuff compliance and cuff pressure, using known mathematical techniques. In preferred embodiments the determined relationship has the form:
• V C C 0S - ⁇ - (7)
• the cuff compliance can be calculated using equation 5 and using equation 6, and the results of the two calculations compared for consistency.
• the quasi-static method provides accurate measurements for the cuff compliance which are free from high frequency artefacts and that can be done during the normal oscillometric NIBP blood pressure measurement.
• Figure 7 shows the measured static volume-pressure curve obtained in these experiments for a particular adult cuff on an arm.
• Figure 8 shows the cuff compliance of the same cuff obtained using the quasi-static method (solid line) and using the static volume-pressure curve of Figure 7 together with equation 6 (dots). It can be seen that the measured cuff compliance obtained using the quasi- static method agrees well with that obtained using the measured static volume-pressure curve.
• step 605 the QS cuff compliance-cuff pressure relationship determined in step 604 is used to correct the blood pressure envelope, in the following manner.
• a pressure envelope is derived from the cuff pressure measurements obtained in step 603 using conventional techniques.
• the cuff-pressure is low-pass filtered to remove high-frequency artefacts (e.g. using a bandwidth of ⁇ 25 Hz) and then high-pass filtered to remove the DC and slow ramp components (e.g. using a cut off frequency of ⁇ 0.25 Hz) and the pressure envelope is derived from this filtered signal.
• artefacts due to, for instance, arrhythmias are removed at this stage.
• correction of the envelope for cuff compliance is done by numerical integration of equation 7.
• the correction can be done using AV(P) ⁇
• a corrected envelope having dimensions of volume is thereby generated.
• This curve can be normalized to dimensionless units (as is done for the pressure curve) by dividing the volume oscillations by the maximum volume oscillation.
• the envelope correction can be enhanced using curve fitting methods. It will be appreciated that the person skilled in the art will be aware of various mathematical techniques which could alternatively be employed in the correction of the envelope.
• the QS cuff compliance-cuff pressure relationship determined in step 604 can beneficially be applied in ways which do not involve correcting a blood pressure envelope. For example, it could be used to compare the compliance behaviour of different cuff designs or brands, and/or to train medical personnel to wrap cuffs in a manner to as to minimize compliance variation. Various other applications will be readily apparent to the skilled person.
• the corrected envelope can be used to determine the diastolic and systolic blood pressure in a conventional manner (shown as an optional step 606 in figure 6). This procedure is not affected by the units of the envelope because it uses dimensionless ratios.
• Figures 9a and 9b show the results of a simulation which illustrates the impact of the envelope correction on blood pressure estimation for a subject with normal blood pressure ( ⁇ 80/120 mmHg) and a severely hypotensive subject (blood pressure ⁇ 30/50) respectively.
• the simulation uses a brachial artery volume-pressure relation from Jeon et. al., World Acad. Sci. Eng. Technol. 2007, 30: 366-371.
• the dashed curve is the uncorrected pressure envelope and the solid curve is the corrected pressure envelope.
• the corrections are negligible for the normotensive patient ( ⁇ 2 mmHg), but for the hypotensive case the corrections are ⁇ 6 mmHg, which is large compared to the measured value. It can also be seen that, besides the changes of systole and diastole, the maximum point of the curve (which in many cases is used as mean blood pressure) is also shifted. The deviations from actual values of the calculated values for the systolic, mean and diastolic blood pressure values based on the uncorrected envelope are clinically relevant ( ⁇ 20%). When the corrected envelope is used, the deviations are significantly smaller.
• the method in Figure 6 enables errors in oscillatory NIBP measurements resulting from variable cuff compliance to be reduced or even entirely eliminated. This is achieved by measuring the cuff compliance for each individual NIBP measurement performed, to obtain cuff compliance data specific to that particular measurement. This data is then used to generate a blood pressure envelope which is corrected for the effects of varying cuff compliance. Blood pressure values estimated using the corrected envelope can therefore be significantly more accurate than blood pressure values estimated using conventional techniques. Furthermore, the method can be implemented by a conventional NIBP device following only minimal changes to its hardware, and does not increase the time or complexity of performing a blood pressure measurement.
• tube resistance the resistance of the tubing to air flow
• tube resistance results in a pressure drop over the tube that can no longer be neglected. This causes a significant additional error (in the range 1 to 10 mmHg), which can be corrected when the tube resistance is known.
• tube resistance should be measured during a NIBP measurement because tube resistance is affected by the temperature, and by the exact path of the tubing (i.e. by any bends or curves in the tubes).
• Tube resistance can be estimated from air volume flow rate, therefore embodiments of the invention also enable oscillatory NIBP measurements to be corrected for tube resistance.
• the measurement time cannot be made arbitrarily short, as a minimum number of heart beats (-10) must be recorded to enable calculation of the blood pressure envelope.
• the ramp rate can be increased as compared to the conventional method (e.g. since 5 heat beats can be recorded during ramp up and 5 heart beats can be recorded during ramp down). As a result the total measurement time can be reduced.
• Figure 10 shows a method for use in oscillatory NIBP
• step 801 a fast ( ⁇ 10-20 mmHg/s) pressure ramp is applied to the cuff.
• step 802 cuff pressure measurements are obtained periodically during the pressure ramp, in the conventional manner.
• step 803 the air volume flow into the cuff during the pressure ramp is measured, as described in relation to step 603 of Figure 6.
• step 804 the tube resistance is determined using one of several possible methods. Three such methods are described:
• the pressure in the tubing 57 is measured (and controlled) and the pressure in the cuff Pc is given by:
• Rm be can be estimated at the end of the ramping period.
• the pressure measured in the tubing 57 will drop because the pressure drop over the tube vanishes. From the observed pressure drop and the known air flow at the end of the ramp up the tube resistance can be estimated using equation 9.
• a drawback of this method is a second pressure drop due to mechanical hysteresis of the cuff. Consequently, only the fast transient pressure drop should be considered.
• this method is applied in the initial phase of the cuff inflation. In such embodiments operating the pump intermittently and measuring the resulting changes in pressure allows the tube resistance to be measured (using Ohms law), since the flow is known.
• the hysteresis loop is partly caused by the flow resistance of the tubing 57 (other contributions come from cuff material hysteresis and increases in arm volume due to blockage of venous flow). To reduce the effects of mechanical hysteresis and change in arm volume, the deflation should be fast.
• the cuff is inflated and deflated, at predetermined ramp rates using a servo controlled system (tube pressure control). In preferred embodiments the ramp down rate is significantly faster than the ramp up rate.
• the air flow and air pressure signals are low pass filtered and artefacts are removed.
• the cuff volume is obtained by integrating the processed air flow over time, and a P-V hysteresis loop is measured as described above.
• the tube resistance for a selected cuff volume (e.g. the volume represented by the horizontal dashed line 84 in Figure 8) can then be determined from:
• flowl and flow2 are the absolute values of the air flow at inflation and deflation for the selected cuff volume and ⁇ is the difference in tube pressure between the forward and backward flows.
• the cuff compliance can be estimated from the static V-P curve.
• the estimation is done using measurements from the extremes of the hysteresis loop (i.e. the highest and lowest pressures) with shortest possible delay time to reduce the impact of arm volume changes and cuff hysteresis. Deflation should be fast for the same reasons. Hence the hysteresis loop method enables both cuff compliance and tube resistance to be obtained in a single measurement.
• step 805 the cuff compliance is evaluated. Once R is known (e.g. from one of the above methods) it is possible to calculate the actual cuff pressure Pc(t) from:
• the cuff compliance Cc can be determined for all cuff pressures Pc as described above in relation to step 604 of figure 6. Hence the oscillatory NIBP measurement can be corrected for both tube resistance and cuff compliance for an arbitrary single lumen cuff, at fast inflation and deflation rates.
• step 806 the cuff compliance data calculated in step 805 and the tube resistance calculated in step 804 are used to correct the blood pressure envelope, using the procedure described above in relation to step 605 of figure 6.
• the corrected envelope can then be used to determine the diastolic and systolic blood pressure in a conventional manner.
• NIBP measurement can be done both fast and accurately using methods and apparatus according to the invention in which both tube resistance errors and cuff compliance transfer characteristics are accounted for.
• Being able to determine the tube resistance means that ramp rates can be significantly increased (up to a level where only 10 beats are observed per NIBP measurement).
• cuff pressure data is collected during both inflation and deflation, further decreasing the total time required for the measurement. This is advantageous as frequent NIBP measurements can be painful for the subject and can even cause them harm.
• measurement speed is determined by the number of beats (or cuff pressure pulses) required for a reliable blood pressure measurement. It will be appreciated that embodiments which enable faster and less obtrusive NIBP measurements are particularly suitable for applications where frequent blood pressure measurements are required (e.g. in hospitals, for ambulatory NIBP, etc.).

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