GB2413499A

GB2413499A - Leak measurement around an uncuffed endo-tracheal tube

Info

Publication number: GB2413499A
Application number: GB0409432A
Authority: GB
Inventors: Andrew J Sims; Jonathan H Smith
Original assignee: Newcastle Upon Tyne Hospitals National Health Service Trust
Current assignee: Newcastle Upon Tyne Hospitals National Health Service Trust
Priority date: 2004-04-28
Filing date: 2004-04-28
Publication date: 2005-11-02
Also published as: WO2005105189A1; GB0409432D0

Abstract

Apparatus comprises a ventilator 1, an uncuffed endo-tracheal tube for delivering gas to the lungs of a patient, a first flow sensor 5 sensing gas flow to a patient upon inspiration and generating a signal representative of the sensed inspiration flow, a pressure sensor 4 for sensing the pressure of gas flowing to a patient upon inspiration and generating a signal representative of the sensed inspiration pressure, a second flow sensor 6 for sensing gas flow from a patient upon expiration and generating a signal representative of the sensed expiration flow, and a controller 8, the controller 8 receiving output signals from the said flow and pressure sensors and processing those signals according to a computer program, and the computer program deriving from the said flow and pressure signals a value for leak resistance. The value for leak resistance may be used to display a value for the quality of fit of the tracheal tube. The computer program may use an algorithm to calculate leak resistance.

Description

241 3499

LEAK MEASUREMENT AROUND AN UNCUFFED ENDO-TRACHEAL TUBE

Field of the Invention

The invention is concerned with endo-tracheal tubes, and in particular tO the measurement of leakage around and uncuffed endo-tracheal tube in a patient and the establishment of qualify of fit of the tube in the patient's trachea.

Background of the Invention

Endo-tracheal tubes are used in the anaesthesia of patients, and in the ventilation of patients suffering from respiratory failure. Endo-tracheal tubes fall into two distinct categories, namely cuffed and uncuffed tubes. Cuffed endo-tracheal tubes have an inflatable balloon (the cuff) around the end of the tube that is inserted into the trachea during incubation. Uncuffed tubes are not equipped with such cuffs. A cuffed tube is used in older children and adults as subglottic area 31 (trachea) beyond the vocal cords 32 has a greater cross-sectional area than the vocal cords Larynx) as shown in Figure 1, where an adult larynx A is illustrated - When a cuffed tube is fitted in the trachea the balloon is inflated so as to occupy the space between the internal surface of the trachea and the external surface of the tube, and therefore all gas passing into a patient's lungs passes through the endo- tracheal tube.

Whilst cuffed tubes provide the benefit of being able to accurately determine the volume of gas transferred to and from the patient's lungs, cuffed tubes are unsuitable for use with neonates, infants and children, because the cross-sectional area of subglottis 31 belong the vocal cords 32 is less than that of the vocal cords (i.e. the relative size of the two is different) as shown in Figure 1, where a paediatric larynx B is illustrated. An uncuffed tube is thus usually sufficient to provide a seal. Too large a tube or insertion of a tube with a balloon can damage the trachea.

In order to avoid the potential damage to the trachea associated with cuffed endo-tracheal tubes, uncuffed endo-tracheal tubes are used for the incubation of neonates, infants and children. In the absence of the abovedescribed cuff, when the tube is inserted into the trachea a gap should exist between the internal surface of the trachea and the external surface of the tube forming a pathway through which gas rnayescape. The presence of such a pathway results in leakage of gas back to atmosphere. The amount of leakage is proportional to the size of the gap, and therefore the size of the gap is of great importance to the well being of the patient. The tube can cause tracheal trauma if the gap is too small, or non-existent. Such trauma may result in bruising, which rnaycause a longer stayin hospital, or in extreme cases granulation tissue or membranes can form that requires a tracheostomyto restore airway patency. This could lead to a longer stayin an intensive care unit. If the gap is too large, too large a volume of gas will escape around the tube to atmosphere, and the patient's lungs will not be inflated properly. Increasing the ventilator pressure can compensate for excessive gaseous escape around the tube; however, it iS not considered safe to increase the inflation pressure beyond a certain threshold.

The vocal cords are two ligaments that connect the arytenoid 34 and thyroid cartilages 35 in the neck, the latter is visible in men as the Adam's apple. These cartilages sit upon the cricoid cartilage and the various muscles that act upon the cords to produce speech have a variety of connections to the cartilages to produce the appropriate movements. The cricoid cartilage is the only complete cartilage ring in the trachea and encloses the subglottic area immediately belong the cords. It is swelling of or damage to the subglottic tissues that is the principal problem related to endotracheal intubation in children; the technique we detail here is concerned with measuring a 'degree of fit', of the endotracheal tube in this space.

Endo-tracheal tubes are available in a number of different sizes. Currently, the age and physical characteristics of the patient are used to determine the appropriate size of tube. It is desirable to fit the correct size of tube first time because each re-fitting of the tube can itself cause trauma to the trachea.

At present, the method used to establish that the tube is not too small, i.e. that leakage is not too great, consists of observation. The chest is observed for movement. If there is no chest movement, the leakage is too great, and therefore the tube too small. A tube is considered to be a good fit when the leak generates an audible sound.

One problem for medical practitioners in this area is that the condition of patients changes from day to day, and a tube that is a good fit one day may not be a good fit the next. Hence there maybe a clinical requirement to re-intubate with a differed sized tube.

Another problem for practitioners in this area is that tubes are supplied bye number of different manufacturers. The tubes they supply are identified by nominal internal diameters, but because the wall thicknesses very from one manufacturer to another, practitioners cannot rely on two tubes of a nominal 2.5 mm internal diameter giving rise to the same leakage when inserted into the trachea.

This variation in external diameters of tubes having the same nominal internal diameters is illustrated

in table 1 below.

Mm Cross sectional area of ETT in mm2 | DirRr Tube 1 Tube 2 Tube 3 | Tube 4 | Tube 5 Tube 6 1 2.50- 1 10.18-10.18 10.18 - 1 9.08 1 -13.20 3.00 14.5214.52 15.21 13.85 16.62 3.50 18.86 20.43 19.63 18.10 1 18.10 1 21.24 l 4.00 24.63 28.27 26.42- 23. 76 1 22.90 1 25.52 4.50 30.19- 33.18- 3421 I 30.19 1 30.19 1 30.19 1 5.00 37.39 37.39 40.72 37.39 1 36.32 5.50 44.18 44.18- 50.27 44.18 1 43.01 1 l 6.00 - 52.81 -1 52.81 - 60.82 1 52.81 52.81

Table 1

The uzzon in anss-s am of arlotral tutesnth mars. The Intel parroter Is Nash inD72 Beam three arfo=-ent sizes In the sane iltiamYersp Stub It would therefore be desirable to be able to establish the quality of fit of the tube to the patient.

It is believed that goodness of fit of an ET tube is related to the shape, size and nature of the spaces between the tube and the trachea. The "leak resistance" is a quantitative measure of the shape, size and nature of these spaces.

The following definitions of terms are used throughout the specification.

"Lung Compliance": an indication of the springiness or elasticity of the lungs. This is routinely defined as the change in lung volume per unit change in the transmural pressure gradient.

It can be measured after the lung has been held at a fixed volume for as long as possible (static), or during the course of normal rhythmic respiration (dynamic) "AirwayResistance": the pneumatic resistance of the patient's airways from the mouth where the ventilator is connected to the internal volume of the lungs. Here we refer to the 'none elastic resistance or the respiratorysystem resistance'. This is the frictional resistance to airflow and thoracic tissue deformation.

"Leak Resistance": a quantitative measure of the shape, size and nature of the spaces between the endo-tracheal tube and the trachea.

"PEEP": "Positive End ExpiratoryPressure" - a technique in which the lungs are not allowed to return to atmospheric pressure at the end of the expiration phase.

"FIC5: "Fraction of Inspired Ckygen" - measured as a percentage.

Apparatus for measuring lung compliance and airway resistance are known as described below US 6,068,602 describes a method and apparatus for determining non-linear airway resistance and lung compliance using an electrical circuit bye method that includes the steps of sensing a gas flow rate through an airway end sensing a gas pressure in the airway. A gas volume is calculated from the gas flow rate, and an invariant exponential based on physical characteristics of the airway. Airway resistance and lung compliance are then calculated based on the gas flow rate, the gas pressure, the gas volume, and the invariant exponential at any flow rate.

US 6,371,113 describes a ventilator of the type having a zero flow condition during a pause phase following mechanical inspiration, and an algorithm for ensuring that a pressure PPI A TEU is determined at zero flow conditions by ensuring that zero flow conditions exist at the end of the pause period.

The ventilator includes an inspiratory flow sensor and an expiratory flow sensor for monitoring the flow of air from the ventilator, the outputs of which communicate with a processor. A flow OUtpUt valve is controlled in response to signals from the inspiratory and expiratory flow sensors so that pressure at the patient connection can be controlled such that a zero flow state exists at the patient connection at the point when the pause period ends. The ventilator described in this patent does not measure tube fit.

US 2002/0026941 describes an exhalation assist device for adjusting airway resistance in an exhalation circuit of a medical ventilator. The ventilator adjusts the resistance within the exhalation circuit by generating a negative pressure around a gas exchange reservoir. The ventilator has the ability to compensate for gas flow resistance into and OUt of the lungs of a patient, and can distinguish between passive reverse airflow and active initiation of inspiration.

US 5,316,009 is concerned with an apparatus for monitoring respiratory muscle activity, the apparatus comprising: a pressure sensor for detecting air pressure in an air passage connecting a lung ventilator and the airwaysystem of a patient; a flow rate sensor for detecting flow rate in the air passage; an arithmetic constant detecting means for detecting resistance and elastance of the s respiratory system including the airway and thorax by using detection signals from the pressure sensor and the flow rate sensor while a lung ventilator is supplying air to the patient whose spontaneous breathing has temporarily slopped.

US 4,031,885 is concerned with a method and apparatus for determining patient lung pressure, compliance and resistance in conjunction with a volume compensated respirator. The patent refers to "volume compensated respiration apparatus" as respirator apparatus including means for compensating for errors in volume delivered to the lungs of a patient.

The prior art does not identify any means of measuring leak resistance, or of establishing qualify of tube fit for an uncuffed endo-tracheal tube. It is therefore an aim of the invention to provide such means.

Summary of the Invention

A first aspect of the invention provides an apparatus for measuring leak resistance as specified in Claim 1.

A second aspect of the invention provides an apparatus for establishing the qualify of tube fit of an uncuffed endo-tracheal tube in a trachea as specified in Maim 11.

A third aspect of the invention provides a method for measuring leak resistance as specified in Claim 15.

A fourth aspect of the invention provides a method for establishing the qualify of tube fit of an uncuffed endo-tracheal tube in a trachea as specified in Claim 19.

Brief Description of Me Drawings

In the drawings, which by way of example, illustrate apparatus and methods for measuring leak around an endo-tracheal tube: Figure 1 is a schematic illustration of an adult larynx A and a paediatric larynx B.; Figure 2 is a circuit diagram of an electrical model of an uncuffed endo- tracheal tube; Figure 3 is a schematic representation of the functional elements of a ventilator; and Figure 4 is a schematic representation of a ventilator control panel.

Detailed Description of the Preferred Embodiments

Referring now to Figure 3, a ventilator 1 comprises a pipe 2 delivering gas to a patient, and a pipe 3 receiving gas from a patient. Pressurised gas is generated by a set of ventilator mechanics 7 (well known to those skilled in the art and therefore not described here in detail), and delivered to the patient via the pipe 2. Two sensors: a flow sensor 5 and a pressure sensor 4 are located in the pipe 2, between the ventilator mechanics 7 and the patient.

The volume of air leaked during each breath depends on the pressure achieved in the lungs, and on the leak resistance. Leak resistance is dependent on the space between the endo-tracheal tube and the larynx.

Gas expired by the patient flows through the pipe 3, through a flow sensor 6 located in the pipe 3, to the ventilator mechanics 7.

The flow sensors 5 and 6, and the pressure sensor 4 generate analogue electrical signals representative of flow and pressure sensor respectively. Output signals from these sensors constitute input signals to the ventilator controller 8.

The ventilator controller includes a micro-processor operated by a computer program. The program causes the processing of the flow and pressure information received from the pressure 4, and flow sensors 5 and 6, according to an algorithm (described in greater detail below), and electronic signals representative of airway resistance, lung compliance and leak resistance are generated. These electronic signal outputs can be transformed in to data comprehensible to a suitably trained healthcare professional, for example by connecting the outputs to a &pray screen, such as a LED screen or a cathode raytube.

The sensors 4, 5 and 6 and the processor are described above as forming part of a ventilator 1. Some ventilators are equipped with a pressure sensor on the expiration side of the circuit. The model of the invention does not require information from such a pressure sensor. In the ventilator illustrated in Figure 3, the pressure sensor 4 is located at the ventilator end of the pipe 2 delivering gas to the patient. The free end of the pipe 2 connects to an endo-tracheal tube by means of an appropriate fitting. It is possible to insert a pressure sensor at the point where the pipe 2 connects the endo- tracheal tube. The use of a pressure sensor in this location prevents anypossible effect of pressure loss in the pipe 2 on the value of leak resistance calculated. However, the affect of pressure loss in the pipe 2 is likelyto be negligible due to its diameter being wide in comparison to the diameter of the endo-tracheal tube. The sensors 4, 5 and 6 and / or the processor may form pert of a stand- alone unit.

The above-mentioned algorithm utilises an electrical model of the lungtrachea circuit in which the air-wayresistance is modelled by electrical resistance, lung compliance by electrical capacitance, air flow by electrical current, and pressure by voltage.

In Figure 2, there is illustrated an electrical circuit comprising a potential (Po) a first resistor \fRET) connected in series with a second resistor TAB) and a capacitor (I), and third resistor By).

The air-way resistances are represented as follows: Resistor (RET) represents the resistance of the endo-tracheal tube, the resistor (RTB) represents the resistance of the tracheo-bronchial tree and lung tissue distal to the endo-tracheal tube, and the resistor RL represents the resistance to leak between the endo-tracheal tube and the trachea. The capacitor (Ci) represents the compliance of the lungs, and P0 the pressure applied bythe ventilator.

The electrical model of the lung-trachea circuit uses a number of equations to establish the leak resistance RL as discussed below A ventilator delivers a time-varying flow of gas by applying an input pressure P0. In a model assuming airway resistance to be linear (i.e. follows Ohm's law), the pressure at the distal end of the endo-tracheal tube, Plug is given by Pk,g(( = Po(t)-Inapt) RET (1) Where Ives is the ventilator flow rate (modelled by electrical current), RET is the resistance of the endo-tracheal tube and tis time.

Pressurised gas flows (Icons) from the ventilator. The flow is either delivered to the lung (Iba; from where it returns to the ventilator via the endo-tracheal tube, or leaks (I,) through the space between the endotracheal tube and the trachea to the atmosphere. Therefore, for all time t: I=,, (t) = Ibis (t) + Ink (t) (2) If the leak resistance is linear, then the following equation relates leak resistance (RL) to lung pressure and flow through the leak.

R = Pb(t) /Ikk (t) (3) Byintegrating the flow rate delivered to the patient Iqo:' and the flow rate of the returned gas I - for time period T. where Tis there duration of one breath, the volume of gas lost in one breath cycle (A can be calculated. Since the flow of gas lost is 1 (t), the integral of IT (t) for time period Tmust be equal to Q,oss Using equation (3) it iS possible to express leak resistance in terms of the pressure at the distal end of the endo-tracheal tube PA as follows: R! S (l/Q,oss) JPbo' (t)dt (4) Equation (1) allows Pb(t) to be substituted by measurable values, therefore providing for the calculation of RL.

RL = (1/Q!aKs) ,(Po (t) - I=r (t) RE7} dt (5) If the REr value indicates that flow is laminar, then the value of (PO (t) - In, (I) RET) can either be established empirically, or derived from the calculation where flow Q= (ins / 871)(p - P2). If however the nature of flowin the ET tube is outside the larninar range of flow, itiS necessaryto establish empirically I, (t) RETvalues for each type of endo-tracheal tube that might potentially tee used with a ventilator for a range of pressure values. These values can be stored in a look up table in the computer program operating the ventilator controller, so that when a practitioner enters a tube type identifier, which could be a tube internal diameter, or a tube internal diameter and a tube length, and a maximum pressure at which the lung is to be ventilated, the ventilator controller knows what value of IN (t) RET to use. Values for RET that are within the laminar range of flow may also be stored in a look up table.

Airway resistance is a combination of the resistance of the endo-tracheal tube and of the tracheobronchial tree and tissues beyond. It has been established that the flow resistance of infant endo-tracheal tubes is equal to, or greater than that of the normal upper airway, for the normal range of infant flows.

The algorithm of the invention involves the following steps over a one breath cycle: a. Measure the instantaneous inspired flow rate throughout a cycle at sampling rate of at least 100 Hz; b. Measure the instantaneous expired flow rate throughout the cycle at a sampling rate of at least 100 Hz; c. Measure the pressure of gas at the ventilator throughout the cycle at a sampling rate of at least 100 Hz; d. Calculate the volume of gas lost to atmosphere ( JTa _ JTb where T is the duration of one breath cycle); e. Calculate the instantaneous pressure at the lungs from a and c, i.e. calculate the instantaneous value of Po (t) - IN (t) RET.

f. Calculate leak resistance from the measured parameters.

The leak resistance is calculated using the equation: R' = (1/Q) (Pb(t)dt.

One way of implementing the equation RL = (1/Q) JTP/(t)dt is to apply the following equation: Rl = (1/Q ((PO (t) - Iota (t) RET) aft, where the algorithm includes the step of calculating, or obtaining a value for, RI. T. RETcan be calculated if flowin the ET tube is larninar, and at low pressures and flow rates as are used in neonates and some infants this may hold true. However, as mentioned above, research indicates that at the flow rates norrnallyused in infants the nature of flow through theETtube is actually transitional, in which case the value of (I,,,(t) RET) must be determined empirically by measuring the pressure at the end of the ET tube. As mentioned previously, these empirically determined values can be stored in a look up table in the computer program operating the ventilator controller, so that when a practitioner enters a tube type identifier, such as its internal diameter or its length and internal diameter, the ventilator controller knows what value of RETto use. Whilst it is possible for the value of RETto be calculated in flow through the tube is laminar, those values could equally be stored in a look up table, and may also be calculated empirically. A range of values of RET for a range of lengths of tube of a given internal diameter may be determined and stored in a look-up table.

The empirically derived data for R,:T may not be stored in a look up table. It is possible that the empirical data for RET may be derived for each tube prior to insertion of the tube into a patient using the measurable parameters on the ventilator.

The invention is particularly advantageous because a) the medical practitioner can quickly establish whether the qualify of fit of the endotracheal tube is good; b) by placing a value on leak resistance, and qualify of fit, it the fit is not good, the practitioner has accurate information on which tO base a selection of a different sized tube; c) by monitoring leak resistance, and hence qualify of fit, through the period of intubation, the medical practitioner has accurate information which he can use in making a clinical decision as to whether it is appropriate to intubate with a different sized tube; and d) by using the invention, practitioners will improve their understanding of tube fit.

Figure 4 illustrates a control panel 10 for controlling a ventilator as illustrated in Figure 3. The control panel comprises a series of dials and a display that allow a healthcare professional to set up, the ventilator for a particular patient, monitor the performance of the ventilator and the patient, and make any necessary adjustments. The series of dials comprises a dial 11 for setting inspiratory pressure, a dial 12 for setting the number of breaths per minute, a dial 13 for setting the inspiration time (i.e. the period during a breathing cycle during which the ventilator delivers air tO the patient), a dial 14 for setting the pause time (i.e. the period in a breathing cycle during which the ventilator does not deliver air to the patient), and dial 15 for setting the PEEP pressure, and a dial 16 for setting the FIO2 level.

The control panel 10 further comprises a display 17. The displayis set up to provide information relating tO a number of different functions of the ventilator, and includes a display clement 18 of Reak peak resistance), a display element 19 of Raw 0 a display clement 20 of C., Qung compliance), a display clement 21 of Pn.a,(maximum pressure in the lung), a displayelement 22 of FIGS, a display element 23 showing bpm (breaths per minute), and a display clement 24 showing Vt the tidal volume of one breath. The display 17 further includes a display element 25 illustrating the variation of flow with respect to time, and a display clement 26 illustrating the variation of pressure with respect to time.

Claims

Claims 1. Apparatus comprising a ventilator, an uncuffed endo-tracheal

tube for delivering gas to the lungs of a patient, a first flow sensor for sensing gas flow to a patient upon inspiration and generating a signal representative of the sensed inspiration flow, a pressure sensor for sensing the pressure of gas flowing to a patient upon inspiration and generating a signal representative of the sensed inspiration pressure, a second flow sensor for sensing gas flow from a patient upon expiration and generating a signal representative of the sensed expiration flow, and a controller, wherein the controller receives output signals from the said flow and pressure sensors and processes those signals according to a computer program, and wherein the computer program derives from the said flow and pressure signals a value for leak resistance.
2. Apparatus according to claim 1, wherein the computer program derives from the said flow and pressure signals a value representative of lung compliance.
3. Apparatus according to Claim 1 or 2, wherein the computer program derives from the said flow and pressure signals a value representative of away resistance.
4. Apparatus according to anypreceding claun, wherein the said flow and pressure signals are electrical signals.
5. Apparatus according to anypreceding claim, wherein the said processor is a micro processor.
6. Apparatus according to anypreceding Claire, wherein the flow and pressure sensors are sensors of the ventilator.
7. Apparatus according to anypreceding claim, wherein the controller is the controller of the ventilator.
8. Apparatus according to any of Claims 1 to 5, wherein the flow and pressure sensors and/or the controller is independent of the ventilator.
9. Apparatus according to anypreceding claim, further comprising a display means for displaying information generated by the computer program
10. Apparatus according the anypreceding claim, wherein apparatus further comprises data input means.
11. Apparatus according to anypreceding claim, wherein the derived value of leak resistance is compared with recorded values of leak resistance correlated to qualify of fit values, and wherein the computer program outputs a qualify of fit associated with the derived leak resistance value.
12. Apparatus according to anypreceding claim, wherein the computer program runs the following algorithm over one breath cycle: a. Measure the instantaneous inspired flow rate throughout a cycle at sampling rate of at least 100 Hz; b. Measure the instantaneous expired flow rate throughout the cycle at a sampling rate of at least 100 Hz; c. Measure the pressure of gas at the ventilator throughout the cycle at a sampling rate of at least 100 Hi; d. Calculate the volume of gas lost to atmosphere ( pa - fib where T is the duration of one breath cycle); e. Calculate the instantaneous pressure at the lungs from a and c, i.e. calculate the instantaneous value of Po (t) - IN (t) RAT.

f. Calculate leak resistance from the measured parameters.
13. Apparatus according to Maim 12, wherein the algorithm calculates leak resistance according to the following equation: RL = (1/Q) JTP - (trait
14. Apparatus according to Claim 12 or 13, wherein the algorithm calculates leak resistance according to the following equation: RL = (1/QSS) JT(PO (I) - IW (9 RET) at
15. A method of using the apparatus as claimed in any of Claims 1 to 14 for measuring leak resistance in a patient intubated with an uncuffed endo-tracheal tube of the said apparatus, and comprising the steps of: a. sensing gas flow to the patient upon inspiration and generating a signal representative of sensed inspiration flow; b. sensing the pressure of gas flowing to the patient upon inspiration and generating a signal representative of sensed inspiration pressure; c. sensing gas flow from the patient upon expiration and generating a signal representative of sensed expiration flow; d. processing the generated flow and pressure signals according to a computer program, the computer program deriving from the said flow and pressure signals a value for leak resistance.
16. A method according to Claim 15, wherein the step (d) includes the performance of the following algorithm a. Measure the instantaneous inspired flow rate throughout a cycle at sampling rate of at least 100 Hz; b. Measure the instantaneous expired flow rate throughout the cycle at a sampling rate of at least 100 Hz; c. Measure the pressure of gas at the ventilator throughout the cycle at a sampling rate of at least 100 Hz; d. Calculate the volume of gas lost to atmosphere ( JTa - Job where T is the duration of one breath cycle); e. Calculate the instantaneous pressure at the lungs from a and c, i.e. calculate the instantaneous value of Po (I) IN (t) RET.

f. Reticulate leak resistance from the measured parameters.
17. A method accor&g to Claim 16, wherein the algorithm calculates leak resistance accor&g to the following equation: RL = (l/QJ0Ks) JTPb(t)dt
18. A method accor&g to Claim 16 or 17, wherein the algorithm calculates leak resistance accor&g to the following equation: RL = (1/Q'Fs) JT(PO (t) Iunt (t) RET) It
19. A method accor&g to accor&g to any of Claims 17 to 18, furtherinclu&g the step of comparing the derived value of leak resistance with recorded values of leak resistance correlated to qualify of fit values, and generating a qualify of fit indication associated with the derived leak resistance value.
20. A method accor&g to Claim 19, wherein the said quality of fit indication is displayed on the said display.
21. Apparatus substantially es shown in, and as described with reference to, the drawings.