WO2005032364A1

WO2005032364A1 - Position monitoring apparatus

Info

Publication number: WO2005032364A1
Application number: PCT/AU2004/001364
Authority: WO
Inventors: Jeoff Parkin; Clive Boyd
Original assignee: Mondo Medical Limited
Priority date: 2003-10-07
Filing date: 2004-10-07
Publication date: 2005-04-14
Also published as: WO2005032364A8

Abstract

A position levelling apparatus having an sensor (10) which is adapted to be located at a position spaced from a required position and an levelling unit (11) which is adapted to be located remote from the sensor which can receive, from the sensor, parameters relating to the required position so that changes in the location of the required position can be ascertained. Blood pressure levelling unit transducers (14) are located at predetermined location on the levelling unit and the levelling unit displays the height above or below the level of the tricuspid valve that tranducers are located. The levelling unit having means (17) whereby it can be moved up or down so that the transducers can be maintained at a height corresponding to that of the tricuspid valve whereby the readings of the transducers are not modified by any variation in vertical location between themselves and the tricuspid valve. Instead of moving the unit, it can provide an adjustment factor for the transducers or an associated monitor (12) as though the transducers were at the level of the tricuspid valve.

Description

POSITION MONITORING APPARATUS

The invention relates to a position monitoring and levelling apparatus and in particular to such apparatus which is adapted to provide an indiction of the position of a patient's tricuspid valve or phlebostatic axis.

The phlebostatic axis defines a horizontal plane that is level to a patient's right atrium at the fourth intercostal space midaxillary line.

Locating external pressure transducer zero tap points at the level of the tricuspid valve ensures accuracy of heart pressure readings and serves as a reference point for consistency when the patient moves.

Zeroing transducer tap points is a standard procedure required to negate the effects of local atmospheric pressure within the closed fluid line so that only patient blood pressure values are measured.

Also, placing transducer tap points on the tricuspid valve mitigates the effect of fluid head offset pressure readings that result when a mismatch in height between two ends of the interconnecting fluid lines occur.

It is the object of the invention to provide a height monitoring and adjustment apparatus which can be used to ascertain the position of a patient's tricuspid valve regardless of the position adopted by the patient.

The invention, in its broadest sense, provides a height monitoring and adjustment apparatus having an sensor which is adapted to be located at a position spaced from a required position and an levelling unit which is adapted to be located remote from the sensor which can receive from the sensor parameters relating to the required position so that changes in the location of the required position can be ascertained. It is preferred that the sensor includes an oil filled member which is in connection with a reservoir in the levelling unit whereby changes in the vertical height between the sensor and the levelling unit cas be ascertained and an accelerometer which can provide an indication of the direction and angle of change, the two enabling the actual position of the remote point to be ascertained.

The levelling unit has a pressure transducer which measures the head of oil in the reservoir.

Changes is the patient's position, such as if he (and we will for convenience refer to the patient as he) rotates or is rotated about a generally longitudinal axis, or sits up or is partially supported in a raised lying position are ascertained by the accelerometer and the levelling unit can calculate from the movement of the accelerometer changes in the vertical position of the tricuspid valve.

The levelling unit is adapted for movement in the vertical plane so that predetermined locations on the levelling unit can be brought into line with the required position. Preferably this movement is along an IV pole or other vertical member. It is preferred that this movement can be readily effected by a user and for this reason a rotatable knob is associated with a member which contacts the vertical member and can cause the levelling unit to move therealong.

The invention is particularly applicable to an apparatus in which the required position is the position of the tricuspid valve of a patient and the predetermined locations on the levelling unit are positions at which blood pressure levelling unit transducers are located, the levelling unit displaying the height above or below the level of the tricuspid valve the blood pressure levelling unit transducers are located, the levelling unit having means whereby it can be moved upwardly and downwardly vertically so that the blood pressure levelling unit transducers can be maintained at a vertical height corresponding to that of the tricuspid valve whereby the readings of the blood pressure levelling unit transducers are not modified by any variation in vertical location between the transducers and the positions at which the blood pressure levelling unit transducers are located.

The device is calibrated (regularised) against the height of the tricuspid valve (the phlebostatic axis) which is a point in space between the spine and the sternum proportional to the transthoracic diameter.

The crux of the use of the invention is to establish the location of the tricuspid valve so that the disposable blood pressure transducers used to measure the actual blood pressure via arterial lines inserted in the patient can be positioned on the same horizontal plane as the tricuspid valve and therefore give accurate actual blood pressure values.

In order that the invention cam be more readily understood, reference will be made to the accompanying drawings, in which:

Fig 1 shows a schematic view of the desiderata for the apparatus when used on a patient;

Fig 2 shows a schematic view of the physical lay out of the apparatus when being used on a patient;

Fig 3 shows a perspective top view of the sensor used with the apparatus; and

Fig 4 Shows one arrangement whereby the levelling unit can be connected 'to and moved along a vertical IV pole or the like.

Where it is necessary to obtain accurate measurements of arterial and venous blood pressures for a patient in intensive care, or for whom it is desirable to have accurate readings, pressure transducers which are connected to lines which pass into the patient's body are used, and in order to obtain an accurate reading of blood pressure it is essential that these be located at the height of the patient's tricuspid valve.

When the patient is supine this position can be calculated being approximately 80% of the distance along the sternum, from the top of the sternum, ( at its jugular or sternal notch) to the lower extremity (tip of xyphoid process). It is also necessary to measure the transthoracic diameter (TTD) which is the height through the body transverse to the body at the position of the heart. The tricuspid valve or phlebostatic axis can be taken, for anglo-saxon patients to be 0.4 of this distance from the chest wall. For patients of different types, the value can be taken to be between 0.35 to 0.45.

Whilst it is basically straightforward to ascertain the position of the phlebostatic axis, whilst the patient relmains in this position, if the patient moves as by a rotation about a longitudinal axes or is caused to sit up, the position of the tricuspid valve, of course, varies.

As mentioned, in order to obtain an accurate blood pressure reading the transducers must be maintained at the same height as the tricuspid valve, and this can be difficult to ascertain, and it is in order to provide a simple method of doing this, that the present invention relates.

The apparatus of the invention includes two basic components, a sensor 10, which is adapted to be-located on the patient's chest, and a levelling unit 11, which is spaced from the sensor and is preferably mounted for vertical movement along an IV pole or the like and which can be connected by line 13 to the sensor.

Levelling unit 11 has connection points 14 to which blood pressure transducers can be connected, and there is a line (which may comprise a number of conductors) 15 whereby the outputs from these transducers can be connected to a pressure monitor 12. The transducers and the pressure monitor are not part of the invention, but, as will be seen, are carried by the levelling unit so that their spatial position can be varied.

The line 13 may include a silicone oil line, one end of which is connected to the sensor, and the other end is in association with a pressure transducer mounted in the levelling unit.

As part of this line there is a resistence-less soft plastic reservoir to and from which oil from the line can pass, depending upon whether the levelling unit is higher than, or lower than, the sensor.

The pressure transducer measures the head of the oil in this reservoir and electronically. from the measurement obtained, the vertical position of the reservoir relative to the sensor can be ascertained.

This head can be calculated in lineal divisions, such as centimetres, and can be displayed as a positive or negative number on the screen 16 of the levelling unit.

This measurement, possibly combined with audio and visual systems, can provide a user with a direct indication of the mismatch in height between the sensor and the levelling unit, or more particularly, a particular part of the levelling unit as will be described, and means, which will be described later, but which include a control knob 17, can be used to adjust the levelling unit until the unit and the sensor are at the same level.

Also associated with the sensor there is a tri axial accelerometer which measures pitch and yaw (twisting about the "x", "y", and "z" axes and the outputs from the accelerometer are fed to the levelling unit, preferably along the line 13 and the unit, mathematically, calculates from changes in the position of the accelerometer along the three axes, the actual position of the tricuspid valve, and this position, in association with that initially ascertained by calibrating the unit using purely the fluid relationship between the sensor and the unit, can provide an accurate figure of the position of the tricuspid valve and an adjustment of the levelling unit 11 by the knob 17 will bring the unit directly into alignment with the tricuspid valve.

We mentioned earlier that it was at a particular position on the levelling unit 11 that the height was required.

Located in this embodiment on the back of the unit there are a number of connectors 14 to which blood pressure transducers may be connected, and it is the horizontal plane through these transducers which needs to be aligned with the tricuspid valve. These transducers are connected by way of lines to the patient's body and it will be appreciated that provided the fluid line which may be connected to the tapping point, which could be a vein or artery close to the heart, to the pressure transducer are at the same height at each end, it is not relevant whether along the path this height varies as there will be an equilisation of pressure at the end.

These transducers, which are not part of the invention, may then be connected to a pressure monitor 12 whereby a real time indication of blood pressure can be provided and, if required, recorded for later examination.

It will be appreciated that by the use of the invention, nursing staff can instantly alter the position of the levelling unit so as to ensure that the blood pressure transducers are aligned with the tricuspid valve and this provides a much more reliable indication of actual and trending blood pressure, which is the basis of clinical decision making, than was the case hereto before, when the patient moved a re measurement needed to be taken and some empirical estimate made of the effect of the movement.

When of if the patient moves again, rolls, sits up, is propped up, the device will recalculate the difference in current height from the former height when the levelling unit had been moved to accommodate the earlier change. The pressure differential associated with the tricuspid sternum offset distance is scaled to the vertical axis and is derived from the patients transthoracic distance.

The operation of the sensor relies on a scale factor between (0 and 1) derived from 3- dimensional spherical acceleration (gravity) vector where a_z=g - (a_x ² + a_y ²) is the vertical component of gravity as defined by two orthogonal, vectors offset by 90 degrees. Derivation of a_z from the acceleration vectors a_x + a_y suffers from the fact that the true value of a₂ can be either positive or negative thereby minimising the ability to derive a vertical offset when the sensor is positioned below the upper hemisphere.

Because of the application of a square root function on the measured a_x & a_y acceleration components the above derivation of a_z can not detect if the patient is aligned in either positive or negative directions relative to gravity.

To resolve the problem of deriving both the positive and negative direction of gravity a_z there is located in the sensor, an acceleration transducer with its directional axis parallel to the direction of gravity g.

When the patient is lying on his back and the sensor assembly positioned at the top of the sternum as previously described the accelerometer axis is aligned normal to gravity, ie an accelerometer detection axis parallel to a line that passes through the centre of mass of the tricuspid valve and a point located on the patient's sternum.

This accelerometer alignment allows both the sign of the vertical gravity vector and the absolute acceleration value to be directly applied in order to generate a sealer S in the range (-a_z I g = S = + a_z I g). The sealer is then applied to a vertical pressure (height) offset that is derived from the length of a patient's transthoracic distance Z^,. This distance being the distance between a patient's sternum and their back via a path that intersects the tricuspid valve. The approximate length of the sternum to the tricuspid valve, tricuspid valve, l_sp, is derived from the transthoracic distance using l_SP = 0.4 * l_πD. For patients of different types, the constant in this formula, 0.4, may be replaced by other constants which would normally be between 0.35 and 0.45.

The derivation of the height of the tricuspid valve h_P relative to the sternum is found by subtracting the measured height of the sternum transducer h_s by a gravity scaled offset where h_P = h_s - S * l_SP.

In the particular embodiment, the relative height of the sternum is, as explained earlier, ascertained by measuring the pressure of a closed hydraulic fluid line.

The apparatus of the invention enables the derivation of the vertical position of a coordinate point within an object by measuring the vertical position of an external coordinate and then offsetting by a sealer derived from gravity.

In the particular application this enables the patient to be moved spatially on any axis whilst, at the same time, providing the derivation of an internal coordinate within the transthoracic cavity which shows variation of that particular coordinate from the original position of the coordinate.

Also, as described earlier, where this is used with a pressure measuring transducer we can then shift the transducer remote from the patient by a distance equal to the vertical distance moved by the patient's tricuspid valve to ensure that the pressure obtained remains accurate. Alternatively, we can use a computer to calculate the effective change in pressure due to the change in the two coordinates and automatically compensate the pressure reading for variation in the position of the patient without the necessity of actually moving the levelling unit to match that at the required coordinate. Referring to Fig 4 we show one means whereby the levelling unit 11, can be connected to a pole 20, which may be an IV pole or any other vertical member.

We provide a clamp 21, which has two components, a fixed component 22 and a moveable component 23, and the fixed component 22 has an over centre lock means 24. In use the lock 24 of the component 22 is released, the device is located about the pole with the clamp 22 abutting the pole and the moveable member 23 is caused to be brought into contact with the pole 20 at which time it can be locked by tightening the screw 25. At this stage the clamp is abutting the pole.

When the over centre lock means 24 is then closed this moves in towards the pole and ensures that the device is firmly held in position on the pole.

Located in association with the moveable member 23 there is an adjustment knob 30 which extends through and has a portion abutting the pole 20 and on rotation of the knob the assembly, and thus the levelling device, can be caused to move upwardly or downwardly, relative to the pole, without further adjustment of the clamp itself.

This is thus basically a simple one handed operation and can be effected by a staff member whilst monitoring the device for vertical height adjustment.

As can be seen from figure 2 there are a number of control buttons 40, adjacent to display 16, and these can include a set-up mode which can be initiated when the device is first brought onto line, and it can enable the introduction of the required transthoracic distance, as measured on the patient, preferably two buttons are provided for this, one to increase and one to decrease the distance, there can be a zero button to reset the device.

There can also be a visual indicator which, when the device is operating can turn from green to some other colour to indicate that adjustment may be necessary and an audible indicator adapted to operate when variation from a coincidence of the location of the phlebostatic valve and the blood pressure transducers reaches a figure which is more than a pre-determined value above or below that which is acceptable.

Whilst we have described the invention specifically in relation to a height levelling uniting apparatus for cardiac pressure reading it will be appreciated that it could have other applications where it is necessary to levelling unit the vertical position of a point over time even if the point itself is not accessible for direct reading.

Also, the measuring means can be varied for example by using geostatic reference, ultrasonic or sonic related sensing, rf signals, pressure sensors and accelerometers or gravity.

Any modifications can be made in the apparatus of the invention without departing from the spirit and the scope thereof.

Claims

The claims defining the invention are as follows:

1. A position levelling uniting apparatus having an sensor which is adapted to be located at a position spaced from a required position and an levelling unit which is adapted to be located remote from the sensor which can receive from the sensor parameters relating to the required position so that changes in the location of the required position can be ascertained.

2. An apparatus as claimed in claim 1 wherein the sensor includes an oil filled member which is in connection with a reservoir in the levelling unit whereby changes in the vertical height between the sensor and the levelling unit cas be ascertained and an accelerometer which can provide an indication of the direction and angle of change, the two enabling the actual position of the remote point to be ascertained.

3. An apparatus as claimed in claim 2 wherein the levelling unit has a pressure transducer which measures the head of oil in the reservoir.

4. An apparatus as claimed in any preceding claim wherein the levelling unit is adapted for movement in the vertical plane so that predetermined locations on the levelling unit can be brought into line with the required position.

5. An apparatus as claimed in any one of claims 1 to 3 wherein, on ascertaining a change in position of the sensor, an adjustment factor can be generated for application to the blood pressure transducers or monitor to compensate for the change.

6. An apparatus as claimed in any one of claims 1 to 4 wherein the required position is the position of the tricuspid valve of a patient and the predetermined locations on the levelling unit are positions at which blood pressure levelling unit transducers are located, the levelling unit displaying the height above or below the level of the tricuspid valve the blood pressure levelling unit transducers are located, the levelling unit having means whereby it can be moved upwardly and downwardly vertically so that the blood pressure levelling unit transducers can be maintained at a vertical height corresponding to that of the tricuspid valve whereby the readings of the blood pressure levelling unit transducers are not modified by any variation in vertical location between the transducers and the positions at which the blood pressure levelling unit transducers are located.

7. An apparatus as claimed in any claim 5 wherein the required position is the position of the tricuspid valve of a patient and the predetermined locations on the levelling unit are positions at which blood pressure levelling unit transducers are located, the levelling unit displaying the height above or below the level of the tricuspid valve the blood pressure levelling unit transducers are located, the levelling unit having means whereby an adjustment factor can be generated for application to the blood pressure transducers or monitor to compensate for the change so that the output of the blood pressure levelling unit transducers can be maintained at a level equivalent to the transducers being at a vertical height corresponding to that of the tricuspid valve whereby the readings of the blood pressure levelling unit transducers are not modified by any variation in vertical location between the transducers and the positions at which the blood pressure levelling unit transducers are located.