WO1991002227A1

WO1991002227A1 - Method and apparatus for measuring the fluid balance of a patient during surgery

Info

Publication number: WO1991002227A1
Application number: PCT/GB1990/001246
Authority: WO
Inventors: Cornelius Andrew Lewis; Stephen William Valentine Coppinger
Original assignee: University College London
Priority date: 1989-08-10
Filing date: 1990-08-09
Publication date: 1991-02-21
Also published as: GB2235776A; GB8918253D0

Abstract

The present invention provides a method and apparatus for measuring the fluid balance of a patient during surgery in which the patient is periodically weighed throughout the operation. The weight values processed to remove unwanted fluctuations and transient effects and an indication of the patient's weight. The patient is preferably weighed using load cells (5) placed in or underneath the operating table (2). The data is preferably displayed graphically relative to a reference weight so that the change in the patient's weight during the course of the operation can be seen.

Description

METHOD AND APPARATUS FOR MEASURING THE FLUID BALANCE OF A PATIENT DURING SURGERY

The present invention relates to a method and apparatus for measuring the fluid balance of a patient during surgery, in particular during transurethral resection of the prostate.

During transurethral resection of the prostate (TURP) it is necessary to have a good view of the prostatic fossa to enable careful guidance of the cutting diathermy loop. An exceptionally clear view is vital because poor vision resulting in a single misplaced cut may cause permanent incontinence in the patient. A clear view of the prostatic cavity is achieved by the provision of a continuous flow of irrigating solution via the resectoscope. The solution dilates the prostatic cavity and washes the resected tissue and blood away; the irrigant is either removed continuously via another channel in the instrument or at intervals by removal of the resectoscope from its sheath.

Regardless of the method of irrigation, variable quantities of the irrigating solution are absorbed during the operation. Large quantities may severely overload the patient's circulation, leading to cardiovascular collapse and toxic effects. Studies have observed that irrigating solution may be absorbed directly into the circulation at a rate of 190 ml/min. Another study has reported a case in which seven litres of irrigant were absorbed in less than one hour while further studies have indicated that the concentrations of irrigating fluid solute in the circulation after surgery can range from 40 ml to 2.8 1. Another factor contributing to the overall fluid balance of the patient is haemorrhage, which also occurs variably. The amount of blood loss is very difficult to estimate due to the dilution of the blood by the irrigating solution and if large losses are not corrected appropriately the patient will be placed at risk of exsanguination. Substantial degrees of both haemorrhage and fluid absorption often coexist duiring the same operation.

Various techniques have been employed in efforts to estimate the amount of irrigating fluid absorbed during TURP. These include:

1) weighing the patient before and after the procedure,'

2) measuring the difference in irrigating fluid in and out of the patient,

3) measuring serum sodium levels either continuously or at intervals,

4) measuring levels of irrigating fluid solute,

5) measuring levels of marker substances,

6) monitoring central venous pressure, and

7) measurement of intravesical pressure.

None of these practices has found universal favour. Some are inaccurate (1,2) or give results which are hard to interpret (3) and others involve invasive procedures or radioisotopes (5,6,7). Some (3,6) only reflect the consequences of massive fluid absorption rather than giving early warning and cannot be quantified. The measurement of levels of irrigating fluid solute (4) is complicated and time-consuming which precludes its use as a rapid bedside assay. Recent studies have shown that marked absorption of irrigant can occur without any appreciable rise in intravesical pressure, calling into doubt the usefulness of this approach. Almost all of these techniques complicate or delay surgery and hence are not generally used in routine surgery. According to the present invention, there is provided a method of measuring the fluid balance of a patient during surgery, comprising the steps of: monitoring the weight of said patient and producing raw data indicative of said weight; processing said raw data to compensate for factors other than changes in said fluid balance to produce a corrected weight; and providing an indication of said corrected weight so that changes in said fluid balance can be observed.

The present invention also provides an apparatus for monitoring the fluid balance of a patient, said apparatus comprising weighing means for weighing said patient and producing raw data indicative of said weight of said patient; processing means for processing said raw data to compensate for factors other than changes in said fluid balance, to produce a corrected weight; means for deriving from the corrected weight an indication of the fluid balance.

Preferably, the means for providing an indication of said weight comprise means for displaying the change in the patient's weight from a reference point established at the beginning of an operation and what is displayed are the changes from that reference during the course of the operation.

The weighing of the patient is preferably carried out using strain gauge transducers and may either involve weighing the whole operating table or, if a special support for the patient is provided, the patient alone. Sudden large changes in weight resulting from extra objects being placed on the operating table also need to be eliminated so that the changes in weight of the patient due to the absorption of irrigating fluid or haemorrhage can be observed. The raw data is subject to low pass filtering to remove high frequency changes and transient fluctuations.

The.present invention will be further described hereinafter with reference to the following description of exemplary embodiments and the accompanying drawings, in which:-

Figure 1 shows an embodiment of the present invention in position during an operation;

Figure 2 is a schematic diagram of an embodiment of the present invention; and

Figure 3 shows another embodiment of the present inventio .

In Figure 1 a patient (1) lies on an operating table (3) upon which the operation is performed. The operating table (3) is supported by pneumatic column (2) which can raise and lower the table (3) . This in turn is mounted on a base (4) which has four feet. Three load cells (5) (such as the commercially available UNI EIGH U 3030) are placed underneath the base, one under each of two feet and one under a bar connecting the other two feet. These give an indication of the weight of the patient which is processed by a processing system (7, 9, 11). Before an operation, the patient is placed on the table and prepared and the system is tared, i.e. the weight reset to zero. Thereafter, the change in weight of the patient from the reference weight during the operation is displayed on a graphical display (11) . The electronics may be conveniently mounted in a cabinet (7) located near the anaesthetic machine. Figure 2 is a schematic diagram showing the processing of dε a. from the load cells (5) . The load cells (5) are connected to a balancing circuit (12) which balances the data from the three cells and passes it to a Digital weighmet: r (13) (for example a Gedge Systems GS 1650) which converts this to a weight value, which is periodically output. The particular weightmeter used produces a value every three seconds. This weight is passed to a bad data filter (15) which eliminates obviously erroneous data by reference to defined criteria. For example data could be rejected on the basis that it falls outside a prescribed range of values. This is especially necessary in TURP as the Diathermal loop used to section the prostate emits a large amount of radio frequency noise which can cause random errors in the weight signal of the order of 20 kg.

The remaining data is passed to a second filter (17) which only accepts a datum if it is within a specified range of the previous data, for example 100 g, representing the largest change in weight when the interval between samples that is likely to be attributable to a change in fluid balance. The output of this is passed to an averaging filte. (19) which takes a mean over a suitable period, for example 15 seconds so as to reduce the effect of transient fluctuations, for example due to the actions of the surgeon, and passes it to the display controller 21 for further smoothing and display on the display (11) .

The display (11) is a graphical display with time along the abscissa and weight along the ordinate. Raw and semi-filtered data may alfϊo be displayed in separate data windows. A numerical display of the corrected weight may also be provided. The corrected weight display is updated at appropriate intervals, for example every 15 seconds or so, and may be capable of displaying about two hours' worth of data at once so as to show the weight change over the whole operation. The various filters have the effect of producing a gently varying corrected weight of the patient which is monitored by one of the operating room staff. A rapid change in weight over a period of several minutes is indicative of a problem.

In TURP the patient is put on the operating table and prepared. The system is then tared, reset to zero, and the patient's bladder then filled. The display therefore indicates an initial weight of between 600 to 1000 g. This then fluctuates during the operation. After the operation the patient's bladder is emptied and the system then gives the amount of irrigant absorbed or blood lost. The system may be tared at any convenient point during the preparation for the operation and may be retared during the operation if required.

While, in the embodiment described, the transducers are placed underneath the whole operating table, alternatively a special support for the patient to rest on might be provided and the transducers mounted so as to weigh this. This would result in a lower total weight being processed, the weight of a typical operating table being approximately 200 kg, so that smaller changes in weight would be detected. Placing the transducers underneath the whgle operating table has the advantage that the system is readily adaptable for use on a large number of different operating tables and does not compromise the electrical or structural safety of the table. Alternatively, as shown in Figure 3, a load cell (6) could be placed in the pneumatic column in a position to bear the weight of the structure and patient above it, simplifying the balancing circuitry required and increasing the accuracy of the system.

Claims

C L A I S

1. An apparatus for monitoring the fluid balance of a pccient, said apparatus comprising weighing means for weighing said patient and producing raw data indicative of said weight of said patient; processing means for processing said raw data to compensate for factors other than changes in said fluid balance, to produce a corrected weight; means for deriving from the corrected weight an indication of the fluid balance.

2. An apparatus according to claim 1, wherein said weighing means comprise at least one strain gauge.

3. An apparatus according to claim 2, wherein said at least one strain gauge is placed underneath said operating table during surgery so as to bear the weight of the operating table.

4. An apparatus according to claim 2, wherein said at least one strain gauge is placed in a central column supporting the operating table so as to support the weight of the patient and the operating table.

5. An apparatus according to claim 1 or 2, wherein said weigh means is incorporated in a stand supporting the operating table.

6. An apparatus according to any one of claims 1 to 5, wherein said weighing means comprises means for balancing the data of said at least one strain gauge and providing balanced data indicative of said weight of said patient.

7. An apparatus according to claim 6, wherein said weighing means includes a digital weighmeter responsive to said balanced data for providing said raw data.

8. An apparatus according to any one of claims 1 to 7, wherein said processing means include a first data filter which eliminates data falling outside a first predetermined range of values.

9. An apparatus according to any one of claims 1 to 15, wherein said processing means includes a second data filter which eliminates data falling outside a second predetermined range of a predetermined number of previous data.

10. An apparatus according to any one of claims 1 to 9, wherein said processing means includes a third data filter which averages said data over a predetermined period.

11. An apparatus according to any one of claims 1 to 10, wherein said means for deriving comprises display means for displaying said corrected weight.

12. An apparatus according to claim 11, wherein said display means displays graphically the corrected weight during the surgery.

13. An apparatus according to claim 11 or 12, wherein said display means includes numerical display means for displaying numerically said corrected weight in real time.

14. An apparatus according to any one of claims 11 to 13, wherein said display means includes means for displaying the raw data and/or partly processed data.

15. A method of measuring the fluid balance of a patient during surgery, comprising the steps of: monitoring the weight of said patient and producing raw data indicative of said weight; processing said raw data to compensate for factors other than changes in said fluid balance to produce a corrected weight; and providing an indication of said corrected weight so that changes in said fluid balance can be observed.

16. A method according to claim 15, wherein said step of processing said raw data includes the step of eliminating data outside a first predetermined range of values.

17. A method according to claim 15 or 16, wherein said step of processing said raw data includes the step of eliminating data which falls outside a second predetermined range of a predetermined number of previous data samples.

18. A method according to claim 15, 16 or 17, wherein said step of processing said raw data includes the step of averaging said data over a predetermined period.

19. A method according to any one of claims 15 to

18, wherein said step of processing said raw data includes the step of taring said weight to provide a reference value such that subsequent corrected weight values each represents the difference between the current weight of the patient and the reference value.

20. A method according to any one of claims 15 to

19, wherein said step of providing an indication of said corrected weight includes the step of providing a gr«_.__.-hical display of said corrected weight and the changes therein during said surgery.

21. A method according to any one of claims 15 to

20, wherein said step of providing an indication includes the step of displaying said raw data and/or partly processed data.

22. A method according to any one of claims 15 to

21, wherein said step of providing an indication of said corrected weight includes the step of displaying a numerical value representing said corrected weight.

23. A method of measuring the fluid balance of a patient during surgery, the method being substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

24. An apparatus for carrying out the method of any one of claims 15 to 23.

25. An apparatus for monitoring the fluid balance of a patient^ such apparatus being constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.