My wife Linda, who is a nurse, was describing the use of bio-impedance to determine the weight of a bed-bound patient. In this procedure a minuscule current, I, is passed from a finger through the body to a toe, and the voltage drop E is measured. The equation R = E/I gives the body's impedance which, given measured body size and the fact that the impedance is related to person's lean/fat ratio, allows a reasonably accurate estimate of body mass.

 

The apparatus is expensive and requires the use of special, disposable electrodes that must be carefully placed in order to get repeatable results. I suggested that it would be simpler to connect the patient to a standard wall outlet with alligator clips, avoiding electrical shock by placing the patient electrically in series with a standard frankfurter. Then, using the watch with a sweep-second hand that every doctor and nurse wears, you would simply measure how long it took to brown the hot dog to a published criterion of brownness. The higher the patient's bio-impedance, the longer it would take.

On careful reflection I realized that this method is potentially dangerous and cruel (alligator clips can pinch painfully), that some patients might be upset by the smell and sound of cooking hot dogs, and that it would be very time-consuming to prepare the frankfurter-browning time vs. lean/fat ratio chart. I studied the situation further, and after some field and library research, I completely solved the problem, as explained in the following article, which is currently awaiting publication.

 

Measuring Body Weight in the Bed-Bound Patient

 

When movement of the patient is contraindicated by extant medical conditions [1] or a weight-training deficit on the part of the attending medical personnel a number of alternative strategies and technologies have become recently available [2, 7, 11]. This article discusses these alternatives and their relative merits in the context of the home. The suitability and applicability of these methods in a hospital or skilled nursing setting should be clear to the experienced practitioner.

 

Method 1: Newtonian

 

Use of the formula f = ma is not at all common in nursing practice, however it is no more complex than calculating a 10 ml / hr drip, with TPN, buffered, in a normal saline infusion. The technology requires an M10 cherry bomb (two, for large patients) and a bed of known mass. Universal precautions are to be augmented with ear covers, lexan (polycarbonate) facial protection, and a flak jacket,. The medical kit should include a high-speed (4000 frames per second) video camera, and a surveyor's rod which is placed vertically on the wall behind the bed.

 

The protocol, which will only be outlined here as it is set out in detail in [3], consists of placing the M10s centrally under the bed, of mass mb, and using a safe source of ignition to promote the initial combustion of the string-like (10 gauge or larger) protrusion from the M10.

 

Prior to this step the patient, along with any family or other persons in attendance, are warned not to be alarmed, and that though the action of the measuring device tends to exhibit abrupt onset, the duration of the measurement is exceedingly brief.

 

The care provider aims the camera at the bed, surveyor's rod, and patient, preferably from an adjacent room, and after initiating the protocol makes sure that the ear and eye protection are secure. The care provider may briefly leave the premises during the actual measurement to insure the privacy of the patient, who may become partially or totally disrobed as one of the sequelae of the technique. Concern for the well-being of the patient is assured as long as the mattress is of good construction and intact prior to the procedure.

 

The quantitative analysis is straightforward. The M10s give a known force f upward (assuming the floor does not buckle downward, in which case a correction must be applied). From the video, the upward acceleration a is found as a function of the height to which the bed-cum-patient is raised, and the time, which is determined by the number of frames of the high-speed video counted between the initial upward movement of the bed and the ultimate point to which it rises. Solving for m, namely m = a/f, and then subtracting mb from the result gives the mass of the patient. The charts in [3] give f for a wide variety of M10s, including those found in Chinatown.

 

Judgment as to whether this procedure is appropriate for patients in other than single-family dwellings must be left to the care provider. There is some anecdotal evidence that this procedure may occasionally be associated with acute myocardial infarction.

 

Method 2: Archimedian

 

For the patient on O2, via nasal cannula, or who has SCUBA [5], the method first described by Archimedes may be applied without undue stress. In other cases the care provider should assure herself by breath volumetric analysis that the patient can sustain a sufficient suspension of respiration to allow the measurement to be completed.

 

In contradistinction to the prior procedure, the bed and any parts with a density of less than 1.0 g / cc are to be firmly attached to the floor.

 

Using duct tape [4l, seal any gaps in doors, walls, floors, or windows. An aliquot of water, measuring 1 kg / liter at 6 degrees Celsius is introduced into the patient's immediate vicinity, and in sufficient quantity to surround the patient. Harlington and Snideman [8] recommend that, for patient comfort, the water be supplied at a somewhat higher temperature, however, this requires the use of their auxiliary temperature / density chart and clinical expertise will be required to evaluate the trade-off between short-term patient discomfort and the added probability of error due to the additional procedure required.

 

The patient is then removed from the room, and the amount by which the water level drops is observed, carefully measured, and charted. A volumetric analysis as outlined in Halliday and Resnick [9] then determines the patients body displacement which can be converted to weight and mass so long as the acceleration of gravity (9.8 m / sec2) is kept in mind.

 

To avoid unnecessary wastage, family or patient can be advised that the water used in the measurement--while no longer potable--is not likely to be sufficiently contaminated to prohibit its further use in agricultural pursuits whether removed by siphon or bilge pump.

 

Method 3: Statistical Verification of Patient Mass

 

This method is preferred if the results are to be subjected to statistical analysis and published. Since a single data point is of little use and of negligible statistical import, the patient is divided into n = 40 portions. In rejecting the null hypothesis, such an n can be used to give results that are significant at the p = .05 level. A larger number of parts, of course, can result in a higher level of confidence. If the work is being compensated under a capitated health plan, the time required for a more statistically significant procedure may not be available.

 

A double-blind technique is recommended to avoid bias and to reduce emesis on the part of the experimenter. One set of difficulties reported [11, 2, 8] consists of errors due to loss of either bodily fluids or semifluids or misplaced small parts. Some solutions to these difficulties have been considered in the literature, including use of a hand-held counter to assure correct part enumeration; a large impermeable membrane, exemplified by a folded, edge-sealed polyethylene sheet usually available as a removable lining for waste enclosures, and which can be securely fastened around the patient during the procedure [13]; a reduction of body temperature until all liquid portions have achieved a solid or near-solid state [12] which greatly eases handling; and making sure that any carnivorous pets are sequestered and/or restrained[14]. Care givers should be advised that if the patient, against medical advice, insists, under the patient bill of rights, on the company of a pet during the procedure, and if the pet is weighed before and after the procedure, then that information can be used to compensate for any ingestion of parts by the pet. The patient's insistence, the medical advice, and the before-and-after weights of the pet should all be carefully charted.

 

Summary:

 

No longer need we be constrained to assess and treat our charges in the absence of one of the most useful vital statistics, body weight, in the case of the bed-bound patient.

 

Once the practitioner has studied and understood these methods, it is likely that he or she will be able to, on his or her own, devise related methods to accommodate local conditions or individual needs. For example, in a remote village where nothing but a see-saw (also known as a teeter-totter) was available, the patient's bed was placed on one arm of the apparatus and the care-giver--in a clever variant of the Newtonian method--jumped onto the other arm from a measured height of two meters, and had the town priest observe the trajectory of the patient.

© Jef Raskin, 1998