The Cameron Heartometer
- The Mechanics Behind the Machine -
From the Journal of the A.M.A., August 6, 1938

The blood pressure determinations are based on the oscillometric method.  Visual indication of the pressure readings is provided by the flashing of gas-filled glow lamps.  A standard blood pressure cuff is fastened over the upper arm in the usual manner and inflated to a pressure above the expected systolic level.  The pressure in the cuff is communicated to a large bellows, causing it to expand.  The elongation of this bellows is about seven-eighths inch from 0 to 300 mm. of mercury.  The movement of the bellows is indicated by a lever and gear system to which is attached a shaft with a small pulley wheel about three-eighths inch in diameter.  A small contact arm is clipped into the groove of this wheel and is held in place by spring tension, so that, when the pulley turns, the contact arm turns with it until it comes up against one of the stationary contacts.  During inflation, one end of the bellows moves up, throwing the contact arm against the contact connected with the blue (argon filled) light.  When deflation is started the contact arm will move toward the contact connected with the red (neon filled) light and will cause a steady red light to appear.  When a point is reached at which the pressure in the cuff is low enough to permit the pulse to penetrate, the oscillations resulting in the bellows will cause the contact arm to oscillate, producing a flashing of the red light synchronous with each heart beat.  The appearance of this flashing is taken as systolic pressure.  It is comparable to the first pulsation in other oscillometric methods.  The pressure level at which this occurs is obtained by a record of a moving pen on the calibrated graph paper.  As the bag is further deflated, according to Marey’s law, the oscillations will become larger in amplitude, and eventually the contact arm will strike both contacts, thus giving alternate flashes of red and blue lights.  With continued deflation, the bellows will shorten and the amplitude of its pulsation will become smaller so that only the blue light will flash.  The highest pressure at which the blue light continues to flash intermittently is taken as the diastolic pressure.  This also is recorded and read on the calibrated graph.

In recording the pulse curve the pressure in the cuff is raised to the level which gives the greatest pulse amplitude.  This, it is stated, is in the neighborhood of the mean pressure (about 10 mm. of mercury above diastolic).  A magnetic clutch is engaged, connecting the bellows to a magnifying lever system.  The graph paper is started in motion by a synchronous motor and a pulse tracing is obtained.  The graph paper is so constructed that one complete revolution takes a minute.  The recording of both pressure and pulse is done in ink with glass pens.  The heart rate is determined from the scale on the graph and a special rule provided; the pulse contour and arrhythmias are interpreted from this curve.

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