BIOMEDICAL RESULTS FROM SKYLAB
APPENDIX A.I.c
Automatic Blood Pressure Measuring System (M092)ROBERT W. NOLTE
The Blood Pressure Measuring System measures blood pressure by the noninvasive Korotkoff sound technique on a continual basis as physical stress is imposed during experiment M092, Lower Body Negative Pressure, and experiment M171, Metabolic Activity.
EquipmentSpecifically the Automatic Blood Pressure Measuring System:
Senses systolic and diastolic blood pressures.
Provides a light display of K-sounds Conditions raw
Routes conditioned data to the Experiment Support System.
Provides a numeric display of blood pressure (SYSTOLIC, DIASTOLIC).
Provides a visual check on blood pressures (through the aneroid gage).
The four components of the Blood Pressure Measuring System are: cuff assembly, electronics module, aneroid gage, and gas umbilical.
Cuff Assembly.—The cuff assembly contains the pressure bladder, gas connector and gas line, pressure transducer, K-sound microphone, pre-amplifier, electrical connector and wiring (fig. A.I.c.-1).
Electronic Module.—Located on the top right side of the Experiment Support System, the electronic module contains the controls to obtain and display brachial blood pressure (fig. A.I.c.-2). SYSTOLIC and DIASTOLIC pressures are displayed on three-digit separate counters. K-SOUNDS detected by the microphone in the Blood Pressure Measuring System cuff assembly are momentarily displayed as a green light as they occur. The CUFF PRESSURE PROGRAM is controlled by the MODE SELECT dial and CUFF INFLATE switch. Pneumatic components to inflate and deflate the cuff assembly are also located within the Electronic module and include a pressure regulator, gas reservoir, solenoid valves, and a bleed down orifice. The UMB GAS connector on the front panel is the interface point for the pneumatics in the Electronic module and the cuff assembly. Signal conditioners receive K-sounds and occlusion cuff pressure from the cuff assembly and condition the data before routing it to the Experiment Support System distributor for transmittal to the Airlock Module Telemetry System and to the Blood Pressure Measuring System displays. Gaseous nitrogen (GN2) comes from the Orbital Workshop via the Experiment Support System.
Aneroid Gage.—In-flight this gage (fig. A.I.c.-3) is mounted on the subject interface box and is connected to the gas umbilical of the Blood Pressure Measuring System. It measures and indicates cuff pressure and is used only as a backup and visual check of the SYSTOLIC and DIASTOLIC displays in the operation of the Blood Pressure Measuring System module.
Gas Umbilical.—The gas umbilical carries gas to the cuff assembly pressure bladder and returns the vented gas from the bladder to the vents at the bottom of the Blood Pressure Measuring System. During experiment activation at the beginning of each mission it is fastened to the vectorcardiograph umbilical by a series of Velcro tabs to simplify the handling of two cables.
The inflight technique for blood pressure determination is similar to the clinical approach using the auscultatory principle discovered by Korotkoff. However, in the Automatic Blood Pressure Measuring System (fig. A.I.c-4) a microphone replaces the stethoscope, a gas pressure and valving system replaces the squeeze bulb, and a quantitative electronics circuit replaces the clinician (Korotkoff sound amplitude and frequency comparison).
The Blood Pressure Measuring System operates from 28 V d.c. and high pressure [150 psia] GN2 which are both supplied at the Experiment Support System interface. The electronic module contains a power supply for its required operating voltages of plus and minus 10 V, +5 V, and +28 V. The high pressure GN2 is regulated down to 400 mm Hg for safe system operation.
A typical Blood Pressure Measuring System operating cycle is automatically repeated for each blood pressure determination as follows:
1. The occlusion cuff is filled to a pressure higher than the anticipated systolic blood pressure by placing the Cuff Inflate switch to START. The design allows for three maximum pressures: 160 mm Hg, 200 mm Hg and 250 mm Hg.
2. Switching is set at the peak of the fill pressure such that the amplitude of filtered (17 to 27.5 Hz) K-sounds can be compared to the amplitude of unfiltered K-sounds.
3. Occlusion cuff pressure begins to decay immediately upon completion of the fill since the "bleed down" orifice is always open.
4. As cuff pressure approaches systolic blood pressure, sound bursts immediately following each heart beat are sensed by the microphone; when the amplitude of the filtered (17 to 27.5 Hz) sounds reaches 45 percent of the amplitude of the unfiltered sound burst, the criterion for systolic blood pressure is achieved.
5. As systolic criterion is achieved, a voltage proportional to cuff pressure is switched to the systolic telemetry output, and the three-digit systolic display is updated with the cuff pressure in millimeters of mercury (Systolic Blood Pressure). The three-digit diastolic display goes blank and switching is changed so that the previously stored amplitude of filtered (40 to 60 Hz) K-sounds can be compared to the peak amplitude of the filtered (40 to 60 Hz) K-sounds.
6. As cuff pressure continues to decay, the amplitude of the filtered sounds (40 to 60 Hz) increases to a peak and then begins to decrease. Each sound burst is compared to the peak and when the amplitude of the decreasing filtered sound bursts is between 17 and 5 percent of the peak amplitude, the diastolic blood pressure criterion is achieved. At the instant of each sound a voltage proportional to cuff pressure is switched to the diastolic telemetry and also stored in the A to D converter.
7. At the instant of the first sound burst, with amplitude less than 17 percent of the peak (diastolic criterion), a 3-second timer is started. If a sound burst with amplitude greater than 17 percent of the peak is subsequently detected during the 3-second interval the timer is restarted and the voltage representing cuff pressure at the diastolic telemetry and A to D converter is updated.
8. Once the 3-second timer completes its 3-second period without additional sound bursts greater than 17 percent of the peak, the voltage representing cuff pressure which was stored at the A to D converter is converted and the three-digit diastolic display is turned on displaying the diastolic blood pressure in millimeters of mercury. The cuff pressure which decreases at about 7 mm Hg per second is then rapidly dumped to ambient at the end of the 3-second period. This completes a typical cycle.
The cycle repeat rate can be selected by the operator to be 30 seconds or 60 seconds. If the 30-second measurement interval is selected the occlusion cuff fills to a pressure of 160 mm Hg. In the 60-second measurement mode two maximum cuff pressures are available to the operator, 200 mm Hg and 250 mm Hg. An additional operating mode, which is not repeated automatically, can be selected. It is activated by the start switch and fills the cuff to 250 mm Hg but does not repeat until the start switch is recycled. Additional features besides the automatically repeated cycle should be noted.
Artifact noise rejection. This feature makes use of a signal supplied by the M093 vectorcardiograph. At each QRS complex of the heart the vectorcardiograph supplies a switch closure which the Blood Pressure Measuring System uses to generate an "operating window." The "operating window" is a time interval during which the filters will accept sound burst signals from the microphone signal preamplifier. There is a 62.5 ms delay from the QRS complex switch closure, which represents minimal propagation time from the heart to the left brachial artery, after which signals are accepted for 275 ms. If no vectorcardiographic heart beat signal is present, the Blood Pressure Measuring System automatically opens the "window" after a 2-second search and then accepts all sound bursts from the arm.
Secondary output voltages. The Blood Pressure Measuring System has parallel 0 to 5 V outputs representing systolic (50 to 250 mm Hg) and diastolic (40 to 140 mm Hg) blood pressure. The primary signal is used by telemetry and the parallel signal is used for driving displays in the Experiment Support System.
Automatic two level calibration feature. Upon receipt of a 28-V signal at the LOW CAL input a shunt resistance is applied to the pressure transducer forcing a known 70 ±2 mm Hg reading on both systolic and diastolic displays as well as the corresponding voltage for telemetry and secondary display outputs. When the 28-V signal is received at the HI CAL input all outputs read 130 ±5 mm Hg.
ConclusionsThe hardware for the Automatic Blood Pressure Measuring System, as used in experiment M092, operated in a satisfactory manner.
AcknowledgmentThe author gratefully acknowledges the assistance of Glen Talcott and his group who were the project engineers for this piece of hardware developed by the Martin Marietta Corporation, Denver, Colorado.
Bibliography Skylab Experiment Operations Handbook. NASA MSC, Vol. I, pp. 1.1-1 through 1.1-47, November 19, 1971. Ground Operating, Maintenance and Handling Procedure. NASA MSC-02754 Rev. B , Jan. 1973
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