The bioinstrumentation system was used on each Mercury flight, at first to measure ECG, respiration, and body temperature, and later to also measure blood pressure (see details below).
MR-3 and MR-4 configuration
ECG sensor: The ECG sensors consisted of rings composed of non-conducting silicone rubber. The rings were constructed to support a disc of 40-mesh stainless steel screen, 30 mm in diameter and approximately 2 mm above the skin. The center conductor of a miniature-type coaxial cable was brought through a strain relieving projection in the rubber ring and soldered to the screen. A piece of thermally shrinking plastic tubing sealed the cable shield at the entrance into the ring to prevent the entry of moisture.
Before the electrode is applied to the washed and shaved skin, a coating of elastoplast adhesive was applied to the bottom surface of the electrode and allowed to dry. The electrode ring cavity was filled with 30 percent calcium chloride in water with a sufficient amount of aluminum silicate powder (Bentonite) to form a paste. The electrode was then applied and the cavity of the ring was checked, voids were eliminated, and then the assembly was sealed with tape. A 4-inch square of moleskin applied over the entire sensor area completed the installation.
Respiration Sensor: Respiration rate was monitored by a thermistor anemometer that detected the flow of the expired air. A single thermistor, heated to approximately 200 degrees Fahrenheit in still air, was mounted in a small plastic enclosure attached to one of the microphones within the helmet. The microphone to which the sensor was attached was pivoted so that it could be adjusted. A funnel-shaped opening facing the astronaut was designed to catch air from the nostrils while expired air from the mouth passed directly across the heated thermistor and through exit vents in the back. A funnel on the microphone would catch air from the nostrils while air from the mouth passed directly across the thermistor. When the flow of air cooled the thermistor, the resistance change caused a voltage variation across the sensor. This voltage change was sensed by a small preamplifier mounted on the cable leading from the sensor and this signal was transmitted through the pressure-suit bioconnector to the spacecraft instrumentation package.
Body temperature: The body temperature probe was a thermistor mounted on the tip of a special rectal catheter. The bulb shape of the rectal thermistor closely fit the anatomy of the rectal sphincter, and the rigidity was sufficient to permit easy introduction. The catheter was a small plastic cylinder about 3 millimeters (mm) in diameter and 25 mm long from which the thermistor projected approximately 2 mm. The thermistor, catheter, and lead wires were dipped in liquid wax to a length of about 20 cm to prevent the entry of moisture. The thermistor formed one arm of a resistance bridge which was excited by 400 cps current and which was located in the spacecraft instrumentation package.
The bioinstrumentation system consisted of two sets of electrocardiographic leads, ECG-1 (auxillary) and ECG-2 (sternal); a rectal temperature thermistor; a respiration-rate thermistor; and the blood-pressure measuring system (BPMS). The bioinstrumentation system was essentially the same as that used during the previous MR-3 and MR-4 suborbital flights with the exception that for the first time the astronaut blood-pressure measuring system (BPMS) was used.
Blood Pressure Measuring System (BPMS): The blood pressure measuring system utilized the same principle used in clinical sphygmomanometry, namely an inflatable occluding cuff on the left arm. The cuff was inflated by gas to a pressure in excess of expected systolic pressure. As the pressure decreased slowly, a microphone placed under the lower half of the cuff over the brachial artery transduced the Korotkoff sounds. The signal from the microphone was amplified and mixed with a signal from a pressure transducer which transmitted the cuff pressure. In order to find the arterial pressure it was necessary to identify the points of inception and cessation of the microphone signal on the cuff pressure signal, which were the systolic and diastolic pressures.
The original concept of the BPMS was an automatic system, which would be initiated from a tracking station through the command receiver, by an automatic sequencing device on board or by the pilot. The automatic system incorporated special safety circuits to dump the cuff pressure if the cuff stayed above 60 mm HG for more than 2 minutes. This feature provided for the situation in which the astronaut was unconscious and the automatic system failed to bleed off the cuff pressure. The cuff pressure in the automatic system was decreased in a linear manner from 220 mm Hg to 60 mm Hg by a special pressure regulator in which the reference spring tension was varied b a motor-driven cam. The pneumatic system consisted of an oxygen storage flash, solenoid fill valve, motor-driven regulator, dump solenoid valve, cuff pressure transducer, and suit reference manifold. The regulator, dump solenoid valve and pressure transducer were referenced to a manifold connected through a flow restrictor to the pressure-suit system to prevent differences between cabin pressure and suit pressure from causing large errors and to allow measurements in the event of the loss of cabin pressure. The cuff was worn inside the suit.
Previous research in blood pressure measuring indicated that if the microphone signal was filtered so that only frequencies between 32 and 40 cps were used, the various artifacts due to movements and ambient noise level are greatly attenuated, while the component that allowed the discrimination of the systolic and diastolic points was passed. For flight, a specially damped, piezoelectric microphone was developed. The instrument was about 3.5 cm in diameter and 0.5 cm thick and was so constructed that sensitivity to noise generating from the side away from the skin was greatly reduced. The microphone signal exited from the suit through the bioconnector and entered the amplifier in the blood-pressure unit. The BPMS amplifier consisted of a shielded preamplifier and two high-gain amplifiers which determined the response characteristics. Each amplifier was designed to have greatly attenuated response outside the 32 to 40 cps pass band by means of resistor-capacitor filtering circuits in each feedback loop. The amplifier output was gated so that unless a signal of sufficient amplitude was present there was no output signal, and this gating resulted in a marked reduction in the output noise level for improved readability of the signal.
Cuff pressure was measured by a potentiometer-type transducer powered by two mercury batteries to give the zero-centered +1.5 volt output necessary for input to the telemeter. The signal from the pressure transducer passed through a miniature transformer where it was mixed with the output from the microphone and then on to the output clipping circuits that protect the telemetry system from excessive voltages that could cause cross-channel interference.
Conflicts with equipment stowage in the interior of the spacecraft resulted in the elimination of the gas pressure source, regulator, and motorprogramer and the installation of a hand-pumped system with a simple orifice to release cuff pressure.
In order to measure arterial pressure without adding telemetry channels, the input to the channel carrying the sternal ECG lead was switched to BPMS during blood-pressure determinations. The band width required for the BPMS was somewhat greater than that required for ECG and it was necessary to modify the receiving equipment to insure legible readout.
The biosensor system consisted of two sets of electrocardiographic leads, ECG-1 (auxillary) and ECG-2 (sternal); a rectal temperature thermistor; a respiration-rate thermistor; and the blood-pressure measuring system (BPMS). The only change from the MA-6 configuration was the replacement of the manual BPMS with a semiautomatic system. The semiautomatic system contained a gas pressure source, regulator, and motorprogramer for cuff inflation.
The BIS for this mission consisted of two sets of electrocardiographic (ECG) leads, a rectal temperature thermistor, an impedance pneumograph, and the blood- pressure measuring system (BPMS). Changes to this system for the MA-8 mission were as follows:
ECG sensors: the ECG electrodes were affixed to the skin with a double-backed adhesive tape, such as that used for a colostomy, and this tape fitted to the rubber ring of the sensor. The sensor paste was changed from the previously used bentonite-calcium chloride compound to a combination of carboxypolymethylene and Ringer's solution. Carboxypolymethylene is a hygroscopic, polymerized carrier for the ions needed to provide electrical continuity, is more soluble, and is easier to work than the bentonite paste. The 10-times-isotonic Ringer's solution not only retained the necessary conductivity and low impedance required, but also afforded decreased skin irritation after prolonged contact.
Blood Pressure Measuring System (BPMS): the BPMS was modified to allow determination of an optimum controller gain setting specially for the flight astronaut. The thickness of the BPMS cuff was decreased without change in bladder size. The diameter of the hose leading from the cuff to the suit connection was also decreased. The microphone, cuff, and controller were all fitted and calibrated specifically for the flight astronaut. These changes did not affect the cuff filling or bleed-down times, and the basic design of the BPMS was unchanged.
Body temperature: Body temperature instrumentation was the same as on previous flights.
Respiration rate: The respiratory trace was obtained by measuring transthoracic impedance which is directly proportional to thoracic volume. The impedance pneumograph system consisted of a 50-kc oscillator, a 50-kc amplifier, a detector, and a low-frequency amplifier. The output of the oscillator was applied across the chest by electrodes in each mid-axillary line at the level of the subject's sixth rib. These electrodes , the conductive paste, and the methods of attachment are identical to the ECG electrode system used during MA-7. The oscillator output varied by means of a potentiometer so that the inspiratory peaks remained on scale on the direct writing recorder except during exceptionally deep breathing. Even when the range was exceeded, respiratory rate could still be determined unless it was accompanied by excessive body movement. The unit did not indicate instantaneous tidal volume, but it did provide a general trend of changes in chest volume. Inspiration was distinguishable even during the thoracic volume changes that occurred with speaking.
Same as MA-8 configuration except that a change was made from continuous rectal to intermittent oral body temperature measurement. The basic thermistor was retained. The thermistor and its lead wires remained within the suit. The sensor was attached to the right ear muff inside the helmet where it was readily accessible. It thereby provided an indication of space suit outlet temperature whenever an oral temperature was not being taken. Then oral temperature was desired, the pilot placed the small thermistor under his tongue for about five minutes.