A specially designed Frank lead vectorcardiograph system was used for all Skylab vectorcardiographic recordings. The recordings were digitized and processed by a computer program called VECTAN, which analyzed the tracings as three-dimensional spatial entities. The program constructed the spatial P-QRS-T vector loops in terms of standard time intervals and vector magnitudes and angles, and computed quantitative parameters such as areas and circumferences for every complex analyzed (one every 5 seconds).
The LBNP test consisted of a resting supine control period, three graded levels of LBNP, and a final period of recovery at ambient pressure. The protocol was adopted from Apollo flights and included the following segments:
(a) 0 mm Hg (resting) for 5 minutes
(b) -8 mm Hg for 1 minute
(c) -16 mm Hg for 1 minute
(d) -30 mm Hg for 3 minutes
(e) -40 mm Hg for 5 minutes
(f) -50 mm Hg for 5 minutes
(g) 0 mm Hg (recovery)for 5 minutes
Three test sessions were scheduled in the month preceding launch, with the last session approximately 5 days before launch. The earliest inflight LBNP tests were performed on the fourth to sixth day of orbit for each crewman; thereafter inflight tests were conducted approximately every third day. Postflight tests were accomplished on recovery day aboard ship as soon as possible after splashdown and on the succeeding two days. Subsequent postflight tests were performed on the fourth and fifth postflight days and on at least three additional days, as late as one to two months after recovery.
All digital recordings were processed by a previously developed computer program called VECTAN which analyzed the three-dimensional spatial entity rather than planar projections in order to obviate perspective distortions. The program basically reconstructs the mathematical elements of the spatial P-QRS-T vector loops which include standard time intervals, vector magnitudes and orientation, calculated areas and circumferences, and other quantitative parameters. These data are computed from the spatial vector for every complex analyzed (one every 5 seconds).
Astronauts of Skylab 3 demonstrated both resting and stressed heart rate elevations inflight, although a fair case may be made for a downward trend as the mission continued. Inflight resting heart rates were not elevated in the Skylab 2 crewmen. Inflight resting heart rates of the three Skylab 4 crewmen, like those of Skylab 3, were elevated significantly over preflight resting rates. The inflight mean increase in heart rate during -50 mm Hg LBNP over resting rates for all nine Skylab crewmen averaged 20.4 beats per minute, a highly significant difference. The data from the Skylab 3 astronauts indicated recovery of heart rate toward preflight values within the first few postflight days.
Elevation of resting systolic blood pressure and pulse pressure along with decreases in diastolic pressure and mean arterial pressure were observed in the majority of Skylab astronauts, though not always to a statistically significant degree. This was thought to be compatible with increased stroke volume and lowered peripheral resistance due to increased cross section of the resistance vessels. In three of nine astronauts, inflight LBNP stressed pulse pressure exceeded preflight stressed pulse pressure, thought to be due a combination of high resting and smaller falls of systolic pressure during inflight LBNP tests. All crewmembers demonstrated inflight calf volume increases during LBNP that greatly exceeded those observed in preflight tests.
Changes in vectorcardiograph wave intervals included modest increases in inflight resting values of the PR interval and the QT interval corrected for heart rate. In all phases of the flight, the application of LBNP decreased the PR interval (4-10%) in all but two crewmen, produced a modest decrement of the QRS duration, and decreased the absolute QT interval (16-15%) consistent with elevated heart rate. Heart rate during LBNP was always elevated 20-50% over resting values regardless of flight phase.
Changes in vectorcardiograph wave magnitudes included significant increases in inflight resting values of P-wave maximum, PmaxMAG, and QRS-complex maximum, QRSmaxMAG. The increase in PmaxMAG was 25% and the increase in QRSmaxMag was 12%, with an increasing inflight trend, returning rather precipitously to preflight levels about 3 days after landing. During LBNP, PmaxMAG was 28-55% higher, with greater changes being seen during and immediately after flight. QRSmaxMAG decreased 7% from resting values in preflight LBNP tests, but demonstrated no significant inflight response. The ST-wave maximum vector magnitude, STmaxMAG, for the group showed non-significant decrements at rest, relative to decrements during LBNP (14% preflight, 25% inflight, and 30% immediately postflight). There were also significant inflight increases both at rest and during LBNP in the total QRS Eigenloop circumference, which is an indicator of the overall QRS depolarization complex. Orientation changes in the QRS maximum vector and the J vector at rest in space appeared different from those caused by the application of LBNP.
In summary, vectorcardiograms from Skylab experiment M092 showed changes that were presumably due to altered autonomic neural inputs to the myocardial conduction system and/or major fluid shifts caused by exposure to weightlessness. All were well within accepted normal limits and consistent with hemodynamic and other related changes, especially the headward fluid shift.