Decreased cardiothoracic transverse diameter ratios were reported following Mercury, Gemini, and Apollo space flights. The majority of crewmembers who exhibited postflight decreases in the cardiac silhouette size also showed a decreased orthostatic tolerance to lower body negative pressure. Similar findings were also reported by Soviet investigators following 30-day bedrest studies and in cosmonauts upon return from space missions. This paper presents further radiological data from all three Skylab manned missions and discusses the physiological factors possibly involved in the cardiac silhouette changes.
Postflight chest X-rays were visually compared with the preflight films for possible changes which might have occurred in pulmonary vasculature, lung parenchyma, bony or soft tissue structures. One or two additional postflight films were taken several days following splashdown to assess trends.
Geometric analysis of the chest X-rays was utilized to quantify changes in cardiac and thoracic areas. The thoracic cage area was obtained using a modified version of the method developed by Barnhard and Loyd. Upon completion of the geometric analysis the following parameters were measured and/or computed:
All postflight data were compared to the respective preflight values using the Student's t-test and regression analysis.
No roentgenological abnormalities were observed on either preflight or postflight films. The chest X-rays of the Skylab 3 Pilot on the day of recovery were of poor quality and not amenable to analysis; all other X-rays were of acceptable quality. Both C/TD and C/TA showed a decrease in the individual values postflight. In general there was more variability in the C/TD responses postflight, some cases in diastole showing a modest increase in the immediate postflight ratios. Comparison of the preflight and postflight cardiac silhouette area differences showed a fairly consistent decrease in the cardiac area on the day of recovery. The Skylab 3 Commander, however, showed a postflight increase in the systolic cardiac area as determined from the products of minor and major diameters (19.76 cm2) and a very slight increase in the diastolic cardiac area as measured by the planimetric method (0.30 cm2).
The mean differences, preflight versus postflight, of the cardiac areas, measured by planimetry, and of the derived measurement (C/TA), were statistically significant (P<0.01 or P<0.05). Return to preflight values was quite variable for all crewmen, but most showed this directional trend by 4 to 5 days after splashdown. There was a significant correlation (r=-0.91) between postflight decrement in systolic heart size as measured by C/TD, and the corresponding augmentation in heart rate responses registered during lower body negative pressure stress. There was no apparent correlation between the duration of orbital stay and the postflight change in heart size.
Previous studies have shown that a significant determinant of cardiac size is the amount of blood returned to the heart. It is quite conceivable that caudad displacement of blood and other fluids together with an absolute decrease in the circulating blood volume could account for the observed decreases in the cardiac silhouette size. At the present time there is certainly no indication that the Skylab crewmen exhibited a greater decrease in their cardiac size than that observed in the Apollo astronauts following shorter duration space missions, nor that the decrease in diastole heart size was of greater magnitude than that of the systolic phase of the cardiac cycle. A small diastolic size might more clearly delineate a deficit in blood return and chamber filling rather than loss of myocardial mass.