Relationships of cervical, thoracic, and lumbar bone mineral density by quantitative CT.

PURPOSE: The purpose of this work was to compare, using quantitative CT (QCT), vertebral bone mineral density (BMD) in the cervical, thoracic, and lumbar spine in healthy volunteers. METHOD: QCT of the vertebral bodies of C2, C5, T12, and L4 was performed on 50 healthy volunteers (25 women, 25 men; mean age 31.7 years). Trabecular BMD analysis was performed at each level. RESULTS: Mean BMDs (mg/cm3 calcium hydroxyapatite) for women and men were highest at C5 (BMD women/men 341.6/300.6 mg/cm3) and C2 (297.2/269.6 mg/cm3) and lowest at T12 (193.1/184.9 mg/cm3) and L4 (186.2/180.1 mg/cm3). The BMD of C2 was statistically significantly different from that of C5, T12, and L4 (p < 0.0001) for both genders. Also, the BMD of C5 differed significantly from that of T12 and L4 (p < 0.0001). The BMD of C5 showed significant gender differences (p = 0.002). Correlation coefficient showed a strong correlation between the BMD of T12 and L4 for both genders (women, r = 0.67; men, r = 0.90). CONCLUSION: Trabecular BMD of C2 and C5 measured by QCT is significantly higher than trabecular BMD of T12 and L4 in nonosteoporotic volunteers of both genders.

Pilot performance of the anti-G straining maneuver: respiratory demands and breathing system effects.

INTRODUCTION: The anti-G straining maneuver (AGSM) is still an important part of pilot protection for G-induced loss of consciousness. The specific requirements for and the effects of breathing systems on the performance of the AGSM are essential elements to designing compatible breathing systems. METHODS: Subject pools of 27 and 34 naval aviators were recruited and used to measure the inhalatory and exhalatory flow requirements for the AGSM and the breathing system effects of mask cavity pressure during AGSM performance on the Naval Air Warfare Center Dynamic Flight Simulator at acceleration levels up to 8 Gz. RESULTS: The mean peak inhalatory flow was 125.5 L.min-1 (n = 135, SD = 42.1) with a maximum value of 274 L.min-1. The mean peak exhalatory flow was 154.4 L.min-1 (n = 135, SD = 49.6) with a maximum value of 308 L.min-1. For the effects of the breathing system on AGSM performance, inhalatory mask cavity pressures were not above 30 mmHg with the majority less than 10 mmHg. Exhalatory mask cavity pressures did not exceed 60 mmHg but predominated in the 20-30 mmHg range. In comparison to accepted guidelines, 67-77% of inhalatory mask cavity pressures were below and 91% of the exhalatory mask cavity pressures were above the Air Standardization and Coordination Committee (ASCC) limit of +/- 14 mmHg. CONCLUSIONS: The difference in the peak inhalatory and exhalatory flows measured during this study and clinically can be attributed to different test conditions and performer techniques. The reduction in inhalatory flow with increasing G is consistent with the increase in breathing difficulty due to the G load and the inflation of the anti-G suit. However, exhalatory mechanics appear unaffected by the G load and the inflation of the anti-G suit. Since 23-33% of the inhalatory mask cavity pressures were above this ASCC limit, improvements in regulator performance are still needed. For exhalatory effects of the breathing system, the main contributor is the mask valve. While no pilot suffered unconsciousness or expressed complaints with the breathing systems used, these exposures were of short duration. The additional work of breathing during a combat engagement may further compromise the pilot's ability to retain consciousness with the AGSM.

Integrated protective systems for operational acceleration-induced loss of consciousness.

Systems that can protect pilots from not only acceleration-induced loss of consciousness (G-LOC) but also from exposure to chemical, biological, and nuclear weapons are discussed. Hazards such as fire, drowning, and ballistic injury are not considered. Physiological stresses, protection methods, and their impact on G capability at increased altitudes are examined, as are stresses induced by the environment (heat stress, cold stress, and cold water immersion). Sustained and short-duration acceleration effects are described. Requirements for protection against chemical, biological, radiological, and laser weapons and the incorporation of mission-enhancement devices are addressed. Concepts for integration of all of these elements are discussed.