Key result
Head-up passive tilt to 60 degrees decreased pulmonary blood volume, cardiac index, and right ventricular diastolic volume by 25-30%, with PBV highly correlated to cardiac index (P<0.001).
Why the study?
How does 60 degrees head-up passive tilt affect pulmonary blood volume, flow, and pressures in cardiopulmonary normal subjects compared to the supine position?
Observational (n=15)
How does 60 degrees head-up passive tilt affect pulmonary blood volume, flow, and pressures in cardiopulmonary normal subjects compared to the supine position?
Head-up tilt causes a 25-30% decrease in pulmonary blood volume and cardiac index, primarily determined by reduced venous return to the right heart.
Supports gravitational effects on central volume in normals; extends physiological models but leaves open translation to disease or orthostasis.
Pulmonary blood volume (PBV), flow, and pressures were measured in 15 cardiopulmonary normal subjects, while supine and at 60 degrees head-up passive tilt. PBV, cardiac index (CI), right ventricular diastolic volume (VDVR), and mean pulmonary arterial (Ppa), venous (Pv), and mean right atrial (Pra) pressures all decreased during the first 4 min of tilting, with little subsequent change thereafter, to 30 min. Relative changes of CI, PBV, and VDVR were of similar magnitude (25--30%; mean Ppa and Pv fell 3.2 and 2.3 Torr, respectively. From measurements of thoracic dimensions and changes in intravascular pressures, we calculate that 11% of the pulmonary vascular bed falls into zone I condition, 34% goes from zone III to zone II, and 55% remains in zone III condition during tilt. A highly significant correlation exists between PBV and CI (P less than 0.001), with no significant change in pulmonary mean transit time during tilt. Significant correlations also exist between PBV and Pra (P less than 0.001), Pv (P less than 0.01), and Ppa (P less than 0.05). We conclude that outflow pressure plays a significant role in determining PBV during head-up tilt, but the major determinant is venous return to the right heart.
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Lewis et al. (1978) conducted an observational in Healthy (cardiopulmonary normal) (n=15). 60 degrees head-up passive tilt vs. Supine position was evaluated on Changes in pulmonary blood volume, flow, and pressures. Head-up passive tilt to 60 degrees decreased pulmonary blood volume, cardiac index, and right ventricular diastolic volume by 25-30%, with PBV highly correlated to cardiac index (P<0.001).
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