title: Changes in whole tissue heme concentration dissociates muscle deoxygenation from muscle oxygen extraction during passive head-up tilt.
abstract: Skeletal muscle deoxygenated hemoglobin and myoglobin concentration ([HHb]), assessed by near-infrared spectroscopy (NIRS), is commonly used as a surrogate of regional O2 extraction (reflecting the O2 delivery-to-consumption ratio, Q̇/V̇o2). However, [HHb] change (Δ[HHb]) is also influenced by capillary-venous heme concentration, and/or small blood vessel volume (reflected in total heme; [THb]). We tested the hypotheses that Δ[HHb] is associated with O2 extraction, and insensitive to [THb], over a wide range of Q̇/V̇o2 elicited by passive head-up tilt (HUT; 10-min, 15° increments, between -10° and 75°). Steady-state common femoral artery blood flow (FBF) was measured by echo-Doppler, and time-resolved NIRS measured [HHb] and [THb] of vastus lateralis (VL) and gastrocnemius (GS) in 13 men. EMG confirmed muscles were inactive. During HUT in VL [HHb] increased linearly (57 ± 10 to 101 ± 16 μM; P < 0.05 above 15°) and was associated (r(2) ∼ 0.80) with the reduction in FBF (618 ± 75 ml/min at 0° to 268 ± 52 ml/min at 75°; P < 0.05 above 30°) and the increase in [THb] (228 ± 30 vs. 252 ± 32 μM; P < 0.05 above 15°). GS response was qualitatively similar to VL. However, there was wide variation within and among individuals, such that the overall limits of agreement between Δ[HHb] and ΔFBF ranged from -35 to +19% across both muscles. Neither knowledge of tissue O2 saturation nor vascular compliance could appropriately account for the Δ[HHb]-ΔFBF dissociation. Thus, under passive tilt, [HHb] is influenced by Q̇/V̇o2, as well as microvascular hematocrit and/or tissue blood vessel volume, complicating its use as a noninvasive surrogate for muscle microvascular O2 extraction.
authors: Rehabilitation Clinical Trials Center, Division of Respiratory and Critical Care Physiology and Medicine, Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, Torrance, California;
authors: Alessandra
authors: A
authors: Adami
authors: Applied Physiology Laboratory, Kobe Design University, Kobe, Japan;
authors: Shunsaku
authors: S
authors: Koga
authors: Laboratory for Applied Human Physiology, Graduate School of Human Development and Environment, Kobe University, Kobe, Japan; and.
authors: Narihiko
authors: N
authors: Kondo
authors: Rehabilitation Clinical Trials Center, Division of Respiratory and Critical Care Physiology and Medicine, Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, Torrance, California;
authors: Daniel T
authors: DT
authors: Cannon
authors: School of Kinesiology and Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada.
authors: John M
authors: JM
authors: Kowalchuk
authors: Laboratory for Applied Human Physiology, Graduate School of Human Development and Environment, Kobe University, Kobe, Japan; and.
authors: Tatsuro
authors: T
authors: Amano
authors: Rehabilitation Clinical Trials Center, Division of Respiratory and Critical Care Physiology and Medicine, Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, Torrance, California; hrossiter@ucla.edu.
authors: Harry B
authors: http://orcid.org/0000-0002-7884-0726
authors: HB
authors: Rossiter
mesh_terms: D000328:Adult; D005263:Femoral Artery / Q000378:metabolism / Q000502:physiology; D006418:Heme / Q000378:metabolism*; D006454:Hemoglobins / Q000378:metabolism; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000502:physiology*; D009211:Myoglobin / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism*; D010101:Oxygen Consumption / Q000502:physiology; D011187:Posture / Q000502:physiology*; D052097:Quadriceps Muscle / Q000378:metabolism / Q000502:physiology; D012039:Regional Blood Flow / Q000502:physiology; D055815:Young Adult
chemical_list: D006454:Hemoglobins; D009211:Myoglobin; D006418:Heme; D010100:Oxygen
keywords: blood flow; echo-Doppler; time-resolved near-infrared spectroscopy; tissue saturation; vascular compliance