Multi-Scale Model of O2 Transport and Metabolism

Relationship between muscle venous blood oxygenation and near-infrared spectroscopy: quantitative analysis of the Hb and Mb contributions

A linear relationship between skeletal muscle venous ([Formula: see text]) and oxygenated (ΔHbMbO2,N) or deoxygenated (ΔHHbMbN) near-infrared spectroscopy (NIRS) signals suggest a main hemoglobin (Hb) contribution to the NIRS signal. However, experimental, and computational evidence supports a significant contribution of myoglobin (Mb) to the NIRS. Venous and NIRS measurements from a canine model of muscle oxidative metabolism (Sun Y, Ferguson BS, Rogatzki MJ, McDonald JR, Gladden LB. Med Sci Sports Exerc 48(10):2013-2020, 2016) were integrated into a computational model of muscle O2 transport and utilization to evaluate whether the relationship between venous and NIRS oxygenation can be affected by a significant Mb contribution to the NIRS signals. The mathematical model predicted well the measure of the changes of [Formula: see text] and NIRS signals for different O2 delivery conditions (blood flow, arterial O2 content) in muscle at rest (T1, T2) and during contraction (T3). Furthermore, computational analysis indicates that for adequate O2 delivery, Mb contribution to NIRS signals was significant (20%-30%) even in the presence of a linear [Formula: see text]-NIRS relationship; for a reduced O2 delivery the nonlinearity of the [Formula: see text]-NIRS relationship was related to the Mb contribution (50%). In this case (T3), the deviation from linearity is observed when O2 delivery is reduced from 1.3 to 0.7 L kg-1·min-1 ([Formula: see text] < 10 mLO2 100 mL-1) and Mb saturation decreased from 85% to 40% corresponding to an increase of the Mb contribution to ΔHHbMbN from 15% to 50% and the contribution to ΔHbMbO2,N from 0% to 30%. In contrast to a common assumption, our model indicates that both NIRS signals (ΔHHbMbN and ΔHbMbO2,N are significantly affected by Hb and Mb oxygenation changes.NEW & NOTEWORTHY Within the near-infrared spectroscopy (NIRS) signal, the contribution from hemoglobin is indistinguishable from that of myoglobin. A computation analysis indicates that a linear relationship between muscle venous oxygen content and NIRS signals does not necessarily indicate a negligible myoglobin contribution to the NIRS signal. A reduced oxygen delivery increases the myoglobin contribution to the NIRS signal. The integrative approach proposed is a powerful way to assist in interpreting the elements from which the NIRS signals are derived.

Blood volume versus deoxygenated NIRS signal: computational analysis of the effects muscle O2 delivery and blood volume on the NIRS signals

Near-infrared spectroscopy (NIRS) signals quantify the oxygenated (ΔHbMbO2) and deoxygenated (ΔHHbMb) heme group concentrations. ΔHHbMb has been preferred to ΔHbMbO2 in evaluating skeletal muscle oxygen extraction because it is assumed to be less sensitive to blood volume (BV) changes, but uncertainties exist on this assumption. To analyze this assumption, a computational model of oxygen transport and metabolism is used to quantify the effect of O2 delivery and BV changes on the NIRS signals from a canine model of muscle oxidative metabolism (Sun Y, Ferguson BS, Rogatzki MJ, McDonald JR, Gladden LB. Med Sci Sports Exerc 48: 2013-2020, 2016). The computational analysis accounts for microvascular (ΔHbO2, ΔHHb) and extravascular (ΔMbO2, ΔHMb) oxygenated and deoxygenated forms. Simulations predicted muscle oxygen uptake and NIRS signal changes well for blood flows ranging from resting to contracting muscle. Additional NIRS signal simulations were obtained in the absence or presence of BV changes corresponding to a heme groups concentration changes (ΔHbMb = 0-48 µM). Under normal delivery (Q = 1.0 L·kg-1·min-1) in contracting muscle, capillary oxygen saturation (So2) was 62% with capillary ΔHbO2 and ΔHHb of ± 41 µΜ for ΔHbMb = 0. An increase of BV (ΔHbMb = 24 µΜ) caused a ΔHbO2 decrease (16µΜ) almost twice as much as the increase observed for ΔHHb (9 µΜ). When So2 increased to more than 80%, only ΔHbO2 was significantly affected by BV changes. The analysis indicates that microvascular So2 is a key factor in determining the sensitivity of ΔHbMbO2 and deoxygenated ΔHHbMb to BV changes. Contrary to a common assumption, the ΔHHbMb is affected by BV changes in normal contracting muscle and even more in the presence of impaired O2 delivery.NEW & NOTEWORTHY Deoxygenated is preferred to the oxygenated near-infrared spectroscopy signal in evaluating skeletal muscle oxygen extraction because it is assumed to be insensitive to blood volume changes. The quantitative analysis proposed in this study indicates that even in absence of skin blood flow effects, both NIRS signals in presence of either normal or reduced oxygen delivery are affected by blood volume changes. These changes should be considered to properly quantify muscle oxygen extraction by NIRS methods.

Impact of venous return on pulmonary oxygen uptake kinetics during dynamic exercise: in silico time series analyses from muscles to lungs

The aim of the present study was to investigate whether a single-compartment (SCM) and a multi-compartment (MCM) venous return model will produce significantly different time-delaying and distortive effects on pulmonary oxygen uptake (V̇o2pulm) responses with equal cardiac outputs (Q̇) and muscle oxygen uptake (V̇o2musc) inputs. For each model, 64 data sets were simulated with alternating Q̇ and V̇o2musc kinetics—time constants (τ) ranging from 10 to 80 s—as responses to pseudorandom binary sequence work rate (WR) changes. Kinetic analyses were performed by using cross-correlation functions (CCFs) between WR with V̇o2pulm and V̇o2musc. Higher maxima of the CCF courses indicate faster system responses—equal to smaller τ values of the variables of interest (e.g., τV̇o2musc). The models demonstrated a highly significant relationship for the resulting V̇o2pulm responses ( r = 0.976, P < 0.001, n = 64). Both models showed significant differences between V̇o2pulm and V̇o2musc kinetics for τV̇o2musc ranging from 10 to 30 s ( P < 0.05 each). In addition, a significant difference in V̇o2pulm kinetics ( P < 0.05) between the models was observed for very fast V̇o2musc kinetics (τ = 10 s). The combinations of fast Q̇ dynamics and slow V̇o2musc kinetics yield distinct deviations in the resultant V̇o2pulm responses compared with V̇o2musc kinetics. Therefore, the venous return models should be used with care and caution if the aim is to infer V̇o2musc by means of V̇o2pulm kinetics. Finally, the resultant V̇o2pulm responses seem to be complex and most likely unpredictable if no cardiodynamic measurements are available in vivo.

NEW & NOTEWORTHY A single-compartment and a multi-compartment venous return model were tested to see whether they result in different pulmonary oxygen uptake (V̇o2pulm) kinetics from equal cardiac output and muscle oxygen uptake (V̇o2musc) kinetics. To infer V̇o2musc kinetics by means of V̇o2pulm kinetics, both models should only be used for V̇o2musc time constants ranging from 40 to 80 s. The resultant V̇o2pulm responses seem to be complex and most likely unpredictable if no cardiodynamic measurements are available.

Gas exchange kinetics following concentric-eccentric isokinetic arm and leg exercise

PURPOSE

To evaluate the effects of exercise velocity (60, 150, 240deg∙s^-1) and muscle mass (arm vs leg) on changes in gas exchange and arterio-venous oxygen content difference (avDO₂) following high-intensity concentric-eccentric isokinetic exercise.

METHODS

Fourteen subjects (26.9±3.1years) performed a 3×20-repetition isokinetic exercise protocol. Recovery beat-to-beat cardiac output (CO) and breath-by-breath gas exchange were recorded to determine post-exercise half-time (t_1/2) for oxygen uptake (V˙O₂pulm), carbon dioxide output (V˙CO₂pulm), and ventilation (V˙_E).

RESULTS

Significant differences of the t_1/2 values were identified between 60 and 150deg∙s^-1. Significant differences in the t_1/2 values were observed between V˙O₂pulm and V˙CO₂pulm and between V˙CO₂pulm and V˙_E. The time to attain the first avDO₂-peak showed significant differences between arm and leg exercise.

CONCLUSIONS

The present study illustrates, that V˙O₂pulm kinetics are distorted due to non-linear CO dynamics. Therefore, it has to be taken into account, that V˙O₂pulm may not be a valuable surrogate for muscular oxygen uptake kinetics in the recovery phases.

Phenylephrine eye drops in pediatric patients undergoing ophthalmic surgery: incidence, presentation, and management of complications during general anesthesia

BACKGROUND

Phenylephrine eye drops are widely used as mydriatic agent to reach the posterior segment of the eye. In literature, many reports suggest a systemic absorption of this agent as a source of severe adverse drug reactions. Hence, we reviewed our experience with topical phenylephrine in ophthalmic surgery.

METHODS

In May 2006, following US guidelines publication, a standard operating procedure was issued in our operating rooms to standardize the use of phenylephrine eye drops in our practice. Two years later, after the occurrence of a cluster of serious adverse drug reactions in infants undergoing surgery, a review of phenylephrine safety and systemic complications incidence was performed.

RESULTS

We observed 451 pediatric patients, and 187 met the inclusions criteria: Among them, 4 experienced hemodynamic complications due to phenylephrine eye drops. The incidence of major complications was 2.1%.

CONCLUSIONS

Two different patterns of side effects occurred. The first one was a cardiovascular derangement with severe hypertension and heart rate alterations; the other one involved exclusively pulmonary circuit causing early edema. These clinical manifestations, their duration, and treatment responses are all explainable by alfa1-adrenergic action of phenylephrine. This hypothetic pathogenesis has been confirmed also by the usefulness of direct vasodilators (anesthetic agents) and by the negative outcome occurred in the past with the use of beta-blockers.

Exercise: Kinetic considerations for gas exchange

The activities of daily living typically occur at metabolic rates below the maximum rate of aerobic energy production. Such activity is characteristic of the nonsteady state, where energy demands, and consequential physiological responses, are in constant flux. The dynamics of the integrated physiological processes during these activities determine the degree to which exercise can be supported through rates of O₂ utilization and CO₂ clearance appropriate for their demands and, as such, provide a physiological framework for the notion of exercise intensity. The rate at which O₂ exchange responds to meet the changing energy demands of exercise--its kinetics--is dependent on the ability of the pulmonary, circulatory, and muscle bioenergetic systems to respond appropriately. Slow response kinetics in pulmonary O₂ uptake predispose toward a greater necessity for substrate-level energy supply, processes that are limited in their capacity, challenge system homeostasis and hence contribute to exercise intolerance. This review provides a physiological systems perspective of pulmonary gas exchange kinetics: from an integrative view on the control of muscle oxygen consumption kinetics to the dissociation of cellular respiration from its pulmonary expression by the circulatory dynamics and the gas capacitance of the lungs, blood, and tissues. The intensity dependence of gas exchange kinetics is discussed in relation to constant, intermittent, and ramped work rate changes. The influence of heterogeneity in the kinetic matching of O₂ delivery to utilization is presented in reference to exercise tolerance in endurance-trained athletes, the elderly, and patients with chronic heart or lung disease.

A validated model of oxygen uptake and circulatory dynamic interactions at exercise onset in humans

At the onset of muscular exercise, the kinetics of pulmonary O2 uptake (Vo2P) reflect the integrated dynamic responses of the ventilatory, circulatory, and neuromuscular systems for O2 transport and utilization. Muscle O2 uptake (Vo2m) kinetics, however, are dissociated from Vo2P kinetics by intervening O2 capacitances and the dynamics of the circulation and ventilation. We developed a multicompartment computational model (MCM) to investigate these dynamic interactions and optimized and validated the MCM using previously published, simultaneously measured Vo2m, alveolar O2 uptake (Vo2A), and muscle blood flow (Qm) in healthy young men during cycle ergometry. The model was used to show that 1) the kinetics of Vo2A during exercise transients are very sensitive to preexercise blood flow distribution and the absolute value of Qm, 2) a low preexercise Qm exaggerates the magnitude of the transient fall in venous O2 concentration for any given Vo2m kinetics, necessitating a tighter coupling of Qm/Vo2m (or a reduction in the available work rate range) during the exercise transient to avoid limits to O2 extraction, and 3) information regarding exercise-related alterations in O2 uptake and blood flow in nonexercising tissues and their effects on mixed venous O2 concentration is required to accurately predict Vo2A kinetics from knowledge of Vo2m and Qm dynamics. Importantly, these data clearly demonstrate that Vo2A kinetics are nonexponential, nonlinear distortions of Vo2m kinetics that can be explained in a MCM by interactions among circulatory and cellular respiratory control processes before and during exercise.

Optimality and thermodynamics determine the evolution of transcriptional regulatory networks

Transcriptional motifs are small regulatory interaction patterns that regulate biological functions in highly-interacting cellular networks. Recently, attempts have been made to explain the significance of transcriptional motifs through dynamic function. However, fundamental questions remain unanswered. Why are certain transcriptional motifs with similar dynamic function abundant while others occur rarely? What are the criteria for topological generalization of these motifs into complex networks? Here, we present a novel paradigm that combines non-equilibrium thermodynamics with multiobjective-optimality for network analysis. We found that energetic cost, defined herein as specific dissipation energy, is minimal at the optimal environmental conditions and it correlates inversely with the abundance of the network motifs obtained experimentally for E. coli and S. cerevisiae. This yields evidence that dissipative energetics is the underlying criteria used during evolution for motif selection and that biological systems during transcription tend towards evolutionary selection of subgraphs which produces minimum specific heat dissipation under optimal conditions, thereby explaining the abundance/rare occurrence of some motifs. We show that although certain motifs had similar dynamical functionality, they had significantly different energetic cost, thus explaining the abundance/rare occurrence of these motifs. The presented insights may establish global thermodynamic analysis as a backbone in designing and understanding complex networks systems, such as metabolic and protein interaction networks.

Models of muscle contraction and energetics

How does skeletal muscle manage to regulate the pathways of ATP synthesis during large-scale changes in work rate while maintaining metabolic homeostasis remains unknown. The classic model of metabolic regulation during muscle contraction states that accelerating ATP utilization leads to increasing concentrations of ADP and Pi, which serve as substrates for oxidative phosphorylation and thus accelerate ATP synthesis. An alternative model states that both the ATP demand and ATP supply pathways are simultaneously activated. Here, we review experimental and computational models of muscle contraction and energetics at various organizational levels and compare them with respect to their pros and cons in facilitating understanding of the regulation of energy metabolism during exercise in the intact organism.