Mechanisms of inspiration that modulate cardiovascular control: the other side of breathing

Evaluating the impact of type 2 diabetes mellitus on pulmonary vascular function and the development of pulmonary fibrosis

The increasing prevalence of type 2 diabetes mellitus (T2DM) is a significant worldwide health concern caused by sedentary lifestyles and unhealthy diets. Beyond glycemic control, T2DM impacts multiple organ systems, leading to various complications. While traditionally associated with cardiovascular and microvascular complications, emerging evidence indicates significant effects on pulmonary health. Pulmonary vascular dysfunction and fibrosis, characterized by alterations in vascular tone and excessive extracellular matrix deposition, are increasingly recognized in individuals with T2DM. The onset of T2DM is often preceded by prediabetes, an intermediate hyperglycemic state that is associated with increased diabetes and cardiovascular disease risk. This review explores the relationship between T2DM, pulmonary vascular dysfunction and pulmonary fibrosis, with a focus on potential links with prediabetes. Pulmonary vascular function, including the roles of nitric oxide (NO), prostacyclin (PGI2), endothelin-1 (ET-1), thromboxane A2 (TxA2) and thrombospondin-1 (THBS1), is discussed in the context of T2DM and prediabetes. Mechanisms linking T2DM to pulmonary fibrosis, such as oxidative stress, dysregulated fibrotic signaling, and chronic inflammation, are explained. The impact of prediabetes on pulmonary health, including endothelial dysfunction, oxidative stress, and dysregulated vasoactive mediators, is highlighted. Early detection and intervention during the prediabetic stage may reduce respiratory complications associated with T2DM, emphasizing the importance of management strategies targeting blood glucose regulation and vascular health. More research that looks into the mechanisms underlying pulmonary complications in T2DM and prediabetes is needed.

Oscillations of Hemodynamic Parameters Induced by Negative Pressure Breathing in Healthy Humans

INTRODUCTION: Negative pressure breathing is breathing with decreased pressure in the respiratory tract without lowering pressure acting on the torso. We lowered air pressure only during inspiration (NPBin). NPBin, used to increase venous return to the heart, is considered a countermeasure against redistribution of body fluids toward the head during spaceflight. We studied NPBin effects on circulation in healthy humans with an emphasis on NPBin-induced oscillations of hemodynamic parameters synchronous with breathing. We propose an approach to analyze the oscillations based on coherent averaging.METHODS: Eight men ages 24-42 yr participated in the NPBin and control series. During the series, to reproduce fluids shift observed under microgravity, subjects were supine and head down (-8°). Duration of NPBin was 20 min, rarefaction -20 cm H₂O. Hemodynamic parameters were measured by Finometer. Electrical impedance measurements were used to estimate changes in blood filling of cerebral vessels.RESULTS: Mean values of hemodynamic parameters virtually did not change under NPBin, but NPBin induced oscillations of the parameters synchronous with respiration. Peak-to-peak amplitude under NPBin were: mean arterial pressure, 4 ± 1 (mmHg); stroke volume, 7 ± 3 (mL); and heart rate, 4 ± 1 (bpm). Electrical impedance of the head increased during inspiration. The increase under NPBin was three times greater than under normal breathing.DISCUSSION: Analysis of oscillations gives more information than analysis of mean values. NPBin induces short-term decrease in left ventricle stroke volume and arterial blood pressure during each inspiration; the decrease is compensated by increase after inspiration. NPBin facilitates redistribution of body fluids away from the head.Semenov YS, Melnikov IS, Luzhnov PV, Dyachenko AI. Oscillations of hemodynamic parameters induced by negative pressure breathing in healthy humans. Aerosp Med Hum Perform. 2024; 95(6):297-304.

Dynamic cerebral autoregulation is preserved during orthostasis and intrathoracic pressure regulation in healthy subjects: A pilot study

Resistance breathing may restore cardiac output (CO) and cerebral blood flow (CBF) during hypovolemia. We assessed CBF and cerebral autoregulation (CA) during tilt, resistance breathing, and paced breathing in 10 healthy subjects. Blood velocities in the internal carotid artery (ICA), middle cerebral arteries (MCA, four subjects), and aorta were measured by Doppler ultrasound in 30° and 60° semi-recumbent positions. ICA blood flow and CO were calculated. Arterial blood pressure (ABP, Finometer), and end-tidal CO2 (ETCO2) were recorded. ICA blood flow response was assessed by mixed-models regression analysis. The synchronization index (SI) for the variable pairs ABP-ICA blood velocity, ABP-MCA velocities in 0.005-0.08 Hz frequency interval was calculated as a measure of CA. Passive tilting from 30° to 60° resulted in 12% decrease in CO (p = 0.001); ICA blood flow tended to fall (p = 0.04); Resistance breathing restored CO and ICA blood flow despite a 10% ETCO2 drop. ETCO2 and CO contributed to ICA blood flow variance (adjusted R2: 0.9, p < 0.0001). The median SI was low (<0.2) indicating intact CA, confirmed by surrogate date testing. The peak SI was transiently elevated during resistance breathing in the 60° position. Resistance breathing may transiently reduce CA efficiency. Paced breathing did not restore CO or ICA blood flow.

Deep Diaphragmatic Breathing-Anatomical and Biomechanical Consideration

Background: Deep diaphragmatic breathing (DDB) involves slow and fully contraction of the diaphragm with expansion of the belly during inhalation, and slow and fully contraction of the abdominal muscles with reduction of the belly during exhalation. It is the key component of the holistic mind-body exercises commonly used for patients with multimorbidity. Purpose: The purpose of this study was to re-visit and address the fundamental anatomical and biomechanical consideration of the DDB with the relevant literature. Method: Peer-reviewed publications from last the 15 years were retrieved, reviewed, and analyzed. Findings: In this article, we described the updated morphological and anatomical characteristics of the diaphragm. Then, we elucidated in a biomechanical approach how and why the DDB can work on the gastrointestinal, cardiopulmonary, and nervous systems as well as on regulating the intra-abdominopelvic pressure and mind-body interaction to coordinate the diaphragm-pelvic floor-abdominal complex for a variety of physical and physiological activities. Conclusion: Understanding of this updated DDB knowledge may help holistic healthcare professionals including holistic nurses provide better patient education and care management during the DDB or DDB-based mind-body intervention time.

Hypoxia evokes a sequence of raphe-pontomedullary network operations for inspiratory drive amplification and gasping

Hypoxia can trigger a sequence of breathing-related behaviors, from tachypnea to apneusis to apnea and gasping, an autoresuscitative behavior that, via large tidal volumes and altered intrathoracic pressure, can enhance coronary perfusion, carotid blood flow, and sympathetic activity, and thereby coordinate cardiac and respiratory functions. We tested the hypothesis that hypoxia-evoked gasps are amplified through a disinhibitory microcircuit within the inspiratory neuron chain and a distributed efference copy mechanism that generates coordinated gasp-like discharges concurrently in other circuits of the raphe-pontomedullary respiratory network. Data were obtained from 6 decerebrate, vagotomized, neuromuscularly-blocked, and artificially ventilated adult cats. Arterial blood pressure, phrenic nerve activity, end-tidal CO2, and other parameters were monitored. Hypoxia was produced by ventilation with a gas mixture of 5% O2 in nitrogen (N2). Neuron spike trains were recorded at multiple pontomedullary sites simultaneously and evaluated for firing rate modulations and short-time scale correlations indicative of functional connectivity. Experimental perturbations evoked reconfiguration of raphe-pontomedullary circuits during tachypnea, apneusis and augmented bursts, apnea, and gasping. The functional connectivity, altered firing rates, efference copy of gasp drive, and coordinated step increments in blood pressure reported here support a distributed brain stem network model for amplification and broadcasting of inspiratory drive during autoresuscitative gasping that begins with a reduction in inhibition by expiratory neurons and an initial loss of inspiratory drive during hypoxic apnea.

Cerebral blood flow response to cardiorespiratory oscillations in healthy humans

Dynamic cerebral autoregulation (CA) characterizes the cerebral blood flow (CBF) response to abrupt changes in arterial blood pressure (ABP). CA operates at frequencies below 0.15 Hz. ABP regulation and probably CA are modified by autonomic nervous activity. We investigated the CBF response and CA dynamics to mild increase in sympathetic activity. Twelve healthy volunteers underwent oscillatory lower body negative pressure (oLBNP), which induced respiratory-related ABP oscillations at an average of 0.22 Hz. We recorded blood velocity in the internal carotid artery (ICA) by Doppler ultrasound and ABP. We quantified variability and peak wavelet power of ABP and ICA blood velocity by wavelet analysis at low frequency (LF, 0.05-0.15 Hz) and Mayer waves (0.08-0.12 Hz), respectively. CA was quantified by calculation of the wavelet synchronization gamma index for the pair ABP-ICA blood velocity in the LF and Mayer wave band. oLBNP increased ABP peak wavelet power at the Mayer wave frequency. At the Mayer wave, ABP peak wavelet power increased by >70 % from rest to oLBNP (p < 0.05), while ICA blood flow velocity peak wavelet power was unchanged, and gamma index increased (from 0.49 to 0.69, p < 0.05). At LF, variability in both ABP and ICA blood velocity and gamma index were unchanged from rest to oLBNP. Despite an increased gamma index at Mayer wave, ICA blood flow variability was unchanged during increased ABP variability. The increased synchronization during oLBNP did not cause less stable CBF or less active CA. Sympathetic activation seems to improve the mechanisms of CA.

Influence of respiratory loading on left-ventricular function in cervical spinal cord injury

KEY POINTS

Cervical spinal cord injury (C-SCI) alters both the cardiac and respiratory systems, however little is known as to how these systems interact following injury. Here, we manipulated inspiratory or expiratory intrathoracic pressure (ITP) to mechanistically test the role of the respiratory pump on circulatory function in highly-trained individuals with C-SCI and an able-bodied reference group. In individuals with C-SCI, greater ITP during expiratory loading caused dynamic hyperinflation that was associated with impaired left-ventricular filling. More negative ITP during inspiratory loading did not significantly alter left-ventricular volumes in either group. Interventions that prevent dynamic hyperinflation and/or enhance the ability to generate expiratory pressures may help preserve left-ventricular filling in individuals with C-SCI.

ABSTRACT

Cervical spinal cord injury (C-SCI) negatively impacts cardiac and respiratory function. As the heart and lungs are linked via the pulmonary circuit these systems are interdependent. Here, we utilized inspiratory and expiratory loading to assess whether augmenting the respiratory pump improves left-ventricular (LV) filling and output in individuals with motor-complete C-SCI. We hypothesized LV end-diastolic volume (LVEDV) would increase and decrease with inspiratory and expiratory loading, respectively. Participants (C-SCI: 7M/1F, 35±7 years; able-bodied: 7M/1F, 32±6 years) were assessed under five conditions during 45° head-up tilt; unloaded, inspiratory loading with -10 and -20cmH₂ O esophageal pressure (Pes) on inspiration, and expiratory loading with +10 and +20cmH₂ O Pes on expiration. An esophageal balloon catheter monitored Pes and LV structure and function were assessed by echocardiography. In C-SCI only, (1) +20cmH₂ O reduced LVEDV vs. unloaded (81±15 vs. 88±11 mL, p = 0.006); (2) heart rate was higher during +20cmH₂ O compared to unloaded (p = 0.001) and +10cmH₂ O (p = 0.002); (3) cardiac output was higher during +20cmH₂ O than unloaded (p = 0.002); and (4) end-expiratory lung volume was higher during +20cmH₂ O vs. unloaded (63±10 vs. 55±13% total lung capacity, p = 0.003) but was unaffected by inspiratory loading. In both groups, -10 and -20cmH₂ O had no significant effect on LVEDV. These findings suggest greater expiratory positive pressure acutely impairs LV filling in C-SCI, potentially via impaired venous return, mediastinal constraint and/or direct ventricular interaction subsequent to dynamic hyperinflation. Inspiratory loading did not significantly improve LV function in C-SCI and neither inspiratory nor expiratory loading affected cardiac function or lung volumes in able-bodied participants. Abstract figure legend Background: Cervical spinal cord injury (C-SCI) alters both the cardiac and respiratory systems. However, expiratory function is compromised to a greater extent than inspiratory function. Experimental set up: To examine how the cardiac and respiratory systems interact following C-SCI we manipulated inspiratory or expiratory intrathoracic pressure (ITP) to mechanistically test how changes in ITP and lung volumes influence cardiac function in highly-trained individuals with C-SCI and an able-bodied reference group. Participants were assessed under five conditions during 45° head-up tilt; unloaded, two inspiratory loading, and two expiratory loading conditions.

KEY FINDINGS

Following C-SCI, greater ITP during expiratory loading increased lung volumes and was associated with impaired left-ventricular filling. Interventions that prevent increases in lung volumes and/or enhance the ability to generate expiratory pressures may help preserve left-ventricular filling in individuals with C-SCI. A portion of this figure was created with biorender.com This article is protected by copyright. All rights reserved.

Perceptual and Ventilatory Responses to Hypercapnia in Athletes and Sedentary Individuals

Purpose

Hypercapnic chemosensitivity traditionally captures the ventilatory response to elevated pressures of carbon dioxide in the blood. However, hypercapnia also contributes to subjective breathing perceptions, and previously we demonstrated a closer matching of perception to changes in ventilation in athletes compared to controls. Here we investigated any potential underlying hypercapnic chemosensitivity differences between groups, and explored whether these measures relate to ventilatory and perceptual responses during exercise as well as trait levels of affect.

Methods

A hypercapnic challenge, incremental maximal exercise test and affective questionnaires were completed by 20 endurance athletes and 20 age-/sex-matched sedentary controls. The hypercapnic challenge involved elevating end-tidal PCO₂ by 0.8% (6.1 mmHg) and 1.5% (11.2 mmHg) for 3 min each (randomised), with constant end-tidal oxygen. Ventilatory and perceptual responses to hypercapnia were compared between groups, and within each group the relationships between hypercapnic chemosensitivity (slope analyses) and exercising ventilation and perceptions were calculated using Spearman’s non-parametric correlations.

Results

While absolute ventilation differences during hypercapnia and exercise were observed, no group differences were found across hypercapnic chemosensitivity (slope) measures. Correlation analyses revealed the anxiety hypercapnic response was related to maximal exercise anxiety, but only in sedentary individuals.

Conclusion

Ventilatory and perceptual hypercapnic chemosensitivity do not differ between athletes and sedentary individuals. However, ventilatory and anxiety hypercapnic chemosensitivities were related to ventilatory and anxiety responses during exercise in untrained individuals only. Athletes may employ additional strategies during exercise to reduce the influence of chemosensitivity on ventilatory and perceptual responses.

Impact of Respiratory Fluctuation on Hemodynamics in Human Cardiovascular System: A 0-1D Multiscale Model

To explore hemodynamic interaction between the human respiratory system (RS) and cardiovascular system (CVS), here we propose an integrated computational model to predict the CVS hemodynamics with consideration of the respiratory fluctuation (RF). A submodule of the intrathoracic pressure (ITP) adjustment is developed and incorporated in a 0-1D multiscale hemodynamic model of the CVS specified for infant, adolescent, and adult individuals. The model is verified to enable reasonable estimation of the blood pressure waveforms accounting for the RF-induced pressure fluctuations in comparison with clinical data. The results show that the negative ITP caused by respiration increases the blood flow rates in superior and inferior vena cavae; the deep breathing improves the venous return in adolescents but has less influence on infants. It is found that a marked reduction in ITP under pathological conditions can excessively increase the flow rates in cavae independent of the individual ages, which may cause the hemodynamic instability and hence increase the risk of heart failure. Our results indicate that the present 0-1D multiscale CVS model incorporated with the RF effect is capable of providing a useful and effective tool to explore the physiological and pathological mechanisms in association with cardiopulmonary interactions and their clinical applications.

Saving the brain after mild-to-moderate traumatic injury: A report on new insights of the physiology underlying adequate maintenance of cerebral perfusion

ABSTRACT

Traumatic brain injury (TBI) is associated with increased morbidity and mortality in civilian trauma and battlefield settings. It has been classified across a continuum of dysfunctions, with as much as 80% to 90% of cases diagnosed as mild to moderate in combat casualties. In this report, a framework is presented that focuses on the potential benefits for acute noninvasive treatment of reduced cerebral perfusion associated with mild TBI by harnessing the natural transfer of negative intrathoracic pressure during inspiration. This process is known as intrathoracic pressure regulation (IPR) therapy, which can be applied by having a patient breath against a small inspiratory resistance created by an impedance threshold device. Intrathoracic pressure regulation therapy leverages two fundamental principles for improving blood flow to the brain: (1) greater negative intrathoracic pressure enhances venous return, cardiac output, and arterial blood pressure; and (2) lowering of intracranial pressure provides less resistance to cerebral blood flow. These two effects work together to produce a greater pressure gradient that results in an improvement in cerebral perfusion pressure. In this way, IPR therapy has the potential to counter hypotension and hypoxia, potentially significant contributing factors to secondary brain injury, particularly in conditions of multiple injuries that include severe hemorrhage. By implementing IPR therapy in patients with mild-to-moderate TBI, a potential exists to provide early neuroprotection at the point of injury and a bridge to more definitive care, particularly in settings of prolonged delays in evacuation such as those anticipated in future multidomain operations.

LEVEL OF EVIDENCE

Report.

Physiology of Human Hemorrhage and Compensation

Hemorrhage is a leading cause of death following traumatic injuries in the United States. Much of the previous work in assessing the physiology and pathophysiology underlying blood loss has focused on descriptive measures of hemodynamic responses such as blood pressure, cardiac output, stroke volume, heart rate, and vascular resistance as indicators of changes in organ perfusion. More recent work has shifted the focus toward understanding mechanisms of compensation for reduced systemic delivery and cellular utilization of oxygen as a more comprehensive approach to understanding the complex physiologic changes that occur following and during blood loss. In this article, we begin with applying dimensional analysis for comparison of animal models, and progress to descriptions of various physiological consequences of hemorrhage. We then introduce the complementary side of compensation by detailing the complexity and integration of various compensatory mechanisms that are activated from the initiation of hemorrhage and serve to maintain adequate vital organ perfusion and hemodynamic stability in the scenario of reduced systemic delivery of oxygen until the onset of hemodynamic decompensation. New data are introduced that challenge legacy concepts related to mechanisms that underlie baroreflex functions and provide novel insights into the measurement of the integrated response of compensation to central hypovolemia known as the compensatory reserve. The impact of demographic and environmental factors on tolerance to hemorrhage is also reviewed. Finally, we describe how understanding the physiology of compensation can be translated to applications for early assessment of the clinical status and accurate triage of hypovolemic and hypotensive patients. © 2021 American Physiological Society. Compr Physiol 11:1531-1574, 2021.

Blood pressure drives multispectral tuning of inspiration via a linked-loop neural network

The respiratory motor pattern is coordinated with cardiovascular system regulation. Inspiratory drive and respiratory phase durations are tuned by blood pressure and baroreceptor reflexes. We hypothesized that perturbations of systemic arterial blood pressure modulate inspiratory drive through a raphe-pontomedullary network. In 15 adult decerebrate vagotomized neuromuscular-blocked cats, we used multielectrode arrays to record the activities of 704 neurons within the medullary ventral respiratory column, pons, and raphe areas during baroreceptor-evoked perturbations of breathing, as measured by altered peak activity in integrated efferent phrenic nerve activity and changes in respiratory phase durations. Blood pressure was transiently (30 s) elevated or reduced by inflations of an embolectomy catheter in the descending aorta or inferior vena cava. S-transform time-frequency representations were calculated for multiunit phrenic nerve activity and some spike trains to identify changes in rhythmic activity during perturbations. Altered firing rates in response to either or both conditions were detected for 474 of 704 tested cells. Spike trains of 17,805 neuron pairs were evaluated for short-time scale correlational signatures indicative of functional connectivity with standard cross-correlation analysis, supplemented with gravitational clustering; ∼70% of tested (498 of 704) and responding neurons (333 of 474) were involved in a functional correlation with at least one other cell. Changes in high-frequency oscillations in the spiking of inspiratory neurons and the evocation or resetting of slow quasi-periodic fluctuations in the respiratory motor pattern associated with oscillations of arterial pressure were observed. The results support a linked-loop pontomedullary network architecture for multispectral tuning of inspiration.NEW & NOTEWORTHY The brain network that supports cardiorespiratory coupling remains poorly understood. Using multielectrode arrays, we tested the hypothesis that blood pressure and baroreceptor reflexes "tune" the breathing motor pattern via a raphe-pontomedullary network. Neuron responses to changes in arterial pressure and identified functional connectivity, together with altered high frequency and slow Lundberg B-wave oscillations, support a model with linked recurrent inhibitory loops that stabilize the respiratory network and provide a path for transmission of baroreceptor signals.

Transferable Benefits of Cycle Hypoventilation Training for Run-Based Performance in Team-Sport Athletes

PURPOSE

To determine whether high-intensity training with voluntary hypoventilation at low lung volume (VHL) in cycling could improve running performance in team-sport athletes.

METHODS

Twenty well-fit subjects competing in different team sports completed, over a 3-week period, 6 high-intensity training sessions in cycling (repeated 8-s exercise bouts at 150% of maximal aerobic power) either with VHL or with normal breathing conditions. Before (Pre) and after (Post) training, the subjects performed a repeated-sprint-ability test (RSA) in running (12 × 20-m all-out sprints), a 200-m maximal run, and the Yo-Yo Intermittent Recovery Level 1 test (YYIR1).

RESULTS

There was no difference between Pre and Post in the mean and best velocities reached in the RSA test, as well as in performance and maximal blood lactate concentration in the 200-m-run trial in both groups. On the other hand, performance was greater in the second part of the RSA test, and the fatigue index of this test was lower (5.18% [1.3%] vs 7.72% [1.6%]; P < .01) after the VHL intervention only. Performance was also greater in the YYIR1 in the VHL group (1468 [313] vs 1111 [248] m; P < .01), whereas no change occurred in the normal-breathing-condition group.

CONCLUSION

This study showed that performing high-intensity cycle training with VHL could improve RSA and possibly endurance performance in running. On the other hand, this kind of approach does not seem to induce transferable benefits for anaerobic performance.

Exercise with End-expiratory Breath Holding Induces Large Increase in Stroke Volume

Eight well-trained male cyclists participated in two testing sessions each including two sets of 10 cycle exercise bouts at 150% of maximal aerobic power. In the first session, subjects performed the exercise bouts with end-expiratory breath holding (EEBH) of maximal duration. Each exercise bout started at the onset of EEBH and ended at its release (mean duration: 9.6±0.9 s; range: 8.6-11.1 s). At the second testing session, subjects performed the exercise bouts (same duration as in the first session) with normal breathing. Heart rate, left ventricular stroke volume (LVSV), and cardiac output were continuously measured through bio-impedancemetry. Data were analysed for the 4 s preceding and following the end of each exercise bout. LVSV (peak values: 163±33 vs. 124±17 mL, p<0.01) was higher and heart rate lower both in the end phase and in the early recovery of the exercise bouts with EEBH as compared with exercise with normal breathing. Cardiac output was generally not different between exercise conditions. This study showed that performing maximal EEBH during high-intensity exercise led to a large increase in LVSV. This phenomenon is likely explained by greater left ventricular filling as a result of an augmented filling time and decreased right ventricular volume at peak EEBH.