Coronary flow velocity changes in response to hypercapnia: assessment by transthoracic Doppler echocardiography

Electro-metabolic signaling

Precise matching of energy substrate delivery to local metabolic needs is essential for the health and function of all tissues. Here, we outline a mechanistic framework for understanding this critical process, which we refer to as electro-metabolic signaling (EMS). All tissues exhibit changes in metabolism over varying spatiotemporal scales and have widely varying energetic needs and reserves. We propose that across tissues, common signatures of elevated metabolism or increases in energy substrate usage that exceed key local thresholds rapidly engage mechanisms that generate hyperpolarizing electrical signals in capillaries that then relax contractile elements throughout the vasculature to quickly adjust blood flow to meet changing needs. The attendant increase in energy substrate delivery serves to meet local metabolic requirements and thus avoids a mismatch in supply and demand and prevents metabolic stress. We discuss in detail key examples of EMS that our laboratories have discovered in the brain and the heart, and we outline potential further EMS mechanisms operating in tissues such as skeletal muscle, pancreas, and kidney. We suggest that the energy imbalance evoked by EMS uncoupling may be central to cellular dysfunction from which the hallmarks of aging and metabolic diseases emerge and may lead to generalized organ failure states-such as diverse flavors of heart failure and dementia. Understanding and manipulating EMS may be key to preventing or reversing these dysfunctions.

Oxygen Supplementation and Hyperoxia in Critically Ill Cardiac Patients: From Pathophysiology to Clinical Practice

Oxygen supplementation has been a mainstay in the management of patients with acute cardiac disease. While hypoxia is known to be detrimental, the adverse effects of artificially high oxygen levels (hyperoxia) have only recently been recognized. Hyperoxia may induce harmful hemodynamic effects, including peripheral and coronary vasoconstriction, and direct cellular toxicity through the production of reactive oxygen species. In addition, emerging evidence has shown that hyperoxia is associated with adverse clinical outcomes. Thus, it is essential for the cardiac intensive care unit (CICU) clinician to understand the available evidence and titrate oxygen therapies to specific goals. This review summarizes the pathophysiology of oxygen within the cardiovascular system and the association between supplemental oxygen and hyperoxia in patients with common CICU diagnoses, including acute myocardial infarction, heart failure, shock, cardiac arrest, pulmonary hypertension, and respiratory failure. Finally, we highlight lessons learned from available trials, gaps in knowledge, and future directions.

The coronary vascular response to the metaboreflex at low-altitude and during acute and prolonged high-altitude in males

Myocardial oxygen delivery is primarily regulated through changes in vascular tone to match increased metabolic demands. In males, activation of the muscle metaboreflex during acute isocapnic hypoxia results in a paradoxical coronary vasoconstriction. Whether coronary blood velocity is reduced by metaboreflex activation following travel and/or adaptation to high-altitude is unknown. This study determined if the response of the coronary vasculature to muscle metaboreflex activation at low-altitude differs from acute (1/2 days) and prolonged (8/9 days) high-altitude. Healthy males (n=16) were recruited and performed isometric handgrip exercise (30 % max) followed by post-exercise circulatory occlusion (PECO) to isolate the muscle metaboreflex at low-altitude and following acute and prolonged high-altitude (3,800 m). Mean left anterior descending coronary artery blood velocity (LADv_mean, transthoracic Doppler echocardiography), heart rate, mean arterial pressure (MAP), ventilation, and respired gases were assessed during baseline and PECO at all time-points. Coronary vascular conductance index (CVC_i) was calculated as LAD_Vmean/MAP. The change in LADv_mean (acute altitude: -1.7 ± 3.9 cm/s, low-altitude: 2.6 ± 3.4 cm/s, P = 0.01) and CVC_i (acute altitude: -0.05 ± 0.04 cm/s/mmHg, low-altitude: -0.01 ± 0.03 cm/s/mmHg, P = 0.005) induced by PECO differed significantly between acute high-altitude and low-altitude. The change in LAD_Vmean and CVC_i induced by PECO following prolonged high-altitude was not different from low-altitude. Our results suggest that coronary vasoconstriction with metaboreflex activation in males is greatest following acute ascent to high-altitude and restored to low-altitude levels following 8-9 days of acclimatization.

Quantitative myocardial first-pass perfusion imaging of CO2 -induced vasodilation in rats

Inducible hypercapnia is an alternative for increasing the coronary blood flow necessary to facilitate the quantification of myocardial blood flow during hyperemia. The current study aimed to quantify the pharmacokinetic effect of a CO₂ gas challenge on myocardial perfusion in rats using high-resolution, first-pass perfusion CMR and compared it with pharmacologically induced hyperemia using regadenoson. A dual-contrast, saturation-recovery, gradient-echo sequence with a Cartesian readout was used on a small-animal 9.4-T scanner; additional cine images during hyperemia/rest were recorded with an ultrashort echo time sequence. The mean myocardial blood flow value at rest was 6.1 ± 1.4 versus 13.9 ± 3.7 and 14.3 ± 4 mL/g/min during vasodilation with hypercapnia and regadenoson, respectively. Accordingly, the myocardial flow reserve value was 2.6 ± 1.1 for the gas challenge and 2.5 ± 1.4 for regadenoson. During hyperemia with both protocols, a significantly increased cardiac output was found. It was concluded that hypercapnia leads to significantly increased coronary flow and yields similar myocardial flow reserves in healthy rats as compared with pharmacological stimulation. Accordingly, inducible hypercapnia can be selected as an alternative stressor in CMR studies of myocardial blood flow in small animals.

Biomarkers for oxidative stress and organ injury during Transnasal Humidified Rapid-Insufflation Ventilatory Exchange compared to mechanical ventilation in adults undergoing microlaryngoscopy: A randomised controlled study

BACKGROUND

Apnoeic oxygenation using Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE) during general anaesthesia prolongs the safe apnoeic period. However, there is a gap of knowledge how THRIVE-induced hyperoxia and hypercapnia impact vital organs. The primary aim of this randomised controlled trial was to characterise oxidative stress and, secondary, vital organ function biomarkers during THRIVE compared to mechanical ventilation (MV).

METHODS

Thirty adult patients, American Society of Anesthesiologists (ASA) 1-2, undergoing short laryngeal surgery under general anaesthesia were randomised to THRIVE, F_I O₂ 1.0, 70 L min^-1 during apnoea or MV. Blood biomarkers for oxidative stress, malondialdehyde and TAC and vital organ function were collected (A) preoperatively, (B) at procedure completion and (C) at PACU discharge.

RESULTS

Mean apnoea time was 17.9 (4.8) min and intubation to end-of-surgery time was 28.1 (12.8) min in the THRIVE and MV group, respectively. Malondialdehyde increased from 11.2 (3.1) to 12.7 (3.1) µM (P = .02) and from 9.5 (2.2) to 11.6 (2.6) µM (P = .003) (A to C) in the THRIVE and MV group, respectively. S100B increased from 0.05 (0.02) to 0.06 (0.02) µg L^-1 (P = .005) (A to C) in the THRIVE group. No increase in TAC, CRP, leukocyte count, troponin-T, NTproBNP, creatinine, eGFRcrea or NSE was demonstrated during THRIVE.

CONCLUSION

While THRIVE and MV was associated with increased oxidative stress, we found no change in cardiac, inflammation or kidney biomarkers during THRIVE. Further evaluation of stress and inflammatory response and cerebral and cardiac function during THRIVE is needed.

When appearances deceive: Echocardiographic changes due to common chest pathology

Most indications for performing echocardiography focus on the evaluation of properties intrinsic to the heart. However, numerous extra-cardiac conditions indirectly convey changes to the echocardiographic appearance through alterations in the governing physiology. Pulmonary embolism increases pulmonary arterial pressure if a sufficient cross-sectional area of the pulmonary vascular bed is occluded. This may result in dilatation of the right ventricle and, in severe cases, concomitant early diastolic septal collapse into the left ventricle. Acute respiratory failure has been shown to yield a similar echocardiographic appearance in experimental conditions due to the resultant pulmonary vasoconstriction. Echocardiography in the presence of pulmonary disease can reveal underlying cardiac pathologies such as pulmonary hypertension that contribute to the clinical severity of respiratory distress. Positive pressure ventilation affects preload, afterload, and compliance of both ventricles. The echocardiographic net result cannot be uniformly anticipated, but provides information on the deciding physiology or pathophysiology. Mediastinal pathology including tumors, herniation of abdominal content, and pleural effusion can often be visualized directly with echocardiography. Mediastinal pathologies adjacent to the heart may compress the myocardium directly, thus facilitating echocardiographic and clinical signs of tamponade in the absence of pericardial effusion. In conclusion, many pathologies of extra-cardiac origin influence the echocardiographic appearance of the heart. These changes do not reflect properties of the myocardium but may well be mistaken for it. Hence, these conditions are essential knowledge to all physicians performing echocardiography across the spectrum from advanced cardiological diagnostics to rapid point-of-care focused cardiac ultrasonography.

Influence of myocardial oxygen demand on the coronary vascular response to arterial blood gas changes in humans

It remains unclear if the human coronary vasculature is inherently sensitive to changes in arterial Po₂ and Pco₂ or if coronary vascular responses are the result of concomitant increases in myocardial O₂ consumption/demand ([Formula: see text]). We hypothesized that the coronary vascular response to Po₂ and Pco₂ would be attenuated in healthy men when [Formula: see text] was attenuated with β₁-adrenergic receptor blockade. Healthy men (age: 25 ± 1 yr, n = 11) received intravenous esmolol (β₁-adrenergic receptor antagonist) or volume-matched saline in a double-blind, randomized crossover study and were exposed to poikilocapnic hypoxia, isocapnic hypoxia, and hypercapnic hypoxia. Measurements made at baseline and after 5 min of steady state at each gas manipulation included left anterior descending coronary blood velocity (LAD_V; Doppler echocardiography), heart rate, and arterial blood pressure. LAD_V values at the end of each hypoxic condition were compared between esmolol and placebo. The rate-pressure product (RPP) and left ventricular mechanical energy (ME_LV) were calculated as indexes of [Formula: see text]. All gas manipulations augmented RPP, ME_LV, and LAD_V, but only RPP and ME_LV were attenuated (4-18%) after β₁-adrenergic receptor blockade ( P < 0.05). Despite attenuated RPP and ME_LV responses, β₁-adrenergic receptor blockade did not attenuate the mean LAD_V vasodilatory response compared with placebo during poikilocapnic hypoxia (29.4 ± 2.2 vs. 27.3 ± 1.6 cm/s) and isocapnic hypoxia (29.5 ± 1.5 vs. 30.3 ± 2.2 cm/s). Hypercapnic hypoxia elicited a feedforward coronary dilation that was blocked by β₁-adrenergic receptor blockade. These results indicate a direct influence of arterial Po₂ on coronary vascular regulation that is independent of [Formula: see text]. NEW & NOTEWORTHY In humans, arterial hypoxemia led to an increase in epicardial coronary artery blood velocity. β₁-Adrenergic receptor blockade did not diminish the hypoxemic coronary response despite reduced myocardial O₂ demand. These data indicate hypoxemia can regulate coronary blood flow independent of myocardial O₂ consumption. A plateau in the mean left anterior descending coronary artery blood velocity-rate-pressure product relationship suggested β₁-adrenergic receptor-mediated, feedforward epicardial coronary artery dilation. In addition, we observed a synergistic effect of Po₂ and Pco₂ during hypercapnic hypoxia.

Arterial CO2 as a Potent Coronary Vasodilator: A Preclinical PET/MR Validation Study with Implications for Cardiac Stress Testing

Myocardial blood flow (MBF) is the critical determinant of cardiac function. However, its response to increases in partial pressure of arterial CO₂ (PaCO₂), particularly with respect to adenosine, is not well characterized because of challenges in blood gas control and limited availability of validated approaches to ascertain MBF in vivo. Methods: By prospectively and independently controlling PaCO₂ and combining it with ¹³N-ammonia PET measurements, we investigated whether a physiologically tolerable hypercapnic stimulus (∼25 mm Hg increase in PaCO₂) can increase MBF to that observed with adenosine in 3 groups of canines: without coronary stenosis, subjected to non-flow-limiting coronary stenosis, and after preadministration of caffeine. The extent of effect on MBF due to hypercapnia was compared with adenosine. Results: In the absence of stenosis, mean MBF under hypercapnia was 2.1 ± 0.9 mL/min/g and adenosine was 2.2 ± 1.1 mL/min/g; these were significantly higher than at rest (0.9 ± 0.5 mL/min/g, P < 0.05) and were not different from each other (P = 0.30). Under left-anterior descending coronary stenosis, MBF increased in response to hypercapnia and adenosine (P < 0.05, all territories), but the effect was significantly lower than in the left-anterior descending coronary territory (with hypercapnia and adenosine; both P < 0.05). Mean perfusion defect volumes measured with adenosine and hypercapnia were significantly correlated (R = 0.85) and were not different (P = 0.12). After preadministration of caffeine, a known inhibitor of adenosine, resting MBF decreased; and hypercapnia increased MBF but not adenosine (P < 0.05). Conclusion: Arterial blood CO₂ tension when increased by 25 mm Hg can induce MBF to the same level as a standard dose of adenosine. Prospectively targeted arterial CO₂ has the capability to evolve as an alternative to current pharmacologic vasodilators used for cardiac stress testing.

The effects of graded changes in oxygen and carbon dioxide tension on coronary blood velocity independent of myocardial energy demand

In humans, coronary blood flow is tightly regulated by microvessels within the myocardium to match myocardial energy demand. However, evidence regarding inherent sensitivity of the microvessels to changes in arterial partial pressure of carbon dioxide and oxygen is conflicting because of the accompanied changes in myocardial energy requirements. This study aimed to investigate the changes in coronary blood velocity while manipulating partial pressures of end-tidal CO2 (Petco2) and O2 (Peto2). It was hypothesized that an increase in Petco2 (hypercapnia) or decrease in Peto2 (hypoxia) would result in a significant increase in mean blood velocity in the left anterior descending artery (LADVmean) due to an increase in both blood gases and energy demand associated with the concomitant cardiovascular response. Cardiac energy demand was assessed through noninvasive measurement of the total left ventricular mechanical energy. Healthy subjects (n = 13) underwent a euoxic CO2 test (Petco2 = -8, -4, 0, +4, and +8 mmHg from baseline) and an isocapnic hypoxia test (Peto2 = 64, 52, and 45 mmHg). LADVmean was assessed using transthoracic Doppler echocardiography. Hypercapnia evoked a 34.6 ± 8.5% (mean ± SE; P < 0.01) increase in mean LADVmean, whereas hypoxia increased LADVmean by 51.4 ± 8.8% (P < 0.05). Multiple stepwise regressions revealed that both mechanical energy and changes in arterial blood gases are important contributors to the observed changes in LADVmean (P < 0.01). In summary, regulation of the coronary vasculature in humans is mediated by metabolic changes within the heart and an inherent sensitivity to arterial blood gases.

Carbon Dioxide and the Heart: Physiology and Clinical Implications

Carbon dioxide (CO2) is an end product of aerobic cellular respiration. In healthy persons, PaCO2 is maintained by physiologic mechanisms within a narrow range (35-45 mm Hg). Both hypercapnia and hypocapnia are encountered in myriad clinical situations. In recent years, the number of hypercapnic patients has increased by the use of smaller tidal volumes to limit lung stretch and injury during mechanical ventilation, so-called permissive hypercapnia. A knowledge and appreciation of the effects of CO2 in the heart are necessary for optimal clinical management in the perioperative and critical care settings. This article reviews, from a historical perspective: (1) the effects of CO2 on coronary blood flow and the mechanisms underlying these effects; (2) the role of endogenously produced CO2 in metabolic control of coronary blood flow and the matching of myocardial oxygen supply to demand; and (3) the direct and reflexogenic actions of CO2 on myocardial contractile function. Clinically relevant issues are addressed, including the role of increased myocardial tissue PCO2 (PmCO2) in the decline in myocardial contractility during coronary hypoperfusion and the increased vulnerability to CO2-induced cardiac depression in patients receiving a β-adrenergic receptor antagonist or with otherwise compromised inotropic reserve. The potential use of real-time measurements of PmO2 to monitor the adequacy of myocardial perfusion in the perioperative period is discussed.

Assessment of myocardial reactivity to controlled hypercapnia with free-breathing T2-prepared cardiac blood oxygen level-dependent MR imaging

PURPOSE

To examine whether controlled and tolerable levels of hypercapnia may be an alternative to adenosine, a routinely used coronary vasodilator, in healthy human subjects and animals.

MATERIALS AND METHODS

Human studies were approved by the institutional review board and were HIPAA compliant. Eighteen subjects had end-tidal partial pressure of carbon dioxide (PetCO2) increased by 10 mm Hg, and myocardial perfusion was monitored with myocardial blood oxygen level-dependent (BOLD) magnetic resonance (MR) imaging. Animal studies were approved by the institutional animal care and use committee. Anesthetized canines with (n = 7) and without (n = 7) induced stenosis of the left anterior descending artery (LAD) underwent vasodilator challenges with hypercapnia and adenosine. LAD coronary blood flow velocity and free-breathing myocardial BOLD MR responses were measured at each intervention. Appropriate statistical tests were performed to evaluate measured quantitative changes in all parameters of interest in response to changes in partial pressure of carbon dioxide.

RESULTS

Changes in myocardial BOLD MR signal were equivalent to reported changes with adenosine (11.2% ± 10.6 [hypercapnia, 10 mm Hg] vs 12% ± 12.3 [adenosine]; P = .75). In intact canines, there was a sigmoidal relationship between BOLD MR response and PetCO2 with most of the response occurring over a 10 mm Hg span. BOLD MR (17% ± 14 [hypercapnia] vs 14% ± 24 [adenosine]; P = .80) and coronary blood flow velocity (21% ± 16 [hypercapnia] vs 26% ± 27 [adenosine]; P > .99) responses were similar to that of adenosine infusion. BOLD MR signal changes in canines with LAD stenosis during hypercapnia and adenosine infusion were not different (1% ± 4 [hypercapnia] vs 6% ± 4 [adenosine]; P = .12).

CONCLUSION

Free-breathing T2-prepared myocardial BOLD MR imaging showed that hypercapnia of 10 mm Hg may provide a cardiac hyperemic stimulus similar to adenosine.

Cerebral and myocardial blood flow responses to hypercapnia and hypoxia in humans

In humans, cerebrovascular responses to alterations in arterial Pco2 and Po2 are well documented. However, few studies have investigated human coronary vascular responses to alterations in blood gases. This study investigated the extent to which the cerebral and coronary vasculatures differ in their responses to euoxic hypercapnia and isocapnic hypoxia in healthy volunteers. Participants ( n = 15) were tested at rest on two occasions. On the first visit, middle cerebral artery blood velocity ( V̄P) was assessed using transcranial Doppler ultrasound. On the second visit, coronary sinus blood flow (CSBF) was measured using cardiac MRI. For comparison with V̄P, CSBF was normalized to the rate pressure product [an index of myocardial oxygen consumption; normalized (n)CSBF]. Both testing sessions began with 5 min of euoxic [end-tidal Po2 (PetO2) = 88 Torr] isocapnia [end-tidal Pco2 (PetCO2) = +1 Torr above resting values]. PetO2 was next held at 88 Torr, and PetCO2 was increased to 40 and 45 Torr in 5-min increments. Participants were then returned to euoxic isocapnia for 5 min, after which PetO2 was decreased from 88 to 60, 52 and 45 Torr in 5-min decrements. Changes in V̄P and nCSBF were normalized to isocapnic euoxic conditions and indexed against PetCO2 and arterial oxyhemoglobin saturation. The V̄P gain for euoxic hypercapnia (%/Torr) was significantly higher than nCSBF ( P = 0.030). Conversely, the V̄P gain for isocapnic hypoxia (%/%desaturation) was not different from nCSBF ( P = 0.518). These findings demonstrate, compared with coronary circulation, that the cerebral circulation is more sensitive to hypercapnia but similarly sensitive to hypoxia.

Coronary blood flow responses to physiological stress in humans

Animal reports suggest that reflex activation of cardiac sympathetic nerves can evoke coronary vasoconstriction. Conversely, physiological stress may induce coronary vasodilation to meet an increased metabolic demand. Whether the sympathetic nervous system can modulate coronary vasomotor tone in response to stress in humans is unclear. Coronary blood velocity (CBV), an index of coronary blood flow, can be measured in humans by noninvasive duplex ultrasound. We studied 11 healthy volunteers and measured beat-by-beat changes in CBV, blood pressure, and heart rate during 1) static handgrip for 20 s at 10% and 70% of maximal voluntary contraction; 2) lower body negative pressure at -10 and -30 mmHg for 3 min each; 3) cold pressor test for 90 s; and 4) hypoxia (10% O(2)), hyperoxia (100% O(2)), and hypercapnia (5% CO(2)) for 5 min each. At the higher level of handgrip, mean blood pressure increased (P < 0.001), whereas CBV did not change [P = not significant (NS)]. In addition, during lower body negative pressure, CBV decreased (P < 0.02; and P < 0.01, for -10 and -30 mmHg, respectively), whereas blood pressure did not change (P = NS). The dissociation between the responses of CBV and blood pressure to handgrip and lower body negative pressure is consistent with coronary vasoconstriction. During hypoxia, CBV increased (P < 0.02) and decreased during hyperoxia (P < 0.01), although blood pressure did not change (P = NS), suggesting coronary vasodilation during hypoxia and vasoconstriction during hyperoxia. In contrast, concordant increases in CBV and blood pressure were noted during the cold pressor test, and hypercapnia had no effects on either parameter. Thus the physiological stress known to be associated with sympathetic activation can produce coronary vasoconstriction in humans. Contrasting responses were noted during systemic hypoxia and hyperoxia where mechanisms independent of autonomic influences appear to dominate the vascular end-organ effects.