Exercise response after rapid intravenous infusion of saline in healthy humans

Evaluation of long-term sequelae by cardiopulmonary exercise testing 12 months after hospitalization for severe COVID-19

BACKGROUND

Cardiopulmonary exercise testing (CPET) is an important clinical tool that provides a global assessment of the respiratory, circulatory and metabolic responses to exercise which are not adequately reflected through the measurement of individual organ system function at rest. In the context of critical COVID-19, CPET is an ideal approach for assessing long term sequelae.

METHODS

In this prospective single-center study, we performed CPET 12 months after symptom onset in 60 patients that had required intensive care unit treatment for a severe COVID-19 infection. Lung function at rest and chest computed tomography (CT) scan were also performed.

RESULTS

Twelve months after severe COVID-19 pneumonia, dyspnea was the most frequently reported symptom although only a minority of patients had impaired respiratory function at rest. Mild ground-glass opacities, reticulations and bronchiectasis were the most common CT scan abnormalities. The majority of the patients (80%) had a peak O₂ uptake (V'O₂) considered within normal limits (median peak predicted O₂ uptake (V'O₂) of 98% [87.2-106.3]). Length of ICU stay remained an independent predictor of V'O₂. More than half of the patients with a normal peak predicted V'O₂ showed ventilatory inefficiency during exercise with an abnormal increase of physiological dead space ventilation (VD/Vt) (median VD/VT of 0.27 [0.21-0.32] at anaerobic threshold (AT) and 0.29 [0.25-0.34] at peak) and a widened median peak alveolar-arterial gradient for O₂ (35.2 mmHg [31.2-44.8]. Peak PetCO₂ was significantly lower in subjects with an abnormal increase of VD/Vt (p = 0.001). Impairments were more pronounced in patients with dyspnea. Peak VD/Vt values were positively correlated with peak D-Dimer plasma concentrations from blood samples collected during ICU stay (r² = 0.12; p = 0.02) and to predicted diffusion capacity of the lung for carbon monoxide (D_LCO) (r² =  - 0.15; p = 0.01).

CONCLUSIONS

Twelve months after severe COVID-19 pneumonia, most of the patients had a peak V'O₂ considered within normal limits but showed ventilatory inefficiency during exercise with increased dead space ventilation that was more pronounced in patients with persistent dyspnea.

TRIAL REGISTRATION

NCT04519320 (19/08/2020).

Effect of Slower vs Faster Intravenous Fluid Bolus Rates on Mortality in Critically Ill Patients: The BaSICS Randomized Clinical Trial

IMPORTANCE

Slower intravenous fluid infusion rates could reduce the formation of tissue edema and organ dysfunction in critically ill patients; however, there are no data to support different infusion rates during fluid challenges for important outcomes such as mortality.

OBJECTIVE

To determine the effect of a slower infusion rate vs control infusion rate on 90-day survival in patients in the intensive care unit (ICU).

DESIGN, SETTING, AND PARTICIPANTS

Unblinded randomized factorial clinical trial in 75 ICUs in Brazil, involving 11 052 patients requiring at least 1 fluid challenge and with 1 risk factor for worse outcomes were randomized from May 29, 2017, to March 2, 2020. Follow-up was concluded on October 29, 2020. Patients were randomized to 2 different infusion rates (reported in this article) and 2 different fluid types (balanced fluids or saline, reported separately).

INTERVENTIONS

Patients were randomized to receive fluid challenges at 2 different infusion rates; 5538 to the slower rate (333 mL/h) and 5514 to the control group (999 mL/h). Patients were also randomized to receive balanced solution or 0.9% saline using a factorial design.

MAIN OUTCOMES AND MEASURES

The primary end point was 90-day survival.

RESULTS

Of all randomized patients, 10 520 (95.2%) were analyzed (mean age, 61.1 years [SD, 17.0 years]; 44.2% were women) after excluding duplicates and consent withdrawals. Patients assigned to the slower rate received a mean of 1162 mL on the first day vs 1252 mL for the control group. By day 90, 1406 of 5276 patients (26.6%) in the slower rate group had died vs 1414 of 5244 (27.0%) in the control group (adjusted hazard ratio, 1.03; 95% CI, 0.96-1.11; P = .46). There was no significant interaction between fluid type and infusion rate (P = .98).

CONCLUSIONS AND RELEVANCE

Among patients in the intensive care unit requiring fluid challenges, infusing at a slower rate compared with a faster rate did not reduce 90-day mortality. These findings do not support the use of a slower infusion rate.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02875873.

Minute ventilation/carbon dioxide production in chronic heart failure

In chronic heart failure, minute ventilation (V'_E) for a given carbon dioxide production (V'_CO2 ) might be abnormally high during exercise due to increased dead space ventilation, lung stiffness, chemo- and metaboreflex sensitivity, early metabolic acidosis and abnormal pulmonary haemodynamics. The V'_E versus V'_CO2 relationship, analysed either as ratio or as slope, enables us to evaluate the causes and entity of the V'_E/perfusion mismatch. Moreover, the V'_E axis intercept, i.e. when V'_CO2 is extrapolated to 0, embeds information on exercise-induced dead space changes, while the analysis of end-tidal and arterial CO₂ pressures provides knowledge about reflex activities. The V'_E versus V'_CO2 relationship has a relevant prognostic power either alone or, better, when included within prognostic scores. The V'_E versus V'_CO2 slope is reported as an absolute number with a recognised cut-off prognostic value of 35, except for specific diseases such as hypertrophic cardiomyopathy and idiopathic cardiomyopathy, where a lower cut-off has been suggested. However, nowadays, it is more appropriate to report V'_E versus V'_CO2 slope as percentage of the predicted value, due to age and gender interferences. Relevant attention is needed in V'_E versus V'_CO2 analysis in the presence of heart failure comorbidities. Finally, V'_E versus V'_CO2 abnormalities are relevant targets for treatment in heart failure.

Pulmonary Limitations in Heart Failure

The heart and lungs are intimately linked. Hence, impaired function of one organ may lead to changes in the other. Accordingly, heart failure is associated with airway obstruction, loss of lung volume, impaired gas exchange, and abnormal ventilatory control. Cardiopulmonary exercise testing is an excellent tool for evaluation of gas exchange and ventilatory control. Indeed, many parameters routinely measured during cardiopulmonary exercise testing, including the level of minute ventilation per unit of carbon dioxide production and the presence of exercise oscillatory ventilation, have been found to be strongly associated with prognosis in patients with heart failure.

Increased physiological dead space at exercise is a marker of mild pulmonary or cardiovascular disease in dyspneic subjects

Background: The characteristics of cardiopulmonary exercise testing (CPET)-derived parameters for the differential diagnosis of exertional dyspnea are not well known.

Objectives: We hypothesized that increased physiological dead space ventilation (VD/Vt) is a marker for mild pulmonary or cardiovascular disease in patients with exertional dyspnea.

Design: We used receiver operating characteristic analysis to determine the performance of individual CPET parameters for identifying subjects with either mild pulmonary or cardiovascular disease, among 77 subjects with mild-to-moderate exertional dyspnea (modified Medical Research Council scale 1–2).

Results: In comparison with subjects without disease, subjects with pulmonary disease (n = 31) had higher VE/V′CO₂ slope, higher VD/Vt, and lower ventilatory reserve. Subjects with cardiovascular disease (n = 14) had lower heart rate and cardiovascular double product and higher VD/Vt at peak exercise. At a threshold of 28%, the sensitivity and specificity of VD/Vt at peak exercise for identifying pulmonary or cardiovascular disease were 89% (95% CI: 64–98%) and 72% (95% CI: 46–89%), respectively.

Conclusions: Increased physiological VD/Vt at exercise is a sensitive and specific marker of mild pulmonary or cardiovascular disease in dyspneic subjects.

Surfactant proteins changes after acute hemodynamic improvement in patients with advanced chronic heart failure treated with Levosimendan

Alveolar-capillary membrane evaluated by carbon monoxide diffusion (DLCO) plays an important role in heart failure (HF). Surfactant Proteins (SPs) have also been suggested as a worthwhile marker. In HF, Levosimendan improves pulmonary hemodynamics and reduces lung fluids but associated SPs and DLCO changes are unknown. Sixty-five advanced HF patients underwent spirometry, cardiopulmonary exercise test (CPET) and SPs determination before and after Levosimendan. Levosimendan caused natriuretic peptide-B (BNP) reduction, peakVO₂ increase and VE/VCO₂ slope reduction. Spirometry improved but DLCO did not. SP-A, SP-D and immature SP-B reduced (73.7 ± 25.3 vs. 66.3 ± 22.7 ng/mL*, 247 ± 121 vs. 223 ± 110 ng/mL*, 39.4 ± 18.7 vs. 34.4 ± 17.9AU*, respectively); while mature SP-B increased (424 ± 218 vs. 461 ± 243 ng/mL, * = p < 0.001). Spirometry, BNP and CPET changes suggest hemodynamic improvement and lung fluid reduction. SP-A, SP-D and immature SP-B reduction indicates a reduction of inflammatory stress; conversely mature SP-B increase suggests alveolar cell function restoration. In conclusion, acute lung fluid reduction is associated with SPs but not DLCO changes. SPs are fast responders to alveolar-capillary membrane condition changes.

The haemodynamic effects of bolus versus slower infusion of intravenous crystalloid in healthy volunteers

PURPOSE

This pilot study aimed to characterise the haemodynamic effect of 1L of IV normal saline (NS) administered as a rapid versus slow infusion on cardiac output (CO), heart rate (HR), systemic blood pressures, and carotid blood flow in six healthy volunteers.

MATERIALS AND METHODS

Six healthy male volunteers aged 18-65years were randomized to receive 1L NS given over 30min or 120min. On a subsequent study session the alternate fluid regimen was administered. Haemodynamic data was gathered using a non-invasive finger arterial pressure monitor (Nexfin®), echocardiography and carotid duplex sonography. Time to micturition and urine volume was also assessed.

RESULTS

Compared to baseline, rapid infusion of 1L of saline over 30min produced a fall in Nexfin®-measured CO by 0.62L/min (p<0.001), whereas there was a marginal but significant increase during infusion of 1L NS over 120min of 0.02L/min (p<0.001). This effect was mirrored by changes in HR and blood pressure (BP) (p<0.001). There were no significant changes in carotid blood flow, time to micturition, or urine volume produced.

CONCLUSIONS

Slower infusion of 1L NS in healthy male volunteers produced a greater increase in CO, HR and BP than rapid infusion.

Diving and pulmonary physiology: Surfactant binding protein, lung fluid and cardiopulmonary test changes in professional divers

Alteration of breathing pattern ranging from an increase of respiratory rate to overt hyperventilation during and after SCUBA diving is frequently reported and is associated with intrathoracic fluid overload. This study was undertaken to assess breathing efficiency after diving and the association with damage of alveolar cells. Ventilation efficiency (VE/VCO₂) during maximal cardiopulmonary exercise test (CPET) before and 2h after a standard protocol dive has been analyzed in twelve professional males divers (39.5±10.5years). Furthermore, within 30min from surfacing, subjects underwent blood sample for surfactant derived proteins (SPs) determination, while thoracic ultrasound was performed at 30, 60, 90 and 120min. Dive consisted in a single quick descend to 18m of sea water, a 47min bottom stay and a direct ascent to the surface. CPET showed a preserved exercise performance with an increase of VE/VCO₂ after diving (21.4±2.9 vs. 22.9±3.3, p<0.05). Mature SP-B increased while other SPs were unchanged. Ultrasound lung comets (ULC) were high in the first post-dive evaluation with a significant, but not complete, progressive reduction at 120min after surfacing. In conclusion we showed that, after a single dive, lung fluid increased with an increase of ventilation inefficiency and of the mature form of SP-B.

Effect of β2-adrenergic receptor stimulation on lung fluid in stable heart failure patients

BACKGROUND

The purpose of this study was to determine: (1) whether stable heart failure patients with reduced ejection fraction (HFrEF) have elevated extravascular lung water (EVLW) when compared with healthy control subjects; and (2) the effect of acute β₂-adrenergic receptor (β₂AR) agonist inhalation on lung fluid balance.

METHODS

Twenty-two stable HFrEF patients and 18 age- and gender-matched healthy subjects were studied. Lung diffusing capacity for carbon monoxide (DLCO), alveolar-capillary membrane conductance (Dm_CO), pulmonary capillary blood volume (V_c) (via re-breathe) and lung tissue volume (V_tis) (via computed tomography) were assessed before and within 30 minutes after administration of nebulized albuterol. EVLW was derived as V_tis - V_c.

RESULTS

Before administration of albuterol, V_tis and EVLW were higher in HFrEF vs control (998 ± 200 vs 884 ± 123 ml, p = 0.041; and 943 ± 202 vs 802 ± 133 ml, p = 0.015, respectively). Albuterol decreased V_tis and EVLW in HFrEF patients (-4.6 ± 7.8%, p = 0.010; -4.6 ± 8.8%, p = 0.018) and control subjects (-2.8 ± 4.9%, p = 0.029; -3.0 ± 5.7%, p = 0.045). There was an inverse relationship between pre-albuterol values and pre- to post-albuterol change for EVLW (r² = -0.264, p = 0.015) and Dm_CO (r² = -0.343, p = 0.004) in HFrEF only.

CONCLUSION

Lung fluid is elevated in stable HFrEF patients relative to healthy subjects. Stimulation of β₂ARs may cause fluid removal in HFrEF, especially in patients with greater evidence of increased lung water at baseline.

Exertional dyspnoea in chronic heart failure: the role of the lung and respiratory mechanical factors

Exertional dyspnoea is among the dominant symptoms in patients with chronic heart failure and progresses relentlessly as the disease advances, leading to reduced ability to function and engage in activities of daily living. Effective management of this disabling symptom awaits a better understanding of its underlying physiology.

Cardiovascular factors are believed to play a major role in dyspnoea in heart failure patients. However, despite pharmacological interventions, such as vasodilators or inotropes that improve central haemodynamics, patients with heart failure still complain of exertional dyspnoea. Clearly, dyspnoea is not determined by cardiac factors alone, but likely depends on complex, integrated cardio-pulmonary interactions.

A growing body of evidence suggests that excessively increased ventilatory demand and abnormal “restrictive” constraints on tidal volume expansion with development of critical mechanical limitation of ventilation, contribute to exertional dyspnoea in heart failure. This article will offer new insights into the pathophysiological mechanisms of exertional dyspnoea in patients with chronic heart failure by exploring the potential role of the various constituents of the physiological response to exercise and particularly the role of abnormal ventilatory and respiratory mechanics responses to exercise in the perception of dyspnoea in patients with heart failure.

Surfactant protein B: From biochemistry to its potential role as diagnostic and prognostic marker in heart failure

Growing interest raised on circulating biomarkers of structural alveolar-capillary unit damage and very recent data support surfactant protein type B (SP-B) as the most promising candidate in this setting. With respect to other proteins proposed as possible markers of lung damage, SP-B has some unique qualities: it is critical for the assembly of pulmonary surfactant, making its lack incompatible with life; it has no other known site of synthesis except alveolar epithelial cells different from other surfactant proteins; and, it undergoes a proteolytic processing in a pulmonary-cell-specific manner. In the recent years circulating SP-B isoforms, mature or immature, have been demonstrated to be detectable in the circulation depending on the magnitude of the damage of alveolar capillary membrane. In the present review, we summarize the recent knowledge on SP-B regulation, function and we discuss its potential role as reliable biological marker of alveolar capillary membrane (dys)function in the context of heart failure.

Altered breathing mechanics and ventilatory response during exercise in children born extremely preterm

Background

Extreme preterm birth confers risk of long-term impairments in lung function and exercise capacity. There are limited data on the factors contributing to exercise limitation following extreme preterm birth. This study examined respiratory mechanics and ventilatory response during exercise in a large cohort of children born extremely preterm (EP).

Methods

This cohort study included children 8–12 years of age who were born EP (≤28 weeks gestation) between 1997 and 2004 and treated in a large regionalised neonatal intensive care unit in western Canada. EP children were divided into no/mild bronchopulmonary dysplasia (BPD) (ie, supplementary oxygen or ventilation ceased before 36 weeks gestational age; n=53) and moderate/severe BPD (ie, continued supplementary oxygen or ventilation at 36 weeks gestational age; n=50). Age-matched control children (n=65) were born at full term. All children attempted lung function and cardiopulmonary exercise testing measurements.

Results

Compared with control children, EP children had lower airway flows and diffusion capacity but preserved total lung capacity. Children with moderate/severe BPD had evidence of gas trapping relative to other groups. The mean difference in exercise capacity (as measured by oxygen uptake (VO₂)% predicted) in children with moderate/severe BPD was −18±5% and −14±5.0% below children with no/mild BPD and control children, respectively. Children with moderate/severe BPD demonstrated a potentiated ventilatory response and greater prevalence of expiratory flow limitation during exercise compared with other groups. Resting lung function did not correlate with exercise capacity.

Conclusions

Expiratory flow limitation and an exaggerated ventilatory response contribute to respiratory limitation to exercise in children born EP with moderate/severe BPD.

Bolus intravenous 0.9% saline, but not 4% albumin or 5% glucose, causes interstitial pulmonary edema in healthy subjects

Rapid intravenous (iv) infusion of 0.9% saline alters respiratory mechanics in healthy subjects. However, the relative cardiovascular and respiratory effects of bolus iv crystalloid vs. colloid are unknown. Six healthy male volunteers were given 30 ml/kg iv 0.9% saline, 4% albumin, and 5% glucose at a rate of 100 ml/min on 3 separate days in a double-blinded, randomized crossover study. Impulse oscillometry, spirometry, lung volumes, diffusing capacity (DLCO), and blood samples were measured before and after fluid administration. Lung ultrasound B-line score (indicating interstitial pulmonary edema) and Doppler echocardiography indices of cardiac preload were measured before, midway, immediately after, and 1 h after fluid administration. Infusion of 0.9% saline increased small airway resistance at 5 Hz (P = 0.04) and lung ultrasound B-line score (P = 0.01) without changes in Doppler echocardiography measures of preload. In contrast, 4% albumin increased DLCO, decreased lung volumes, and increased the Doppler echocardiography mitral E velocity (P = 0.001) and E-to-lateral/septal e' ratio, estimated blood volume, and N-terminal pro B-type natriuretic peptide (P = 0.01) but not lung ultrasound B-line score, consistent with increased pulmonary blood volume without interstitial pulmonary edema. There were no significant changes with 5% glucose. Plasma angiopoietin-2 concentration increased only after 0.9% saline (P = 0.001), suggesting an inflammatory mechanism associated with edema formation. In healthy subjects, 0.9% saline and 4% albumin have differential pulmonary effects not attributable to passive fluid filtration. This may reflect either different effects of these fluids on active signaling in the pulmonary circulation or a protective effect of albumin.

Dopamine receptor blockade improves pulmonary gas exchange but decreases exercise performance in healthy humans

Pulmonary gas exchange, as evaluated by the alveolar-arterial oxygen difference (A-aDO2), is impaired during intense exercise, and has been correlated with recruitment of intrapulmonary arteriovenous anastomoses (IPAVA) as measured by agitated saline contrast echocardiography. Previous work has shown that dopamine (DA) recruits IPAVA and increases venous admixture (Q̇s/Q̇t) at rest. As circulating DA increases during exercise, we hypothesized that A-aDO2 and IPAVA recruitment would be decreased with DA receptor blockade. Twelve healthy males (age: 25 ± 6 years, V̇O2 max : 58.6 ± 6.5 ml kg(-1) min(-1) ) performed two incremental staged cycling exercise sessions after ingestion of either placebo or a DA receptor blocker (metoclopramide 20 mg). Arterial blood gas, cardiorespiratory and IPAVA recruitment (evaluated by agitated saline contrast echocardiography) data were obtained at rest and during exercise up to 85% of V̇O2 max . On different days, participants also completed incremental exercise tests and exercise tolerance (time-to-exhaustion (TTE) at 85% of V̇O2 max ) with or without dopamine blockade. Compared to placebo, DA blockade did not change O2 consumption, CO2 production, or respiratory exchange ratio at any intensity. At 85% V̇O2 max , DA blockade decreased A-aDO2, increased arterial O2 saturation and minute ventilation, but did not reduce IPAVA recruitment, suggesting that positive saline contrast is unrelated to A-aDO2. Compared to placebo, DA blockade decreased maximal cardiac output, V̇O2 max and TTE. Despite improving pulmonary gas exchange, blocking dopamine receptors appears to be detrimental to exercise performance. These findings suggest that endogenous dopamine is important to the normal cardiopulmonary response to exercise and is necessary for optimal high-intensity exercise performance.

Effects of blood transfusion on exercise capacity in thalassemia major patients

Anemia has an important role in exercise performance. However, the direct link between rapid changes of hemoglobin and exercise performance is still unknown.To find out more on this topic, we studied 18 beta-thalassemia major patients free of relevant cardiac dysfunction (age 33.5±7.2 years,males = 10). Patients performed a maximal cardiopulmolmonary exercise test (cycloergometer, personalized ramp protocol, breath-by-breath measurements of expired gases) before and the day after blood transfusion (500 cc of red cell concentrates). After blood transfusion, hemoglobin increased from 10.5±0.8 g/dL to 12.1±1.2 (p<0.001), peak VO₂ from 1408 to 1546mL/min (p<0.05), and VO₂ at anaerobic threshold from 965 to 1024mL/min (p<0.05). No major changes were observed as regards heart and respiratory rates either at peak exercise or at anaerobic threshold. Similarly, no relevant changes were observed in ventilation efficiency, as evaluated by the ventilation vs. carbon dioxide production relationship, or in O₂ delivery to the periphery as analyzed by the VO₂ vs. workload relationship. The relationship between hemoglobin and VO₂ changes showed, for each g/dL of hemoglobin increase, a VO₂ increase = 82.5 mL/min and 35 mL/min, at peak exercise and at anaerobic threshold, respectively. In beta-thalassemia major patients, an acute albeit partial anemia correction by blood transfusion determinates a relevant increase of exercise performance, observed both at peak exercise and at anaerobic threshold.

Acute volume loading and exercise capacity in postural tachycardia syndrome

Postural tachycardia syndrome (POTS) is associated with exercise intolerance, hypovolemia, and cardiac atrophy, which may contribute to reduced stroke volume and compensatory exaggerated heart rate (HR) increases. Acute volume loading with intravenous (iv) saline reduces HR and improves orthostatic tolerance and symptoms in POTS, but its effect on exercise capacity is unknown. In this study, we determined the effect of iv saline infusion on peak exercise capacity (VO2peak) in POTS. Nineteen patients with POTS participated in a sequential study. VO2peak was measured on two separate study days, following administration of placebo or 1 liter of i.v. saline (NaCl 0.9%). Patients exercised on a semirecumbent bicycle with resistance increased by 25 W every 2 min until maximal effort was achieved. Patients exhibited blood volume deficits (-13.4 ± 1.4% ideal volume), consistent with mild to moderate hypovolemia. At baseline, saline significantly increased stroke volume (saline 80 ± 8 ml vs. placebo 64 ± 4 ml; P = 0.010), increased cardiac output (saline 6.9 ± 0.5 liter/min vs. placebo 5.7 ± 0.2 liter/min; P = 0.021), and reduced systemic vascular resistance (saline 992.6 ± 70.0 dyn-s/cm(5) vs. placebo 1,184.0 ± 50.8 dyn-s/cm(5); P = 0.011), with no effect on HR or blood pressure. During exercise, saline did not produce differences in VO2peak (saline 26.3 ± 1.2 mg·kg(-1)·min(-1) vs. placebo 27.7 ± 1.8 mg·kg(-1)·min(-1); P = 0.615), peak HR [saline 174 ± 4 beats per minute (bpm) vs. placebo 175 ± 3 bpm; P = 0.672] or other cardiovascular parameters. These findings suggest that acute volume loading with saline does not improve VO2peak or cardiovascular responses to exercise in POTS, despite improvements in resting hemodynamic function.

High-altitude pulmonary edema

High-altitude pulmonary edema (HAPE), a not uncommon form of acute altitude illness, can occur within days of ascent above 2500 to 3000 m. Although life-threatening, it is avoidable by slow ascent to permit acclimatization or with drug prophylaxis. The critical pathophysiology is an excessive rise in pulmonary vascular resistance or hypoxic pulmonary vasoconstriction (HPV) leading to increased microvascular pressures. The resultant hydrostatic stress causes dynamic changes in the permeability of the alveolar capillary barrier and mechanical injurious damage leading to leakage of large proteins and erythrocytes into the alveolar space in the absence of inflammation. Bronchoalveolar lavage and hemodynamic pressure measurements in humans confirm that elevated capillary pressure induces a high-permeability noninflammatory lung edema. Reduced nitric oxide availability and increased endothelin in hypoxia are the major determinants of excessive HPV in HAPE-susceptible individuals. Other hypoxia-dependent differences in ventilatory control, sympathetic nervous system activation, endothelial function, and alveolar epithelial active fluid reabsorption likely contribute additionally to HAPE susceptibility. Recent studies strongly suggest nonuniform regional hypoxic arteriolar vasoconstriction as an explanation for how HPV occurring predominantly at the arteriolar level causes leakage. In areas of high blood flow due to lesser HPV, edema develops due to pressures that exceed the dynamic and structural capacity of the alveolar capillary barrier to maintain normal fluid balance. This article will review the pathophysiology of the vasculature, alveolar epithelium, innervation, immune response, and genetics of the lung at high altitude, as well as therapeutic and prophylactic strategies to reduce the morbidity and mortality of HAPE.

Gas exchange consequences of left heart failure

This review explores the pathophysiology of gas exchange abnormalities arising consequent to either acute or chronic elevation of pulmonary venous pressures. The initial experimental studies of acute pulmonary edema outlined the sequence of events from lymphatic congestion with edema fluid to frank alveolar flooding and its resultant hypoxemia. Clinical studies of acute heart failure (HF) suggested that hypoxemia was associated only with the final stage of alveolar flooding. However, in patients with chronic heart failure and normal oxygenation, hypoxemia could be produced by the administration of potent pulmonary vasodilators, suggesting that hypoxic pulmonary vasoconstriction is an important reflex for these patients. Patients with chronic left HF commonly manifest a reduced diffusing capacity, an abnormality that appears to be a consequence of chronic elevation of left atrial pressure. That reduction in diffusing capacity does not appear to be primarily attributable to increases in lung water but is improved by any sustained treatment that improves overall cardiac function. Patients with heart failure may also manifest an abnormally elevated VE/VCO2 during exercise, and that exercise ventilation abnormality arises as a consequence of both alveolar hyperventilation and elevated physiologic dead space. That elevated exercise VE/VCO2 in an HF patient has proven to be a powerful predictor of an adverse outcome and hence it has received sustained attention in the HF literature. At least three of the classes of drugs used to treat HF will normalize the exercise VE/VCO2, suggesting that the excessive ventilation response may be linked to elevated sympathetic activity.

Performance benefits of rehydration with intravenous fluid and oral glycerol

PURPOSE

Intravenous (IV) saline has been used by athletes attempting to accelerate rehydration procedures. The diuresis from IV rehydration may be circumvented through the concomitant use of oral glycerol. We aimed to examine the effects of rehydrating with four different regimens of IV fluid and oral glycerol on subsequent 40-km cycling time trial performance.

METHODS

Nine endurance-trained men were dehydrated by 4% bodyweight via exercise in the heat. They then rehydrated with 150% of the fluid lost via four protocols using a randomized crossover design: 1) oral = sports drink and water; 2) oral glycerol = sports drink, water, and glycerol; 3) IV = half as normal saline, half of sports drink, and water; and 4) IV with oral glycerol = half as normal saline, half as sports drink, water, and glycerol. After this, they completed a 40-km cycling performance test in the heat.

RESULTS

Compared with oral rehydration, there were significant performance benefits (P < 0.05) when rehydrating with oral glycerol (improved time to complete 40 km by 3.7%), IV (3.5%), and IV with oral glycerol (4.1%). Plasma volume restoration was highest in IV with oral glycerol, then IV, then oral glycerol, then oral (P < 0.01 for all of these comparisons). There were no differences in HR, tympanic/skin temperatures, sweat rate, blood lactate concentration, thermal stress, or RPE between groups.

CONCLUSIONS

Combining IV fluid with oral glycerol resulted in the greatest fluid retention; however, it did not improve exercise performance compared with either modality alone.

Role of alveolar β2-adrenergic receptors on lung fluid clearance and exercise ventilation in healthy humans

Background

In experimental conditions alveolar fluid clearance is controlled by alveolar β₂-adrenergic receptors. We hypothesized that if this occurs in humans, then non-selective β-blockers should reduce the membrane diffusing capacity (D_M), an index of lung interstitial fluid homeostasis. Moreover, we wondered whether this effect is potentiated by saline solution infusion, an intervention expected to cause interstitial lung edema. Since fluid retention within the lungs might trigger excessive ventilation during exercise, we also hypothesized that after the β₂-blockade ventilation increased in excess to CO₂ output and this was further enhanced by interstitial edema.

Methods and Results

22 healthy males took part in the study. On day 1, spirometry, lung diffusion for carbon monoxide (DLCO) including its subcomponents D_M and capillary volume (V_Cap), and cardiopulmonary exercise test were performed. On day 2, these tests were repeated after rapid 25 ml/kg saline infusion. Then, in random order 11 subjects were assigned to oral treatment with Carvedilol (CARV) and 11 to Bisoprolol (BISOPR). When heart rate fell at least by 10 beats·min⁻¹, the tests were repeated before (day 3) and after saline infusion (day 4). CARV but not BISOPR, decreased D_M (−13±7%, p = 0.001) and increased V_Cap (+20±22%, p = 0.016) and VE/VCO₂ slope (+12±8%, p<0.01). These changes further increased after saline: −18±13% for D_M (p<0.01), +44±28% for V_Cap (p<0.001), and +20±10% for VE/VCO₂ slope (p<0.001).

Conclusions

These findings support the hypothesis that in humans in vivo the β₂-alveolar receptors contribute to control alveolar fluid clearance and that interstitial lung fluid may trigger exercise hyperventilation.

Pulmonary gas exchange and acid-base balance during exercise

As the first step in the oxygen-transport chain, the lung has a critical task: optimizing the exchange of respiratory gases to maintain delivery of oxygen and the elimination of carbon dioxide. In healthy subjects, gas exchange, as evaluated by the alveolar-to-arterial PO2 difference (A-aDO2), worsens with incremental exercise, and typically reaches an A-aDO2 of approximately 25 mmHg at peak exercise. While there is great individual variability, A-aDO2 is generally largest at peak exercise in subjects with the highest peak oxygen consumption. Inert gas data has shown that the increase in A-aDO2 is explained by decreased ventilation-perfusion matching, and the development of a diffusion limitation for oxygen. Gas exchange data does not indicate the presence of right-to-left intrapulmonary shunt developing with exercise, despite recent data suggesting that large-diameter arteriovenous shunt vessels may be recruited with exercise. At the same time, multisystem mechanisms regulate systemic acid-base balance in integrative processes that involve gas exchange between tissues and the environment and simultaneous net changes in the concentrations of strong and weak ions within, and transfer between, extracellular and intracellular fluids. The physicochemical approach to acid-base balance is used to understand the contributions from independent acid-base variables to measured acid-base disturbances within contracting skeletal muscle, erythrocytes and noncontracting tissues. In muscle, the magnitude of the disturbance is proportional to the concentrations of dissociated weak acids, the rate at which acid equivalents (strong acid) accumulate and the rate at which strong base cations are added to or removed from muscle.

Lungs in heart failure

Lung function abnormalities both at rest and during exercise are frequently observed in patients with chronic heart failure, also in the absence of respiratory disease. Alterations of respiratory mechanics and of gas exchange capacity are strictly related to heart failure. Severe heart failure patients often show a restrictive respiratory pattern, secondary to heart enlargement and increased lung fluids, and impairment of alveolar-capillary gas diffusion, mainly due to an increased resistance to molecular diffusion across the alveolar capillary membrane. Reduced gas diffusion contributes to exercise intolerance and to a worse prognosis. Cardiopulmonary exercise test is considered the “gold standard” when studying the cardiovascular, pulmonary, and metabolic adaptations to exercise in cardiac patients. During exercise, hyperventilation and consequent reduction of ventilation efficiency are often observed in heart failure patients, resulting in an increased slope of ventilation/carbon dioxide (VE/VCO₂) relationship. Ventilatory efficiency is as strong prognostic and an important stratification marker. This paper describes the pulmonary abnormalities at rest and during exercise in the patients with heart failure, highlighting the principal diagnostic tools for evaluation of lungs function, the possible pharmacological interventions, and the parameters that could be useful in prognostic assessment of heart failure patients.

Assessing exercise limitation using cardiopulmonary exercise testing

The cardiopulmonary exercise test (CPET) is an important physiological investigation that can aid clinicians in their evaluation of exercise intolerance and dyspnea. Maximal oxygen consumption ([Formula: see text]) is the gold-standard measure of aerobic fitness and is determined by the variables that define oxygen delivery in the Fick equation ([Formula: see text] = cardiac output × arterial-venous O(2) content difference). In healthy subjects, of the variables involved in oxygen delivery, it is the limitations of the cardiovascular system that are most responsible for limiting exercise, as ventilation and gas exchange are sufficient to maintain arterial O(2) content up to peak exercise. Patients with lung disease can develop a pulmonary limitation to exercise which can contribute to exercise intolerance and dyspnea. In these patients, ventilation may be insufficient for metabolic demand, as demonstrated by an inadequate breathing reserve, expiratory flow limitation, dynamic hyperinflation, and/or retention of arterial CO(2). Lung disease patients can also develop gas exchange impairments with exercise as demonstrated by an increased alveolar-to-arterial O(2) pressure difference. CPET testing data, when combined with other clinical/investigation studies, can provide the clinician with an objective method to evaluate cardiopulmonary physiology and determination of exercise intolerance.

Rapid intravenous infusion of 20 mL/kg saline alters the distribution of perfusion in healthy supine humans

Rapid intravenous saline infusion, a model meant to replicate the initial changes leading to pulmonary interstitial edema, increases pulmonary arterial pressure in humans. We hypothesized that this would alter lung perfusion distribution. Six healthy subjects (29 ± 6 years) underwent magnetic resonance imaging to quantify perfusion using arterial spin labeling. Regional proton density was measured using a fast-gradient echo sequence, allowing blood delivered to the slice to be normalized for density and quantified in mL/min/g. Contributions from flow in large conduit vessels were minimized using a flow cutoff value (blood delivered > 35% maximum in mL/min/cm(3)) in order to obtain an estimate of blood delivered to the capillary bed (perfusion). Images were acquired supine at baseline, after infusion of 20 mL/kg saline, and after a short upright recovery period for a single sagittal slice in the right lung during breath-holds at functional residual capacity. Thoracic fluid content measured by impedance cardiography was elevated post-infusion by up to 13% (p<0.0001). Forced expiratory volume in 1s was reduced by 5.1% post-20 mL/kg (p=0.007). Infusion increased perfusion in nondependent lung by up to 16% (6.4 ± 1.6 mL/min/g baseline, 7.3 ± 1.8 post, 7.4 ± 1.7 recovery, p=0.03). Including conduit vessels, blood delivered in dependent lung was unchanged post-infusion; however, was increased at recovery (9.4 ± 2.7 mL/min/g baseline, 9.7 ± 2.0 post, 11.3 ± 2.2 recovery, p=0.01). After accounting for changes in conduit vessels, there were no significant changes in perfusion in dependent lung following infusion (7.8 ± 1.9 mL/min/g baseline, 7.9 ± 2.0 post, 8.5 ± 2.1 recovery, p=0.36). There were no significant changes in lung density. These data suggest that saline infusion increased perfusion to nondependent lung, consistent with an increase in intravascular pressures. Dependent lung may have been "protected" from increases in perfusion following infusion due to gravitational compression of the pulmonary vasculature.

Rapid intravenous infusion of 20 ml/kg saline does not impair resting pulmonary gas exchange in the healthy human lung

Rapid infusion of intravenous saline, a model of pulmonary interstitial edema, alters the distribution of pulmonary perfusion, raises pulmonary capillary blood volume, and increases bronchial wall thickness in humans. We hypothesized that infusion would disrupt pulmonary gas exchange by increasing ventilation/perfusion ((.)VA/(.)Q) inequality as opposed to a diffusive impairment in O2 exchange. Seven males (26 +/- 3 yr; FEV1: 110 +/- 16% predicted.) performed spirometry and had (.)VA/(.)Q mismatch measured using the multiple inert gas elimination technique, before and after 20 ml/kg iv of normal saline delivered in approximately 30 min. Infusion increased thoracic fluid content from transthoracic impedance by 12% (P < 0.0001) and left FVC unchanged but reduced expiratory flows (FEF(25-75) falling from 5.1 +/- 0.4 to 4.2 +/- 0.4 l/s, P < 0.05). However, (.)VA/(.)Q mismatch as measured by the log standard deviation of the ventilation (LogSD(.)V) and perfusion (LogSD(.)Q) distributions remained unchanged; LogSD(.)V: 0.40 +/- 0.03 pre, 0.38 +/- 0.04 post, NS; LogSD(.)Q: 0.38 +/- 0.03 pre, 0.37 +/- 0.03 post, NS. There was no significant change in arterial PO2 (99 +/- 2 pre, 99 +/- 3 mmHg post, NS) but arterial PCO2 was decreased (38.7 +/- 0.6 pre, 36.8 +/- 1.2 mmHg post, P < 0.05). Thus, infusion compressed small airways and caused a mild degree of hyperventilation. There was no evidence for a diffusive limitation to O2 exchange, with the measured-predicted alveolar-arterial oxygen partial pressure difference being unaltered by infusion at FIO2 = 0.125 (4.3 +/- 1.0 pre, 5.2 +/- 1.0 post, NS). After infusion, the fraction of perfusion going to areas with (.)VA/(.)Q < 1 was increased when a subject breathed a hyperoxic gas mixture [0.72 +/- 0.06 (FIO2 = 0.21), 0.80 +/- 0.06 (FIO2 = 0.30), P < 0.05] with similar effects on ventilation in the face of unchanged (.)VA and (.)Q. These results suggest active control of blood flow to regions of decreased ventilation during air breathing, thus minimizing the gas exchange consequences.

Influence of rapid fluid loading on airway structure and function in healthy humans

BACKGROUND

The present study examined the influence of rapid intravenous fluid loading (RFL) on airway structure and pulmonary vascular volumes using computed tomography imaging and the subsequent impact on pulmonary function in healthy adults (n = 16).

METHODS AND RESULTS

Total lung capacity (DeltaTLC = -6%), forced vital capacity (DeltaFVC = -14%), and peak expiratory flow (DeltaPEF = -19%) decreased, and residual volume (DeltaRV = +38%) increased post-RFL (P < .05). Airway luminal cross-sectional area (CSA) decreased at the trachea, and at airway generation 3 (P < .05), wall thickness changed minimally with a tendency for increasing in generation five (P = .13). Baseline pulmonary function was positively associated with airway luminal CSA; however, this relationship deteriorated after RFL. Lung tissue volume and pulmonary vascular volumes increased 28% (P < .001) post-RFL, but did not fully account for the decline in TLC.

CONCLUSIONS

These data suggest that RFL results in obstructive/restrictive PF changes that are most likely related to structural changes in smaller airways or changes in extrapulmonary vascular beds.

Isovolumic Acceleration Measured by Tissue Doppler Echocardiography Is Preload Independent in Healthy Subjects

BACKGROUND

Isovolumic acceleration (IVA) as assessed by Tissue Doppler Imaging (TDI) has been proposed as a measure of left ventricular (LV) contractility. IVA is believed to be less dependent on preload than previously proposed estimates. IVA has been measured at different locations, and studies have shown conflicting results.

OBJECTIVES

We investigated the impact of increased preload on modern echocardiographic estimates of contractility, including IVA performed at different locations, in healthy volunteers.

METHODS

Seventeen young healthy individuals (male 13, age 31(+/- 9) years) with no prior history of cardiovascular or metabolic diseases had a Doppler and Tissue Doppler echocardiographic study performed at baseline and after a rapid infusion of 30 ml/kg of bodyweight of isotonic saline. Results are given as mean +/- standard deviation (SD), differences tested by paired t-test.

RESULTS

Echocardiographic parameters used to determine changes in preload, altered significantly. E/e' increased both at the lateral (5 +/- 1 vs 7 +/- 1 P < 0.01) and at the septal side of the annulus (7 +/- 2 vs 9 +/- 2, P < 0.01). Afterload remained unchanged. IVA was unchanged regardless of the measurement location: in the basal free wall (1.21 +/- 0.58 vs 0.98 +/- 0.41, not significant (NS)) or in the mitral annulus (1.18 +/- 0.56 vs 1.15 +/- 0.33, NS). Peak systolic strain, measured at the basal segment of LV septum, increased significantly (15.4 +/- 5.0 vs 20.7 +/- 5, P < 0.05), while all other measurements for strain or strain rate (SR) remained unchanged.

CONCLUSION

IVA is unchanged following significant increases in preload in healthy subjects, and thus is a potentially useful measure of global LV contractility.

Genetic variation of the β2-adrenergic receptor is associated with differences in lung fluid accumulation in humans

The β2-adrenergic receptors (β2AR) play an important role in lung fluid regulation. Previous research has suggested that subjects homozygous for arginine at amino acid 16 of the β2AR (Arg16) may have attenuated receptor function relative to subjects homozygous for glycine at the same amino acid (Gly16). We sought to determine if the Arg16Gly polymorphism of the β2AR influenced lung fluid balance in response to rapid saline infusion. We hypothesized that subjects homozygous for Arg at amino acid 16 ( n = 14) would have greater lung fluid accumulation compared with those homozygous for Gly ( n = 15) following a rapid intravenous infusion of isotonic saline (30 ml/kg over 17 min). Changes in lung fluid were determined using measures of lung density and tissue volume (computerized tomography imaging) and measures of pulmonary capillary blood volume (Vc) and alveolar-capillary conductance (DM, determined from the simultaneous assessment of the diffusing capacities of the lungs for carbon monoxide and nitric oxide). The saline infusion resulted in elevated catecholamines in both genotype groups (Arg16 283 ± 117% vs. Gly16 252 ± 118%, P > 0.05). The Arg16 group had a larger decrease in DMand increase in lung tissue volume and lung water after saline infusion relative to the Gly16 group (DM−13 ± 14 vs. 0 ± 26%, P < 0.05; lung tissue volume 13 ± 11 vs. 3 ± 11% and lung water +90 ± 66 vs. +48 ± 144 ml, P = 0.10, P < 0.05, for Arg vs. Gly16, respectively, means ± SD). These data suggest that subjects homozygous for Arg at amino acid 16 of the β2AR have a greater susceptibility for lung fluid accumulation relative to subjects homozygous for Gly at this position.

Carvedilol reduces exercise-induced hyperventilation: A benefit in normoxia and a problem with hypoxia

AIMS

To evaluate whether carvedilol influences exercise hyperventilation and the ventilatory response to hypoxia in heart failure (HF).

METHODS AND RESULTS

Fifteen HF patients participated to this double blind, randomised, placebo controlled, cross-over study. Patients were evaluated by quality of life questionnaire, echocardiography, pulmonary function and cardiopulmonary exercise tests (ramp and constant workload) both in normoxia (FiO2 = 21%) and hypoxia (FiO2 = 16%, equivalent to a simulated altitude of 2000 m). Carvedilol improved clinical condition and reduced left ventricle size, but had no effect on lung mechanics. In normoxia during exercise, ventilation was lower, V(CO2) unchanged and PaCO2 (constant workload) or PetCO2 (ramp) higher with carvedilol, exercise capacity was unchanged (peak workload 92+/-22 and 90+/-22W for placebo and carvedilol, respectively). Abnormal V(E)/V(CO2) slope was reduced by carvedilol. Hypoxia increased ventilation but less with carvedilol; exercise capacity decreased to 87+/-21W (placebo) and to 80+/-11 W (carvedilol, p < 0.01). With hypoxia, carvedilol decreased V(E)/V(CO2) slope. At constant workload exercise with hypoxia, PaO2 decreased to 69+/-6 mm Hg (placebo) and to 64+/-5 (carvedilol, p < 0.01).

CONCLUSION

Carvedilol reduced hyperventilation possibly by reducing peripheral chemoreflex sensitivity as suggested by PaCO2 increase with normoxia and PaO2 decrease with hypoxia without V(CO2) and V(D)/V(T) changes. Lessening hyperventilation is beneficial when breathing normally, but detrimental when hyperventilation is needed for exercise at high altitude.

Effect of acute increases in pulmonary vascular pressures on exercise pulmonary gas exchange

The purpose of this study was to determine the effect of acute increases in pulmonary vascular pressures, caused by the application of lower-body positive pressure (LBPP), on exercise alveolar-to-arterial Po2 difference (A-aDo2), anatomical intrapulmonary (IP) shunt recruitment, and ventilation. Eight healthy men performed graded upright cycling to 90% maximal oxygen uptake under normal conditions and with 52 Torr (1 psi) of LBPP. Pulmonary arterial (PAP) and pulmonary artery wedge pressures (PAWP) were measured with a Swan-Ganz catheter. Arterial blood samples were obtained from a radial artery catheter, cardiac output was calculated by the direct Fick method, and anatomical IP shunt was determined by administering agitated saline during continuous two-dimensional echocardiography. LBPP increased both PAP and PAWP while upright at rest, and at all points during exercise (mean increase in PAP and PAWP 3.7 and 4.0 mmHg, respectively, P < 0.05). There were no differences in exercise oxygen uptake or cardiac output between control and LBPP. Despite the increased PAP and PAWP with LBPP, A-aDo2 was not affected. In the upright resting position, there was no evidence of shunt in the control condition, whereas LBPP caused shunt in one subject. At the lowest exercise workload (75 W), shunt occurred in three subjects during control and in four subjects with LBPP. LBPP did not affect IP shunt recruitment during subsequent higher workloads. Minute ventilation and arterial Pco2 were not consistently affected by LBPP. Therefore, small acute increases in pulmonary vascular pressures do not widen exercise A-aDo2 or consistently affect IP shunt recruitment or ventilation.

Impact of Preload and Afterload on Global and Regional Right Ventricular Function and Pressure: A Quantitative Echocardiography Study

BACKGROUND

Several quantitative echocardiographic measures of global and regional right ventricular (RV) function have been proposed, but knowledge of the impact of increases in preload and afterload is limited.

METHODS

Seventeen healthy participants were exposed to increased preload by rapid infusion of 30 mL/kg of saline over 15 minutes, and to increased afterload simulated in an 16- to 18-hour stay in a controlled hypoxic environment (fractional concentration of oxygen in inspired gas = 12.3%). Two-dimensional, Doppler, and Doppler tissue echocardiography evaluations were performed to evaluate global and regional RV function, with changes evaluated by paired analysis.

RESULTS

Peak tricuspid regurgitation velocity increased in both conditions, whereas the RV end-diastolic diameter and acceleration time of the pulmonary forward flow only increased with increased preload and afterload, respectively. Estimates of RV function and contractility remained stable: no changes in the RV isovolumic acceleration (1.6 +/- 0.6 vs 1.6 +/- 0.4 and 1.3 +/- 0.4 cm/s2) or tricuspid annular plane systolic excursion (2.5 +/- 0.4 vs 2.5 +/- 0.3 and 2.6 +/- 0.3 cm) were seen (baseline compared with increased afterload and preload, respectively). The RV index of myocardial performance was increased with increased afterload (0.26 +/- 0.08 vs 0.34 +/- 0.13, P < .05), whereas no changes with increased preload were seen. Changes in loading conditions did not affect the regional strain.

CONCLUSION

Moderate volume and pressure loading of the RV induces detectable changes in the RV pressure and morphology. Modern echocardiographic measures of systolic RV function seem stable with moderate increases in preload and afterload.

Exercise-Associated Hyponatraemia

In 1958, Edelman and colleagues empirically showed plasma sodium concentration ([Na+]p) to be primarily a function of the sum of exchangeable sodium and potassium (E) divided by total body water (TBW). Based on Edelman's equation, Nguyen and Kurtz derived an equation to show how [Na+]p changes as a function of TBW, change in TBW (DeltaTBW), and change in the sum of exchangeable sodium and potassium (DeltaE). Using the Nguyen-Kurtz equation, the present study examines the sensitivity of [Na+]p to these parameters: [Na+]p is very sensitive to DeltaTBW and moderately sensitive to DeltaE, and is modulated by TBW. For example, for a person with 50 L TBW, a net increase of 1L water lowers [Na+]p by 3.2 mEq/L, but for a person with 25 L TBW it lowers [Na+]p by 6.3 mEq/L (assuming initial [Na+]p is 140 mEq/L). In each case, a loss of 159 mEq of sodium plus potassium (roughly equivalent to 1.5 teaspoons of table salt) would be required to produce the same effect as the net increase of 1 L water. The present review demonstrates why fluid overload predominates over electrolyte loss in the aetiology of exercise-associated hyponatraemia (EAH), and why the excretion of electrolyte-dilute urine is highly effective in correcting EAH (nonetheless, loss of sodium and potassium is significant in long events in warm weather). Sports drinks will, if overconsumed, result in hyponatraemia. Administration of a sports drink to an athlete with fluid overload hyponatraemia further lowers [Na+]p and increases fluid overload. Administration of either a sports drink or normal (0.9%) saline increases fluid overload.