Is the dyspnea during adenosine cardiac stress test caused by bronchospasm?

Adenosine as a Marker and Mediator of Cardiovascular Homeostasis: A Translational Perspective

Adenosine, a purine nucleoside, is produced broadly and implicated in the homeostasis of many cells and tissues. It signals predominantly via 4 purinergic adenosine receptors (ADORs) - ADORA1, ADORA2A, ADORA2B and ADOosine signaling, both through design as specific ADOR agonists and antagonists and as offtarget effects of existing anti-platelet medications. Despite this, adenosine has yet to be firmly established as either a therapeutic or a prognostic tool in clinical medicine to date. Herein, we provide a bench-to-bedside review of adenosine biology, highlighting the key considerations for further translational development of this proRA3 in addition to non-ADOR mediated effects. Through these signaling mechanisms, adenosine exerts effects on numerous cell types crucial to maintaining vascular homeostasis, especially following vascular injury. Both in vitro and in vivo models have provided considerable insights into adenosine signaling and identified targets for therapeutic intervention. Numerous pharmacologic agents have been developed that modulate adenmising molecule.

Successful Reversal of Bradycardia and Dyspnea With Aminophylline After Ticagrelor Load

A 69-year-old male underwent elective percutaneous coronary intervention requiring placement of a drug-eluting stent to the first obtuse marginal artery. Four hours following the administration of a ticagrelor loading dose, he developed dyspnea and sinus pauses. Aminophylline was administered and resulted in immediate and sustained symptom resolution. Ticagrelor has been associated with dyspnea and bradyarrhythmias, both attributed to increased adenosine exposure. Ticagrelor inhibits reuptake of intracellular adenosine. Adenosine antagonists aminophylline and theophylline have been utilized to reverse the effects of adenosine and may relieve adenosine-mediated adverse effects induced by ticagrelor therapy. Aminophylline may be considered for reversal of dyspnea and bradyarrhythmia associated with ticagrelor therapy through alterations in adenosine exposure.

Potential role of endogenous adenosine in ticagrelor-induced dyspnea

Ticagrelor, a recently approved platelet antagonist indicated for the reduction of thrombotic cardiovascular events in patients with acute coronary syndrome (ACS), has been reported to cause dyspnea in more than 13% of patients. Dyspnea is not a clinically relevant adverse event with other medications indicated for ACS. One suggested mechanism of ticagrelor-induced dyspnea involves an increase in systemic adenosine concentrations through adenosine deaminase inhibition. Dyspnea, a subjective finding resulting from physiologic and sensory mechanisms, may be a consequence of increased systemic adenosine concentrations, leading to amplified and prolonged receptor activity. Current literature suggests, however, that pulmonary status is not compromised, with no reduction of efficacy seen in patients with ticagrelor-induced dyspnea, thus allowing clinicians to continue therapy without reservation. Still, patients with a history of asthma and chronic obstructive pulmonary disease may be more susceptible to ticagrelor-induced dyspnea, potentially leading to nonadherence and exacerbations of morbidity. Therefore, it is paramount that health care providers continually monitor these patients with the aims of maintaining medication therapy adherence and providing relevant options if dyspnea becomes intolerable.

Prognosis in patients having chronic obstructive pulmonary disease with significant coronary artery lesion angina

BACKGROUND/AIMS

Many studies have investigated angina and its relationship with chronic obstructive pulmonary disease (COPD). However, angina was diagnosed only by noninvasive tests or only by clinical symptoms in most of these studies. The aim of this study was to compare the prognosis, including rate of hospitalization and death from significant coronary artery lesion and nonsignificant coronary artery lesion angina, in patients with COPD.

METHODS

Patients with COPD who underwent coronary angiography (CAG) due to angina were reviewed retrospectively at a tertiary referral hospital. COPD is defined as post-bronchodilator forced expiratory volume in 1 sec/forced vital capacity (FEV(1)/FVC) of < 70%. A significant coronary lesion is defined as at least 50% diameter stenosis of one major epicardial artery in CAG.

RESULTS

In total, 113 patients were enrolled. Mean follow-up duration was 39 ± 21 months. Of the patients, 52 (46%) had mild COPD and 48 (42%) had moderate COPD. Sixty-nine (61%) patients had significant stenosis in CAG. The death rate in the follow-up period was 2.21 per 100 patient-years. No significant difference was observed among the all-cause mortality rate, admission rate, or intensive care unit admission rate in patients who had COPD with or without significant coronary artery disease. Pneumonia or acute exacerbation of COPD was the most common cause of admission.

CONCLUSIONS

In patients having COPD with angina who underwent CAG, no significant difference was observed in mortality or admission events depending on the presence of a significant coronary artery lesion during the 2-year follow-up period.

Hemodynamic response, arrhythmic risk, and overall safety of regadenoson as a pharmacologic stress agent for myocardial perfusion imaging in chronic obstructive pulmonary disease and bronchial asthma patients

Regadenoson (REG) is a A2a receptor selective pharmacologic SPECT imaging agent. Its safety in unselected chronic obstructive pulmonary disease (COPD) or asthma (AM) undergoing SPECT imaging has not been well evaluated. We retrospectively identified 228 patients (COPD n = 126 and AM n = 102, Grp 1) undergoing REG SPECT from Jan to Nov 2009 and compared to 1,142 patients without COPD and AM (control, Grp 2). A standard 400 μg REG bolus was used and gated Tc-99 m tetrofosmin SPECT done. Patient demographics, REG SPECT data, side effects, arrhythmia occurrences, and any exacerbation of COPD or AM leading to treatment, hospitalization or death were evaluated. The side effect profile of Grp 1 was also compared to a historical cohort who underwent intravenous dipyridamole thallium-201 imaging and adenosine SPECT. Both groups were comparable with regards to baseline characteristics. There was 0% incidence of clinical exacerbation of COPD or AM after REG. COPD patients had more non-significant arrhythmias (58.3% vs. Grp 2, 43%, P = 0.004). There was 0% incidence of any atrio-ventricular block and only 2 instances of brief supraventricular tachycardia. When compared to the historical cohort of COPD who underwent IV dipyridamole thallium imaging, COPD in Grp 1, had more dyspnea and flushing and when compared to COPD/AM patients who underwent adenosine SPECT, Grp 1 pts had more of flushing and headache (24.9% vs. 2.8%, P = <0.001) but less of bronchospasm (1.3% vs. 6.9%, P = 0.022) and AV block (0% vs. 4.2%, P = 0.014). REG SPECT can be safely performed in COPD and AM population.

Ticagrelor inhibits adenosine uptake in vitro and enhances adenosine-mediated hyperemia responses in a canine model

AIMS

A routine secondary pharmacology screen indicated that reversibly binding oral P2Y(12) receptor antagonist ticagrelor could inhibit adenosine uptake in human erythrocytes, suggesting that ticagrelor may potentiate adenosine-mediated responses in vivo. The aim of this study was to further characterize the adenosine uptake inhibition in vitro and study possible physiological consequences of adenosine uptake inhibition by ticagrelor in an anesthetized dog model of coronary blood flow compared to dipyridamole.

METHODS AND RESULTS

We measured [2-3H]adenosine uptake in purified human erythrocytes and several cell lines in the presence of ticagrelor or the known uptake inhibitor dipyridamole as a comparator. Using an open-chest dog model (beagles), we measured the left anterior descending (LAD) coronary artery blood flow during reactive hyperemia after 1 minute occlusion or intracoronary infusion of adenosine before and after administration of vehicle, ticagrelor, or dipyridamole (each n = 8). Ticagrelor concentration-dependently inhibited adenosine uptake in human erythrocytes and in cell lines of rat, canine, or human origin. In the dog model, ticagrelor and dipyridamole dose-dependently augmented reactive hyperemia after LAD occlusion, as assessed by percentage repayment of flow debt relative to control (both Ps < .05). Ticagrelor and dipyridamole also dose-dependently augmented intracoronary adenosine-induced increases in LAD blood flow relative to control (both Ps < .05).

CONCLUSION

Ticagrelor inhibits adenosine uptake in vitro and subsequently augments cardiac blood flow in a canine model of reactive hypoxia- or adenosine-induced blood flow increases. These findings suggest that ticagrelor may have additional benefits in patients with acute coronary syndrome beyond inhibition of platelet aggregation.

Lessons from regadenoson and low-level treadmill/regadenoson myocardial perfusion imaging: initial clinical experience in 1263 patients

BACKGROUND

Regadenoson is a pharmacologic stress agent, which was recently approved for stress myocardial perfusion imaging (MPI). Aside from the initial protocol-driven studies, clinical experience with this stress agent is limited. Furthermore, low-level treadmill testing in a large population with regadenoson has not previously been evaluated. We describe our experience in the first 6 months of routine inpatient and outpatient clinical use.

METHODS

Between 7/1/08 and 12/04/08, 1263 patients underwent regadenoson stress testing (596 with low-level treadmill, 667 supine). Past medical history, clinical symptoms during stress, and changes in systolic blood pressure were prospectively recorded.

RESULTS

Low-level treadmill testing was well tolerated in our patient population. Shortness of breath, dizziness, palpitations, transient heart block, and nausea were less frequent when patients were able to exercise. In our population, 51% experienced a drop in systolic blood pressure greater than 10 mmHg, with 9% experiencing a decrease in more than 30 mmHg. This decrease in blood pressure was more common in patients who underwent low-level treadmill testing vs those that were supine (56% vs 47%, P-value < 0.001). The frequency of symptoms in our entire population was as follows: chest pain 35%, dizziness/lightheadedness 16%, shortness of breath 27%, headache 1.4%, nausea 2.3%, and palpitations 5%. There were no major hemodynamic or conduction abnormalities. In the 16% of patients, a history of COPD/Asthma Regadenoson MPI was well tolerated. There were no procedural deaths.

CONCLUSION

Regadenoson and Regadenoson combined with low level exercise MPI appear to be safe and well tolerated. An asymptomatic fall in systolic blood pressure seems to be more common in patients who undergo low-level treadmill testing.

Regadenoson: a new myocardial stress agent

Vasodilator stress myocardial perfusion imaging (MPI) accounts for up to 50% of all stress MPI studies performed in the U.S. In 2008, the Food and Drug Administration approved regadenoson for stress testing in conjunction with MPI. Regadenoson, unlike adenosine, is a selective A(2A) agonist that is given as an intravenous bolus at a fixed dose, with less undesirable side effects including atrioventricular block and bronchospasm. Unlike adenosine, regadenoson could be used in patients with mild-to-moderate reactive airway disease. This review will summarize the pre-clinical and clinical data on regadenoson, as they specifically relate to its use as a vasodilator stress agent, currently the only approved selective A(2A) agonist.

Adenosine for wide-complex tachycardia: efficacy and safety

OBJECTIVES

: To determine whether adenosine is useful and safe as a diagnostic and therapeutic agent for patients with undifferentiated wide QRS complex tachycardia. The etiology of sustained monomorphic wide QRS complex tachycardia is often uncertain acutely.

DESIGN

: A retrospective observational study.

SETTING

: Treatment associated with emergency visits at nine urban hospitals.

PATIENTS

: Consecutive patients treated with adenosine for regular wide QRS complex tachycardia between 1991 and 2006.

INTERVENTIONS

: Treatment with adenosine infusion.

MEASUREMENTS AND MAIN RESULTS

: Measured outcomes included rhythm response to adenosine, if any, and all adverse effects. A positive response was defined as an observed change in rhythm including temporary atrioventricular conduction block or tachycardia termination. A primary adverse event was defined as emergent electrical or medical therapy instituted in response to an adverse adenosine effect. A rhythm diagnosis was made in each case. The characteristics of adenosine administration as a test for a supraventricular as opposed to ventricular tachycardia were determined, and the adverse event rates were calculated. A total of 197 patients were included: 104 (90%) of 116 (95% confidence interval, 83%-95%) and two (2%) of 81 (95% confidence interval, 0.3%-9%) supraventricular tachycardia and ventricular tachycardia patients demonstrated a response to adenosine, respectively. The odds of supraventricular tachycardia increased by a factor of 36 (95% confidence interval, 9-143) after a positive response to adenosine. The odds of ventricular tachycardia increased by a factor of 9 (95% confidence interval, 6-16) when there was no response to adenosine. The rate of primary adverse events for patients with supraventricular tachycardia and ventricular tachycardia was 0 (0%) of 116 (95% confidence interval, 0%-3%) and 0 (0%) of 81 (95% confidence interval, 0%-4%), respectively.

CONCLUSIONS

: Adenosine is useful and safe as a diagnostic and therapeutic agent for patients with regular wide QRS complex tachycardia.

Pharmacological stress myocardial perfusion scintigraphy: use of a modified adenosine protocol in patients with asthma

BACKGROUND

Stress radionuclide myocardial perfusion scintigraphy (MPS) using adenosine pharmacological vasodilatation is the preferred method in many centres because of its convenience, safety and speed. It can, however, cause bronchospasm and hence its use is avoided in patients with known or suspected bronchospasm. Owing to service pressures, we use technologist-led adenosine stressing for patients referred for MPS studies. We use a modified adenosine infusion protocol under medical supervision for patients with asthma to prevent and minimize adenosine-induced bronchospasm. In this study, we audited our use of this modified protocol in asthmatic patients and compared the side-effect profile with the standard adenosine protocol used in nonasthmatic patients.

METHODS

We audited 50 consecutive patients with asthma attending our department for stress MPS. All patients were taking regular inhalers+/-oral steroids. Patients who had exacerbation of asthma requiring hospital admission during the preceding 6 months were excluded. Before commencing the infusion, two inhaled puffs of salbutamol were administered. A modified adenosine infusion protocol was used, starting initially at a rate of 70 microg/kg/min and increasing to the standard 140 microg/kg/min within 1 min and then maintained for a further 5 min. Technetium-99m tetrofosmin was injected at 3 min. Blood pressure (BP), pulse rate (PR), oxygen saturation and ECG were monitored before, during and at the end of the infusion. All side effects were recorded. Fifty-eight consecutive patients without asthma were included as controls and received the standard 140 microg/kg/min infusion over 6 min.

RESULTS

One hundred and eight patients, 50 with asthma and 58 without asthma, were entered into the study. The test was stopped early in two patients (4%) with asthma and 11 patients (19%) without asthma (chi=5.679; P=0.017). Proportionally, more nonasthmatics developed shortness of breath (SOB) (47 of 58, 81% without asthma vs. 35 of 50, 70% with asthma); however, this did not reach statistical significance (chi=1.788, P=NS). Three out of 50 (6%) patients in the asthma group experienced severe SOB but only one of those 50 patients (2%) developed bronchospasm, manifesting as wheeze. In the nonasthma group, five of 58 patients (8.6%) experienced severe SOB but none developed a wheeze. Less flushing (16 of 50, 32% vs. 36 of 58, 62%; P=0.002), dizziness (12 of 50, 24% vs. 26 of 58, 45%; P=0.023) and neck/throat pain (5 of 50, 10% vs. 16 of 58, 28%; P=0.021) was observed in the modified infusion group with asthma compared with the standard infusion group without asthma. Statistical significance was observed in these three side effects. No significant difference in other side effects was noted. A similar decrease in mean diastolic BP, and an increase in mean PR were observed during the infusion in both asthmatic and nonasthmatic groups. The mean systolic BP decreased significantly in nonasthmatic patients (P<0.001) but not in the asthmatic group. No significant change in oxygen saturation was seen during infusion in the asthmatic group.

CONCLUSION

The modified adenosine infusion protocol with salbutamol premedication can be used in patients with asthma. This protocol resulted in fewer side effects and changes in BP and PR in asthmatic patients compared with nonasthmatic patients who received the standard adenosine infusion.

Cardiac disease in chronic obstructive pulmonary disease

The cardiac manifestations of chronic obstructive pulmonary disease (COPD) are numerous. Impairments of right ventricular dysfunction and pulmonary vascular disease are well known to complicate the clinical course of COPD and correlate inversely with survival. The pathogenesis of pulmonary vascular disease in COPD is likely multifactorial and related to alterations in gas exchange and vascular biology, as well as structural changes of the pulmonary vasculature and mechanical factors. Several modalities currently exist for the assessment of pulmonary vascular disease in COPD, but right heart catheterization remains the gold standard. Although no specific therapy other than oxygen has been generally accepted for the treatment of pulmonary hypertension in this population, there has been renewed interest in specific pulmonary vasodilators. The coexistence of COPD and coronary artery disease occurs frequently. This association is likely related to shared risk factors as well as similar pathogenic mechanisms, such as systemic inflammation. Management strategies for the care of patients with COPD and coronary artery disease are similar to those without COPD, but care must be given to address their respiratory limitations. Arrhythmias occur frequently in patients with COPD, but are rarely fatal and can generally be treated medically. Use of beta-blockers in the management of cardiac disease, while a theoretical concern in patients with increased airway resistance, is generally safe with the use of cardioselective agents.

Safety of regadenoson, an adenosine A2A receptor agonist for myocardial perfusion imaging, in mild asthma and moderate asthma patients: a randomized, double-blind, placebo-controlled trial

BACKGROUND

Patients with reactive airways are at risk for adenosine-induced bronchoconstriction, mediated via A(2B) and/or A(3) adenosine receptors.

METHODS AND RESULTS

To examine the effects of regadenoson, a selective adenosine A(2A) receptor agonist, on airway resistance, we conducted a randomized, double-blind, placebo-controlled crossover trial in asthmatic patients with a positive adenosine monophosphate challenge test. The mean ratio of the forced expiratory volume in 1 second (FEV(1)) at each tested time point relative to the baseline FEV(1) was significantly higher after treatment with regadenoson compared with placebo from 10 to 60 minutes after treatment. One patient had a substantial but asymptomatic FEV(1) reduction (-36.2%) after regadenoson that reversed spontaneously. The most common adverse events with regadenoson were tachycardia (66%), dizziness (53%), headache (45%), and dyspnea (34%). The mean heart rate significantly increased with regadenoson (maximum of +10.4 beats/min) versus placebo.

CONCLUSIONS

In this pilot safety study of 48 patients with mild or moderate asthma who had bronchial reactivity to adenosine monophosphate, regadenoson was safe and well tolerated.

Safety of regadenoson, a selective adenosine A2A agonist, in patients with chronic obstructive pulmonary disease: A randomized, double-blind, placebo-controlled trial (RegCOPD trial)

BACKGROUND

Patients with reactive airways are at risk for adenosine-induced bronchoconstriction, mediated via A(2B) and/or A(3) adenosine receptors.

METHODS AND RESULTS

In this randomized, double-blind, placebo-controlled crossover trial, we examined the safety of regadenoson, a selective adenosine A(2A) receptor agonist, in patients with moderate chronic obstructive pulmonary disease (COPD) (n = 38) and patients with severe COPD (n = 11) with a baseline mean forced expiratory volume in 1 second (FEV(1)) of 1.74 +/- 0.50 L and 1.0 +/- 0.35 L, respectively, 37% of whom had dyspnea during activities of daily living. Patients receiving glucocorticoids or oxygen and those with pretreatment wheezing were included. Short-acting bronchodilators were withheld for at least 8 hours before treatment. No differences emerged between regadenoson and placebo on multiple lung function parameters, including repeated FEV(1) and forced vital capacity, respiratory rate, pulmonary examinations, and oxygen saturation. The mean maximum decline in FEV(1) was 0.11 +/- 0.02 L and 0.12 +/- 0.02 L (P = .55) in patients after regadenoson and placebo, respectively, and new-onset wheezing was observed in 6% and 12%, respectively (P = .33). No patient required acute treatment with bronchodilators or oxygen.

CONCLUSIONS

This pilot study showed the overall safety of regadenoson in 49 compromised outpatients with clinically stable moderate and severe chronic obstructive pulmonary disease.

Respiratory resistance of patients during cardiac stress testing with adenosine: is dyspnea a sign of bronchospasm?

BACKGROUND

Adenosine is widely used for stress-testing in myocardial perfusion imaging. During adenosine infusion, dyspnea is one of the main complaints of patients. The aim of this study was to determine whether dyspnea during adenosine infusion is caused by bronchospasm.

METHODS

Fifty-four patients were enrolled in the study. Seven of these 54 suffered from mild chronic obstructive pulmonary disease (COPD). We continuously measured respiratory resistance (Rrs), using impulse oscillometry. Respiratory resistance was measured before, during, and after a continuous infusion of 140 microg/kg/min adenosine.

RESULTS

Sixty-seven percent of patients suffered from dyspnea during adenosine infusion. In patients with mild COPD, Rrs was higher compared with other patients (0.48 vs 0.27 kPa/L/s, P < .05). Neither patients with COPD nor those without COPD exhibited a significant increase in Rrs during adenosine infusion. The Rrs of patients with dyspnea was insignificantly lower compared with patients without dyspnea (P = .469).

CONCLUSIONS

Dyspnea as a side effect of adenosine infusion is not correlated with impaired respiratory resistance in nonasthmatic patients and in patients with mild COPD. Thus bronchospasm is ruled out as cause of this clinical symptom. Despite the small number of COPD patients enrolled in the study, adenosine infusion might be possible in patients with mild COPD.

Side effect profile and tolerability of adenosine myocardial perfusion scintigraphy in patients with mild asthma or chronic obstructive pulmonary disease

BACKGROUND

Adenosine may cause bronchoconstriction in subjects with asthma or chronic obstructive pulmonary disease (COPD). Recent evidence suggests that this effect may be dependent on the severity of disease. This study investigates the tolerability of adenosine stress in patients with mild asthma or COPD undergoing myocardial perfusion scintigraphy.

METHODS AND RESULTS

In this case-control study patients with known or suspected mild asthma or COPD were pretreated with an inhaled beta(2)-adrenergic agonist and adenosine titrated up to the maximal dose of 140 microg x kg(-1) x min(-1) over a period of 6 minutes. The occurrence of side effects and test tolerability were compared between the airway disease group and 72 control subjects. Of 1261 patients, 124 had known or suspected airway disease; of these, 72 (58%) were suitable for adenosine stress. The proportion of tests completed as per protocol in the asthma/COPD group was similar to that of control subjects (93% vs 100%, P = .06). Dyspnea (n = 38 [53%] in asthma/COPD group vs n = 25 [35%] in control group, P = .03) and chest pain (n = 14 [19%] in asthma/COPD group vs n = 16 [22%] in control group, P = .7) were the most common side effects, and these were mostly mild and well tolerated. Bronchospasm occurred in 5 patients with asthma/COPD but reverted shortly after discontinuation of the adenosine infusion. Aminophylline was not required in any case.

CONCLUSIONS

A stepwise 6-minute adenosine infusion with prophylactic beta(2)-adrenergic agonist is safe and well tolerated in patients with mild asthma or COPD.

The pulmonary effects of intravenous adenosine in asthmatic subjects

BACKGROUND

We have shown that intravenous adenosine in normal subjects does not cause bronchospasm, but causes dyspnea, most likely by an effect on vagal C fibers in the lungs [Burki et al. J Appl Physiol 2005; 98:180-5]. Since airways inflammation and bronchial hyperreactivity are features of asthma, it is possible that intravenous adenosine may be associated with an increased intensity of dyspnea, and may cause bronchospasm, as noted anecdotally in previous reports.

METHODS

We compared the effects of placebo and 10 mg intravenous adenosine, in 6 normal and 6 asthmatic subjects.

RESULTS

Placebo injection had no significant (p > 0.05) effect on the forced expiratory spirogram, heart rate, minute ventilation (Ve), or respiratory sensation. Similarly, adenosine injection caused no significant changes (p > 0.05) in the forced expiratory spirogram; however, there was a rapid development of dyspnea as signified visually on a modified Borg scale, and a significant (p < 0.05) tachycardia in each subject (Asthmatics +18%, Normals + 34%), and a significant (p < 0.05) increase in Ve (Asthmatics +93%, Normals +130%). The intensity of dyspnea was significantly greater (p < 0.05) in the asthmatic subjects.

CONCLUSION

These data indicate that intravenous adenosine does not cause bronchospasm in asthmatic subjects, and supports the concept that adenosine-induced dyspnea is most likely secondary to stimulation of vagal C fibers in the lungs. The increased intensity of adenosine-induced dyspnea in the asthmatic subjects suggests that airways inflammation may have sensitized the vagal C fibers.

Intravenous adenosine and dyspnea in humans

Intravenous adenosine for the treatment of supraventricular tachycardia is reported to cause bronchospasm and dyspnea and to increase ventilation in humans, but these effects have not been systematically studied. We therefore compared the effects of 10 mg of intravenous adenosine with placebo in 21 normal subjects under normoxic conditions and evaluated the temporal sequence of the effects of adenosine on ventilation, dyspnea, and heart rate. The study was repeated in 11 of these subjects during hyperoxia. In all subjects, adenosine resulted in the development of dyspnea, assessed by handgrip dynamometry, without any significant change ( P > 0.1) in lung resistance as measured by the interrupter technique. There were significant increases ( P < 0.05) in ventilation and heart rate in response to adenosine. The dyspneic response occurred slightly before the ventilatory or heart rate responses in every subject, but the timing of the dyspneic, ventilatory, and heart rate responses was not significantly different when the group data were analyzed (18.9 ± 5.8, 20.3 ± 5.5, and 19.7 ± 4.5 s, respectively). During hyperoxia, adenosine resulted in similar effects, with no significant differences in the magnitude of the ventilatory response; however, compared with the normoxic state, the intensity of the dyspneic response was significantly ( P < 0.05) reduced, whereas the heart rate response increased significantly ( P < 0.05). These data indicate that intravenous adenosine-induced dyspnea is not associated with bronchospasm in normal subjects. The time latency of the response indicates that the dyspnea is probably not a consequence of peripheral chemoreceptor or brain stem respiratory center stimulation, suggesting that it is most likely secondary to stimulation of receptors in the lungs, most likely vagal C fibers.