Effects of Chronic Hypoxemia on Chemosensitivity in Patients With Univentricular Heart

AbstractObjectives. We sought to compare the arterial blood gas chemosensitivity in relation to exercise ventilatory response in patients with univentricular heart and cyanosis and in patients with univentricular heart and Fontan-type circulation without cyanosis.Background. Patients with univentricular heart demonstrate excessive ventilation during exercise. Chronic hypoxemia may alter chemoreceptor function, affecting ventilation.Methods. Cardiopulmonary exercise testing was performed in 10 patients with rest or stress-induced cyanosis (cyanotic group: mean age ± SE 30.5 ± 2.3 years; 5 men), 8 patients without cyanosis with Fontan-type circulation (Fontan group: mean age 29.4 ± 1.5 years; 4 men) and 10 healthy control subjects (normal group: mean age 30.7 ± 1.9 years; 5 men). Hypoxic and hypercapnic chemosensitivity were assessed by using transient inhalations of pure nitrogen and the rebreathing of 7% CO2in 93% O2, respectively.Results. Peak O2consumption was comparable in both patient groups (21.7 ± 2.5 [cyanotic group] vs. 21.0 ± 1.9 ml/kg per min [Fontan group]) but was lower than that in the normal group (34.7 ± 1.9 ml/kg per min). The ventilatory response to exercise, characterized by the regression slope relating minute ventilation to CO2output, was higher in the cyanotic group (43.4 ± 4.0) than in the Fontan group (31.4 ± 3.0, p = 0.02) and the normal group (23.1 ± 1.1). Hypoxic chemosensitivity was blunted in the cyanotic group compared with that in the Fontan and normal groups (0.148 vs. 0.448 [p = 0.02] vs. 0.311 liter/min per percent arterial O2saturation, respectively) and did not correlate with the ventilatory response to exercise (r = −0.36, p = 0.29). In contrast, hypercapnic chemosensitivity represented by the slope of the hypercapnic-ventilatory response line was similar in the cyanotic, Fontan and normal groups (1.71 vs. 1.76 vs. 1.70 liter/min per mm Hg, respectively), but the response line had shifted to the left in the cyanotic group (x intercept = 31.9 vs. 39.9 mm Hg [p = 0.026]), compared with 45.2 mm Hg in normal subjects. These findings suggest that in the cyanotic group, ventilation is greater for a given level of arterial CO2tension and thus may partly explain the increased exercise ventilatory response in this group.Conclusions. Hypoxic chemosensitivity is blunted in patients with univentricular heart and cyanosis and does not determine the exercise ventilatory response. CO2elimination appears more important. The blunting of hypoxic chemosensitivity is reversible once chronic hypoxemia is relieved, as evident in the Fontan group

Clinical Correlates and Prognostic Significance of the Ventilatory Response to Exercise in Chronic Heart Failure

AbstractObjectives. This study sought to investigate the clinical characteristics of patients with chronic heart failure and an increased ventilatory response to exercise and to examine the prognostic usefulness of this response.Background. The ventilatory response to exercise is increased in many patients with chronic heart failure and may be characterized by the regression slope relating minute ventilation to carbon dioxide output (V̇e–V̇co2slope) during exercise.Methods. One hundred seventy-three consecutive patients (155 men; mean [±SD] age 59.8 ± 11.5 years; radionuclide left ventricular ejection fraction [LVEF] 28.4 ± 14.6%) underwent cardiopulmonary exercise testing (peak oxygen consumption 18.5 ± 7.3 ml/kg per min; V̇e–V̇co2slope 34.8 ± 10.6) over a 2-year period. Using 1.96 standard deviations above the mean V̇e–V̇co2slope of 68 healthy age-matched subjects (mean slope 26.3 ± 4.1), we defined a high ventilatory response to exercise as a slope >34.Results. Eighty-three patients (48%) had an increased V̇e–V̇co2slope (mean 43.1 ± 8.9). There was a difference in age (62.2 vs. 57.3 years, p = 0.005), New York Heart Association functional class (2.9 vs. 2.1, p < 0.001), LVEF (24.7 vs. 31.9%, p = 0.0016), peak oxygen consumption (14.9 vs. 21.7 ml/kg per min, p < 0.0001) and radiographic cardiothoracic ratio (0.58 vs. 0.55, p = 0.002) between these patients and those with a normal slope. In the univariate Cox proportional hazards model, the V̇e–V̇co2slope was an important prognostic factor (p < 0.0001). In the multivariate Cox analyses using several variables (age, peak oxygen consumption, V̇e–V̇co2slope and LVEF), the V̇e–V̇co2slope gave additional prognostic information (p = 0.018) beyond peak oxygen consumption (p = 0.022). Kaplan-Meier survival curves at 18 months demonstrated a survival rate of 95% in patients with a normal V̇e–V̇co2slope compared with 69% in those with a high slope (p < 0.0001).Conclusions. A high V̇e–V̇co2slope selects patients with more severe heart failure and is an independent prognostic marker. The V̇e–V̇co2slope may be used as a supplementary index in the assessment of patients with chronic heart failure.(J Am Coll Cardiol 1997;29:1585–90

Chronic Heart Failure and Factors Contributing to the Increased Ventilatory response to exercise

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Modeling Diffusion in Social Networks Using Network Properties

Diffusion of items occurs in social networks due to spreading of items through word of mouth and exogenous factors. These items may be news, products, videos, advertisements or contagious viruses. Previous research has studied diffusion process at both the macro and micro levels. The former models the number of item adopters in the diffusion process while the latter determines which individuals adopt item. In this paper, we establish a general probabilistic framework, which can be used to derive macro-level diffusion models, including the well known Bass Model (BM). Using this framework, we develop several other models considering the social network&rsquo;s degree distribution coupled with the assumption of linear influence by neighboring adopters in the diffusion process. Through some evaluation on synthetic data, this paper shows that degree distribution actually changes during the diffusion process. We therefore introduce a multi-stage diffusion model to cope with variable degree distribution. By conducting experiments on both synthetic and real datasets, we show that our proposed diffusion models can recover the diffusion parameters from the observed diffusion data, which allows us to model diffusion with high accuracy