Modulation of ventilatory reflex control by cardiac resynchronization therapy

Autonomic and respiratory consequences of altered chemoreflex function: clinical and therapeutic implications in cardiovascular diseases

The importance of chemoreflex function for cardiovascular health is increasingly recognized in clinical practice. The physiological function of the chemoreflex is to constantly adjust ventilation and circulatory control to match respiratory gases to metabolism. This is achieved in a highly integrated fashion with the baroreflex and the ergoreflex. The functionality of chemoreceptors is altered in cardiovascular diseases, causing unstable ventilation and apnoeas and promoting sympathovagal imbalance, and it is associated with arrhythmias and fatal cardiorespiratory events. In the last few years, opportunities to desensitize hyperactive chemoreceptors have emerged as potential options for treatment of hypertension and heart failure. This review summarizes up to date evidence of chemoreflex physiology/pathophysiology, highlighting the clinical significance of chemoreflex dysfunction, and lists the latest proof of concept studies based on modulation of the chemoreflex as a novel target in cardiovascular diseases.

The risk of post-operative pulmonary complications in lung resection candidates with normal forced expiratory volume in 1 s and diffusing capacity of the lung for carbon monoxide: a prospective multicentre study

Introduction

According to the guidelines for preoperative assessment of lung resection candidates, patients with normal forced expiratory volume in 1 s (FEV₁) and diffusing capacity of the lung for carbon monoxide (D _LCO) are at low risk for post-operative pulmonary complications (PPC). However, PPC affect hospital length of stay and related healthcare costs. We aimed to assess risk of PPC for lung resection candidates with normal FEV₁ and D _LCO (>80% predicted) and identify factors associated with PPC.

Methods

398 patients were prospectively studied at two centres between 2017 and 2021. PPC were recorded from the first 30 post-operative days. Subgroups of patients with and without PPC were compared and factors with significant difference were analysed by uni- and multivariate logistic regression.

Results

188 subjects had normal FEV₁ and D _LCO. Of these, 17 patients (9%) developed PPC. Patients with PPC had significantly lower pressure of end-tidal carbon dioxide (P _ETCO2 ) at rest (27.7 versus 29.9; p=0.033) and higher ventilatory efficiency (V'_E/V'_CO2 ) slope (31.1 versus 28; p=0.016) compared to those without PPC. Multivariate models showed association between resting P _ETCO2 (OR 0.872; p=0.035) and V'_E/V'_CO2 slope (OR 1.116; p=0.03) and PPC. In both models, thoracotomy was strongly associated with PPC (OR 6.419; p=0.005 and OR 5.884; p=0.007, respectively). Peak oxygen consumption failed to predict PPC (p=0.917).

Conclusions

Resting P _ETCO2 adds incremental information for risk prediction of PPC in patients with normal FEV₁ and D _LCO. We propose resting P _ETCO2 be an additional parameter to FEV₁ and D _LCO for preoperative risk stratification.

Inhibition of peripheral chemoreceptors improves ventilatory efficiency during exercise in heart failure with preserved ejection fraction − a role of tonic activity and acute reflex response

Peripheral chemoreceptors (PChRs) play a significant role in maintaining adequate oxygenation in the bloodstream. PChRs functionality comprises two components: tonic activity (PChT) which regulates ventilation during normoxia and acute reflex response (peripheral chemosensitivity, PChS), which increases ventilation following a specific stimulus. There is a clear link between augmented PChS and exercise intolerance in patients with heart failure with reduced ejection fraction. It has been also shown that inhibition of PChRs leads to the improvement in exercise capacity. However, it has not been established yet: 1) whether similar mechanisms take part in heart failure with preserved ejection fraction (HFpEF) and 2) which component of PChRs functionality (PChT vs. PChS) is responsible for the benefit seen after the acute experimental blockade. To answer those questions we enrolled 12 stable patients with HFpEF. All participants underwent an assessment of PChT (attenuation of minute ventilation in response to low-dose dopamine infusion), PChS (enhancement of minute ventilation in response to hypoxia) and a symptom-limited cardiopulmonary exercise test on cycle ergometer. All tests were placebo-controlled, double-blinded and performed in a randomized order. Under resting conditions and at normoxia dopamine attenuated minute ventilation and systemic vascular resistance (p = 0.03 for both). These changes were not seen with placebo. Dopamine also decreased ventilatory and mean arterial pressure responses to hypoxia (p < 0.05 for both). Inhibition of PChRs led to a decrease in V˙E/V˙CO₂ comparing to placebo (36 ± 3.6 vs. 34.3 ± 3.7, p = 0.04), with no effect on peak oxygen consumption. We found a significant relationship between PChT and the relative decrement of V˙E/V˙CO₂ on dopamine comparing to placebo (R = 0.76, p = 0.005). There was a trend for correlation between PChS (on placebo) and V˙E/V˙CO₂ during placebo infusion (R = 0.56, p = 0.059), but the relative improvement in V˙E/V˙CO₂ was not related to the change in PChS (dopamine vs. placebo). We did not find a significant relationship between PChT and PChS. In conclusion, inhibition of PChRs in HFpEF population improves ventilatory efficiency during exercise. Increased PChS is associated with worse (higher) V˙E/V˙CO₂, whereas PChT predicts an improvement in V˙E/V˙CO₂ after PChRs inhibition. This results may be meaningful for patient selection in further clinical trials involving PChRs modulation.

Prediction of Postoperative Complications: Ventilatory Efficiency and Rest End-tidal Carbon Dioxide

BACKGROUND

Cardiopulmonary exercise testing parameters including ventilatory efficiency (V_E/VCO₂ slope) are used for risk assessment of lung resection candidates. However, many patients are unable or unwilling to undergo exercise. V_E/VCO₂ slope is closely related to the partial pressure of end-tidal carbon dioxide (P_ETCO₂). We hypothesized P_ETCO₂ at rest predicts postoperative pulmonary complications.

METHODS

Consecutive lung resection candidates were included in this prospective multicenter study. Postoperative respiratory complications were assessed from the first 30 postoperative days or from the hospital stay. Student t test or Mann-Whitney U test was used for comparison. Multivariate stepwise logistic regression analysis was used to analyze association with the development of postoperative pulmonary complications. The De Long test was used to compare area under the curve (AUC). Data are summarized as median (interquartile range).

RESULTS

Three hundred fifty-three patients were analyzed, of which 59 (17%) developed postoperative pulmonary complications. P_ETCO₂ at rest was significantly lower (27 [24-30] vs 29 [26-32] mm Hg; P < .01) and V_E/VCO₂ slope during exercise significantly higher (35 [30-40] vs 29 [25-33]; P < .01) in patients who developed postoperative pulmonary complications. Both rest P_ETCO₂ with odds ratio 0.90 (95% confidence interval [CI] 0.83-0.97); P = .01 and V_E/VCO₂ slope with odds ratio 1.10 (95% CI 1.05-1.16); P < .01 were independently associated with postoperative pulmonary complications by multivariate stepwise logistic regression analysis. There was no significant difference between AUC of both models (rest P_ETCO₂: AUC = 0.79 (95% CI 0.74-0.85); V_E/VCO₂ slope: AUC = 0.81 (95% CI 0.75-0.86); P = .48).

CONCLUSIONS

P_ETCO₂ at rest has similar prognostic utility as V_E/VCO₂ slope, suggesting rest P_ETCO₂ may be used for postoperative pulmonary complications prediction in lung resection candidates.

Mitigation of Exercise Oscillatory Ventilation Score by Cardiac Resynchronization Therapy

BACKGROUND

Exercise oscillatory ventilation (EOV) is a consequence of ventilatory control system instability and is commonly observed in patients with advanced heart failure (HF); it is associated with adverse prognosis. The goal of this study was to evaluate the effects of cardiac resynchronization therapy (CRT) on oscillatory ventilation as quantified by a proposed EOV score.

METHODS AND RESULTS

Consecutive patients with HF (N = 35) who underwent clinically indicated CRT, cardiopulmonary exercise testing and carbon dioxide (CO₂) chemosensitivity by rebreathe before and 4-6 months after CRT were included in this post hoc analysis. With CRT, EOV scores improved in 22 patients (63%). In these patients, left ventricular ejection fraction, left atrial volume, brain natriuretic peptide concentration, and CO₂ chemosensitivity significantly improved after CRT (P < 0.05). Furthermore, minute ventilation per unit CO₂ production significantly decreased, and end-tidal CO₂ increased at rest and at peak exercise post-CRT. Multiple regression analysis showed only the change of CO₂ chemosensitivity to be significantly associated with the improvement of the EOV score (b = 0.64; F = 11.3; P = 0.004). In the group without EOV score improvement (n = 13), though left ventricular ejection fraction significantly increased with CRT (P = 0.015), no significant changes in ventilation or gas exchange were observed.

CONCLUSION

The EOV score was mitigated by CRT and was associated with decreased CO₂ chemosensitivity.

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.

Resting End-Tidal Carbon Dioxide Predicts Respiratory Complications in Patients Undergoing Thoracic Surgical Procedures

BACKGROUND

Ventilatory efficiency (V˙e/V˙co₂ slope [minute ventilation to carbon dioxide output slope]) has been shown to predict morbidity and mortality in lung resection candidates. Patients with increased V˙e/V˙co₂ during exercise also exhibit an increased V˙e/V˙co₂ ratio and a decreased end-tidal CO₂ at rest. This study hypothesized that ventilatory values at rest predict respiratory complications and death in patients undergoing thoracic surgical procedures.

METHODS

Inclusion criteria for this retrospective, multicenter study were thoracotomy and cardiopulmonary exercise testing as part of routine preoperative assessment. Respiratory complications were assessed from the medical records (from the hospital stay or from the first 30 postoperative days). For comparisons, Student's t test or the Mann-Whitney U test was used. Logistic regression and receiver operating characteristic analyses were performed for evaluation of measurements associated with respiratory complications. Data are summarized as mean ± SD; p <0.05 is considered significant.

RESULTS

Seventy-six subjects were studied. Postoperatively, respiratory complications developed in 56 (74%) patients. Patients with postoperative respiratory complications had significantly lower resting tidal volume (0.8 ± 0.3 vs 0.9 ± 0.3L; p = 0.03), lower rest end-tidal CO₂ (28.1 ± 4.3vs 31.5 ± 4.2 mm Hg; p < 0.01), higher resting V˙e/V˙co₂ ratio (45.1 ± 7.1 vs 41.0 ± 6.4; p = 0.02), and higher V˙e/V˙co₂ slope (34.9 ± 6.4 vs 31.2 ± 4.3; p = 0.01). Logistic regression (age and sex adjusted) showed resting end-tidal CO₂ to be the best predictor of respiratory complications (odds ratio: 1.21; 95% confidence interval: 1.06 to 1.39; area under the curve: 0.77; p = 0.01).

CONCLUSIONS

Resting end-tidal CO₂ may identify patients at increased risk for postoperative respiratory complications of thoracic surgical procedures.

Exercise oscillatory ventilation: Mechanisms and prognostic significance

Alteration in breathing patterns characterized by cyclic variation of ventilation during rest and during exercise has been recognized in patients with advanced heart failure (HF) for nearly two centuries. Periodic breathing (PB) during exercise is known as exercise oscillatory ventilation (EOV) and is characterized by the periods of hyperpnea and hypopnea without interposed apnea. EOV is a non-invasive parameter detected during submaximal cardiopulmonary exercise testing. Presence of EOV during exercise in HF patients indicates significant impairment in resting and exercise hemodynamic parameters. EOV is also an independent risk factor for poor prognosis in HF patients both with reduced and preserved ejection fraction irrespective of other gas exchange variables. Circulatory delay, increased chemosensitivity, pulmonary congestion and increased ergoreflex signaling have been proposed as the mechanisms underlying the generation of EOV in HF patients. There is no proven treatment of EOV but its reversal has been noted with phosphodiesterase inhibitors, exercise training and acetazolamide in relatively small studies. In this review, we discuss the mechanistic basis of PB during exercise and the clinical implications of recognizing PB patterns in patients with HF.