Reinnervation of pulmonary stretch receptors

Effects of bilateral lung transplantation on cardiac autonomic modulation and cardiorespiratory coupling: a prospective study

Background

Although cardiac autonomic modulation has been studied in several respiratory diseases, the evidence is limited on lung transplantation, particularly on its acute and chronic effects. Thus, we aimed to evaluate cardiac autonomic modulation before and after bilateral lung transplantation (BLT) through a prospective study on patients enrolled while awaiting transplant.

Methods

Twenty-two patients on the waiting list for lung transplantation (11 women, age 33 [24–51] years) were enrolled in a prospective study at Ospedale Maggiore Policlinico Hospital in Milan, Italy. To evaluate cardiac autonomic modulation, ten minutes ECG and respiration were recorded at different time points before (T0) and 15 days (T1) and 6 months (T2) after bilateral lung transplantation. As to the analysis of cardiac autonomic modulation, heart rate variability (HRV) was assessed using spectral and symbolic analysis. Entropy-derived measures were used to evaluate complexity of cardiac autonomic modulation. Comparisons of autonomic indices at different time points were performed.

Results

BLT reduced HRV total power, HRV complexity and vagal modulation, while it increased sympathetic modulation in the acute phase (T1) compared to baseline (T0). The HRV alterations remained stable after 6 months (T2).

Conclusion

BLT reduced global variability and complexity of cardiac autonomic modulation in acute phases, and these alterations remain stable after 6 months from surgery. After BLT, a sympathetic predominance and a vagal withdrawal could be a characteristic autonomic pattern in this population.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-021-01752-6.

Pulmonary volume-feedback and ventilatory pattern after bilateral lung transplantation using neurally adjusted ventilatory assist ventilation

BACKGROUND

Bilateral lung transplantation results in pulmonary vagal denervation, which potentially alters respiratory drive, volume-feedback, and ventilatory pattern. We hypothesised that Neurally Adjusted Ventilatory Assist (NAVA) ventilation, which is driven by diaphragm electrical activity (EAdi), would reveal whether vagally mediated pulmonary-volume feedback is preserved in the early phases after bilateral lung transplantation.

METHODS

We prospectively studied bilateral lung transplant recipients within 48 h of surgery. Subjects were ventilated with NAVA and randomised to receive 3 ventilatory modes (baseline NAVA, 50%, and 150% of baseline NAVA values) and 2 PEEP levels (6 and 12 cm H₂O). We recorded airway pressure, flow, and EAdi.

RESULTS

We studied 30 subjects (37% female; age: 37 (27-56) yr), of whom 19 (63%) had stable EAdi. The baseline NAVA level was 0.6 (0.2-1.0) cm H₂O μV^-1. Tripling NAVA level increased the ventilatory peak pressure over PEEP by 6.3 (1.8), 7.6 (2.4), and 8.7 (3.2) cm H₂O, at 50%, 100%, and 150% of baseline NAVA level, respectively (P<0.001). EAdi peak decreased by 10.1 (9.0), 9.5 (9.4) and 8.8 μV (8.7) (P<0.001), accompanied by small increases in tidal volume, 8.3 (3.0), 8.7 (3.6), and 8.9 (3.3) ml kg^-1 donor's predicted body weight at 50%, 100%, and 150% of baseline NAVA levels, respectively (P<0.001). Doubling PEEP did not affect tidal volume.

CONCLUSIONS

NAVA ventilation was feasible in the majority of patients during the early postoperative period after bilateral lung transplantation. Despite surgical vagotomy distal to the bronchial anastomoses, bilateral lung transplant recipients maintained an unmodified respiratory pattern in response to variations in ventilatory assistance and PEEP.

CLINICAL TRIAL REGISTRATION

NCT03367221.

Cough reflex in lung transplant recipients

Lung transplantation has become the standard of care for particular individuals with advanced lung disease. However, this surgical procedure involves interruption of the lower vagal nerve fibers which leads to loss of the protective cough reflex. Injury of the neural pathways involved with the sensory limb of the cough reflex is associated with an increased risk of complications involving the allograft. While loss of the cough reflex was once considered permanent, recent evidence indicates functional and structural restoration is a time-dependent process that occurs 6-12 months after lung transplantation. The implication that the cough reflex may be reestablished in lung transplant recipients provides insight into the dynamic response to airway neural injury that may lead to improvements in allograft tissue repair.

Lung transplantation and lung volume reduction surgery

Since the publication of the last edition of the Handbook of Physiology, lung transplantation has become widely available, via specialized centers, for a variety of end-stage lung diseases. Lung volume reduction surgery, a procedure for emphysema first conceptualized in the 1950s, electrified the pulmonary medicine community when it was rediscovered in the 1990s. In parallel with their technical and clinical refinement, extensive investigation has explored the unique physiology of these procedures. In the case of lung transplantation, relevant issues include the discrepant mechanical function of the donor lungs and recipient thorax, the effects of surgical denervation, acute and chronic rejection, respiratory, chest wall, and limb muscle function, and response to exercise. For lung volume reduction surgery, there have been new insights into the counterintuitive observation that lung function in severe emphysema can be improved by resecting the most diseased portions of the lungs. For both procedures, insights from physiology have fed back to clinicians to refine patient selection and to scientists to design clinical trials. This section will first provide an overview of the clinical aspects of these procedures, including patient selection, surgical techniques, complications, and outcomes. It then reviews the extensive data on lung and muscle function following transplantation and its complications. Finally, it reviews the insights from the last 15 years on the mechanisms whereby removal of lung from an emphysema patient can improve the function of the lung left behind.

Sympathetic reinnervation of unilaterally denervated rat lung

The effect of unilateral sympathetic denervation and reinnervation of the lung on a variety of circulatory parameters was investigated in urethane-anaesthetized rats. The left main stem bronchus together with its surrounding nerves was cut and reanastomosed in 40 rats: 12 intact rats served as controls. Final experiments were performed after 0 days to 10 months: the left stellate ganglion was stimulated. The effect was greatest at 20 Hz. Pulmonary arterial pressure increased significantly (P < 0.05) by 10% and pulmonary flow decreased significantly (P < 0.05) by 16% in the control rats; no effect on these parameters was found in acutely denervated rats. The stimulus-elicited change in pulmonary arterial pressure reappeared 1 week after unilateral hilar stripping and gradually returned during reinnervation. After 10 months, the increase in pressure was significantly (P < 0.05) larger than that of the control by 50%, whereas the noradrenaline content of the reinnervated lung was significantly (P < 0.05) smaller than that of the intact side by 48%. This discrepancy may reflect denervation hypersensitivity of vascular muscle cells. The operations had no effect on systemic circulation: heart rate and systemic arterial pressure increased and aortic blood flow decreased to the same extent in all experimental groups during nerve stimulation. These results suggest that sympathetic reinnervation of the rat lung starts within 1 week but that reinnervation is still incomplete 10 months after unilateral hilar stripping.

Apnoea following normocapnic mechanical ventilation in awake mammals: a demonstration of control system inertia

1. Inhibition of inspiratory muscle activity from volume-related feedback during mechanical ventilation has been shown previously. To determine if this neuromechanical inhibition displays a memory effect, the duration of expiration immediately following cessation of mechanical ventilation was assessed in eight normal subjects. The subjects were passively mechanically ventilated via a nasal mask until the end-tidal CO2 (PET,CO2) was a minimum of 30 mmHg and inspiratory effort was no longer detected, as evidenced by stabilization of mouth pressure and disappearance of surface diaphragm EMG activity. The ventilator output was held constant at a mean tidal volume (VT) of 1.0 l and breath duration of 4.6 s and PET,CO2 was increased 1-1.5 mmHg/min (via increased inspired CO2 fraction, FI,CO2) until inspiratory muscle activity returned. The PET,CO2 at which activation first occurred was defined as the CO2 recruitment threshold (PCO2,RT). The mechanical ventilation protocol was repeated and the PET,CO2 increased 1-1.5 mmHg/min until it was a mean of 1.1 mmHg above spontaneous PET,CO2 and 3.6 mmHg below PCO2,RT. After 4-6 min of mildly hypercapnic mechanical ventilation, the mechanical ventilation was terminated. 2. Following termination of mechanical ventilation, the duration of the subsequent apnoea was 14.6 +/- 2.8 s (mean +/- S.E.M.) or 453 +/- 123% > spontaneous TE and 178 +/- 62% > the TE chosen by the subject during 'assist control' ventilation at VT = 1.0 l. 3. To test the hypothesis that the apnoea following cessation of mechanical ventilation was due to a vagally mediated memory effect, the study was repeated in five double-lung transplant patients with similar PCO2,RT to normal subjects. These pulmonary vagally denervated patients also displayed an apnoea (14.5 +/- 4.0 s) upon cessation of mechanical ventilation (at a PET,CO2 2.0 mmHg > eupnoea and 2.4 mmHg < PCO2,RT), that was 367 +/- 162% > spontaneous TE. 4. We also found significant apnoea in the awake dog immediately following mildly hypercapnic passive mechanical ventilation, and this was similar before and after bilateral vagal blockade (15.7 +/- 1.3 and 19.7 +/- 4.7 s, respectively). 5. We conclude that neuromechanical inhibition of inspiratory muscle activity, produced by passive mechanical ventilation at high VT, exhibits a memory effect reflected in TE prolongation, which persists in the face of substantial increases in chemoreceptor stimuli. This effect is not dependent on vagal feedback from lung receptors. 6. We hypothesize that this persistent apnoea represents an inherent 'inertia', characteristic of the ventilatory control system.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency of mechanical ventilation and respiratory activity after double lung transplantation

We investigated the contribution of pulmonary afferent nerve fibers to the control of inspiratory activity in awake humans. Eight double lung transplant outpatients and eight normal subjects were hyperventilated with a mechanical ventilator. Respiratory frequency was increased until no respiratory activity was detectable. Then, by either adding CO2 in the inspired gas or decreasing respiratory frequency, end-tidal PCO2 (PETCO2) was increased until inspiratory activity (i.e. change in inspiratory airway pressure peak and/or time profile) was detected. In normal subjects, PETCO2 threshold for inspiratory muscle recruitment was significantly lower when frequency was decreased than when CO2 was added (31.3 +/- 6.8 Torr vs. 38.2 +/- 8.1 Torr respectively, P < 0.005). This was not the case in the double lung transplant group (31.5 +/- 6.5 Torr vs. 32.9 +/- 5.8 Torr). These findings suggest that pulmonary afferent nerves have an inhibitory effect on inspiratory activity in humans.

Respiratory modulation of muscle sympathetic nerve activity in intact and lung denervated humans

We determined the influences of breathing-induced changes in intrathoracic and intravascular pressures, central respiratory drive, and pulmonary vagal feedback on the within-breath variation in skeletal muscle sympathetic nerve activity (MSNA) in humans. MSNA (peroneal microneurography), arterial blood pressure (Finapres finger monitor), and tidal volume (VT) were recorded continuously in six normal subjects and four heart-lung transplant patients during: 1) spontaneous air breathing; 2) increased FICO2; 3) voluntary augmentation of VT with and without inspiratory resistance; and 4) positive pressure, passive mechanical ventilation. During conditions 3 and 4, which were performed under isocapnic conditions with a high MSNA background (either high resting activity or nonhypotensive lower body suction), subjects breathed at control or elevated VT with normal or prolonged inspiratory time (TI); breathing frequency was 12 breaths per minute. During control breathing in normal subjects there was a distinct within-breath pattern of MSNA, with approximately 70% of the activity occurring during low lung volumes (initial half of inspiration and latter half of expiration). This within-breath variation of MSNA was potentiated with increased VT breathing (> 85% of activity occurring during low lung volumes; p < 0.05 versus control breathing) and was similar during the voluntary and CO2-induced hyperpneas. MSNA decreased progressively and markedly from onset to late inspiration; fell slightly further, reaching its nadir at end-inspiration/onset-expiration; and rose sharply during mid-late expiration. Only the nadir of MSNA was associated with any change in arterial pressure. Resistive breathing, especially at elevated VT, caused a fall in arterial pressure and increased respiratory drive during inspiration, yet MSNA still declined as lung volume increased. Normal within-breath modulation of MSNA also was observed during control and elevated VT induced via positive pressure with passive ventilation, which reversed lung inflation/deflation-induced intrathoracic pressure changes and reduced or removed respiratory motor output. During control breathing in transplant patients the specific within-breath pattern of MSNA was somewhat different than that of the normal subjects, but on average, the overall low lung volume to high lung volume MSNA ratio was similar to normal subjects. In contrast to the normal subjects, however, there was no potentiation of the within-breath variation of MSNA with elevated tidal breathing. These findings indicate that during normal levels of tidal breathing most of the respiratory phase influence on muscle sympathetic outflow observed in normal conscious humans is independent of baroreceptor-sensed fluctuations in intrathoracic or intravascular pressures and of lung inflation-stimulated vagal afferent activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Breathing during wakefulness and sleep after human heart-lung transplantation

To study the effects of pulmonary denervation on breathing during sleep, sleep studies were conducted on seven heart-lung transplant recipients (H-LT) and a comparable number of sex-matched normal subjects of similar age. Four of the H-LT patients had a restrictive pattern on spirometry. The time since transplantation ranged from 45 to 1,102 days. There were no significant differences between the groups with respect to total sleep time or distribution of sleep stages. There were no significant differences between the H-LT recipients and normal subjects with respect to baseline awake oxyhemoglobin saturation (SaO2) or the nadirs of SaO2 during REM and non-REM sleep, the absolute number and frequency (number per hour of sleep) of apneas, hypopneas, desaturation events, both over the whole night of study or separately during non-REM and REM sleep. Across wakefulness and all sleep stages, the H-LT patients tended to have shorter total respiratory cycle times (Ttot) (p = 0.052) and more rapid breathing frequency (F) than the normal subjects. This was associated with significantly shorter inspiratory times (Tl) (p less than 0.001) and smaller duty cycles (Tl/Ttot) (p less than 0.005) in the H-LT recipients. During non-REM and REM sleep, F tended to be higher in the H-LT recipients with pulmonary restriction than in the nonrestricted patients. There were no significant differences between the H-LT recipients and the normal subjects with regard to the periodicity of breathing, either in terms of timing parameters or breath amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)