What is the Hering‑Breuer reflex and its clinical relevance in mechanical ventilation, neuromuscular disease, pediatric anesthesia, and obstructive sleep apnea?

Hering-Breuer Reflex: Definition and Clinical Relevance

Core Mechanism

The Hering-Breuer reflex is a protective respiratory reflex mediated by slowly adapting pulmonary stretch receptors (SARs) that terminates inspiration and prolongs expiration when the lungs reach adequate inflation. 1

Anatomical Basis

• SARs are vagal afferent nerve fibers that terminate primarily in intrapulmonary airways, with some species variation in extrapulmonary distribution 1
• These receptors are myelinated fibers with conduction velocities of 15-32 m/s and maintain continuous discharge activity of 10-40 impulses/second during normal breathing 1
• SARs are closely associated with airway smooth muscle, continuously sensing tension within myoelastic airway components caused by lung inflation, smooth muscle contraction, and tethering of small airways to lung parenchyma 2

Physiological Function

• SAR activity increases sharply during inspiration and peaks just before expiration begins 1
• When activated by sustained lung inflation, SARs trigger central inhibition of respiration and inhibition of cholinergic drive 1
• The reflex produces expiratory prolongation by activating expiratory-decrementing (E-Dec) neurons in the ventral respiratory group while simultaneously hyperpolarizing and silencing inspiratory neurons 3

Clinical Relevance in Specific Populations

Mechanical Ventilation

In mechanically ventilated adults, the Hering-Breuer reflex can be assessed by end-inspiratory airway occlusion, with positive reflex indicated when occluded expiratory time exceeds maximal baseline expiratory time. 4

• The reflex is demonstrable in all mechanically ventilated adults, though sensitivity varies with underlying lung pathology 4
• Normal adults show progressive increase in expiratory time prolongation with increasing lung volumes (from 167.5% at normal FRC to 474% at PEEP 20 cmH₂O) 4
• COPD patients demonstrate markedly reduced reflex sensitivity compared to normal controls, with expiratory time prolongation increasing only from 125.2% to 193.7% across the same volume range 4
• This blunted response in COPD may contribute to altered breathing patterns and reduced protective mechanisms against hyperinflation 4

Practical Application in Ventilator Management

• During pressure support ventilation, the reflex influences inspiratory termination and patient-ventilator synchrony 5
• In patients with obstructive lung disease, intrinsic PEEP (PEEPi) of 10-15 cmH₂O can develop, though EPAP levels above 5 cmH₂O are rarely tolerated 1
• The reflex can be utilized during weaning by applying end-inspiratory occlusions to assess readiness for spontaneous breathing, with prolonged expiratory times indicating intact protective reflexes 1

Pediatric Anesthesia and Neonatal Care

The Hering-Breuer deflationary reflex is highly active in newborn infants and serves a critical protective role in maintaining functional residual capacity. 6

• In 85% of forced expiratory maneuvers in term newborns, inspiratory effort occurs within 166 msec of lung deflation, with mean intrathoracic pressure reduction of 23.4 cmH₂O (2.5 times normal tidal breathing) 6
• When lung deflation occurs at low volumes (end-expiration), the inspiratory response is significantly faster (133 msec) and stronger (reducing intrathoracic pressure to -15.8 cmH₂O) compared to deflation at end-inspiration (181 msec, -25.2 cmH₂O) 6
• This rapid, powerful response protects against excessive FRC reduction in infants 6

Measurement in Infant Respiratory Function Testing

• The reflex strength can be quantified during plethysmographic measurements using airway occlusion at end-expiration or end-inspiration 1
• HBR-EE (end-expiratory occlusion) is calculated as: 100×(tI,occ - tI,FRC)/tI,FRC 1
• HBR-EI (end-inspiratory occlusion) is calculated as: 100×(tE,occ - tE,FRC)/tE,FRC 1
• Activation of the end-inspiratory Hering-Breuer reflex in sedated infants can be used to evoke apnea needed for low-frequency forced oscillation technique measurements 1

Neuromuscular Disease

Patients with neuromuscular disease may have insufficient respiratory effort to trigger ventilator breaths, particularly during sleep, necessitating timed backup modes that bypass normal reflex control. 1

• The reflex remains intact in neuromuscular disease, but weak inspiratory muscles cannot generate adequate negative pressure to trigger ventilator support 1
• Timed or assist/control ventilation modes are essential, providing mandatory breaths similar to the patient's unsupported ventilatory pattern 1
• Backup respiratory rates of 10-14 breaths/min should be set to ensure adequate ventilation when spontaneous effort is insufficient 5, 7

Obstructive Sleep Apnea

While the Hering-Breuer reflex itself is not directly implicated in OSA pathophysiology, reduced pharyngeal tone and obtunded upper airway reflexes in OSA patients increase susceptibility to respiratory complications during sedation and the postoperative period. 8

• OSA patients experience multiple prolonged oxygen desaturations that increase sensitivity to opioid-induced respiratory depression 8
• Reduced pharyngeal tone causes upper airway collapse, particularly in obese OSA patients who are more sensitive to residual anesthesia and opioids 1, 8
• Late airway obstruction following opioid administration is a recognized problem in OSA patients, requiring reduced opioid dosing and extended monitoring 1, 8
• Protective laryngeal reflexes remain impaired after tracheal extubation, increasing aspiration risk 1, 8

Pathological Alterations

Asthma

During acute asthma attacks, the normal Hering-Breuer reflexes that regulate breathing are abolished, and increased stretch receptor activity paradoxically enhances tachypnea rather than inhibiting inspiration. 9

• The lung inflation reflex (which normally terminates inspiration) no longer functions during bronchospasm 9
• Instead, the lung deflation reflex becomes dominant, with expiratory self-compression of the lungs driving increased respiratory frequency 9
• This pathological response contributes to dynamic hyperinflation and increased work of breathing during asthma exacerbations 9

Interstitial Fibrosis

Patients with interstitial fibrosis maintain normal or enhanced Hering-Breuer reflex sensitivity despite altered lung mechanics. 4

• Unlike COPD patients, IF patients show preserved reflex responses to volume changes 4
• This preserved sensitivity may contribute to the characteristic rapid, shallow breathing pattern in restrictive lung disease 4

Critical Distinctions from Other Reflexes

The Hering-Breuer reflex mediated by SARs is distinct from cough reflexes and should not be confused with rapidly adapting receptors (RARs) or C-fiber activation. 1

• SARs are spontaneously active during normal breathing and do not directly initiate coughing 1
• RARs (often incorrectly grouped with "irritant receptors") respond primarily during dynamic lung inflation and deflation but are largely ineffective at initiating cough 1
• Sustained lung inflations and negative intraluminal pressures activate stretch receptors but do not reliably trigger cough in animals or humans 1
• Both SAR and RAR activity likely modulates sensitivity to tussive stimuli and regulates the duration and magnitude of cough phases, but neither plays a primary role in cough initiation 1