Respiratory mechanics
Respiratory mechanics is the framework of elastic recoil, compliance, and airway resistance that determines the work of breathing and the pattern of respiratory failure a disease produces. A falling compliance drives restrictive physiology treated with lung-protective ventilation (tidal volume 4-8 mL/kg, plateau pressure below 30 cm H2O), while rising airway resistance drives obstructive physiology, auto-PEEP, and is treated with bronchodilators and a slower respiratory rate to lengthen exhalation.
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What this article covers
- Overview
Definition: respiratory mechanics is the framework, elastic recoil, compliance, and airway resistance, that determines how much pressure the muscles must generate to move a given volume of air, and therefore the work of breathing.
- Epidemiology
Applies to any patient with dyspnea, tachypnea, or respiratory failure, not a discrete disease with its own incidence; used every time breathing effort is assessed at the bedside or on a ventilator.
- Risk factors
Emphysema and severe COPD, which lower elastic recoil and airway integrity together, producing the largest simultaneous rise in compliance and resistance.
- Pathophysiology
Elastic recoil is the lung's tendency to collapse inward, opposed by the chest wall's tendency to spring outward; at functional residual capacity (FRC) these forces balance, airway and alveolar…
- Clinical features
Increased work of breathing is recognized at the bedside: tachypnea, accessory muscle use (sternocleidomastoid, scalene, intercostal retractions), and nasal flaring are the earliest visible signs.
- Differential diagnosis
COPD/emphysema: irreversible obstruction from loss of elastic recoil and high resistance; unlike asthma, the postbronchodilator ratio of forced expiratory volume in 1 second to forced vital capacity…
- Diagnosis
Abnormal mechanics is characterized, not diagnosed, by classifying the pattern (obstructive, restrictive, or pump failure) and quantifying severity: obstruction is confirmed when FEV1/FVC falls below…
- Management
First, treat the mechanical derangement driving increased work of breathing, not a number: escalate support for severe distress (inability to speak in sentences, rate above 30/min, SpO2 below 90%…
- Complications
Ventilator-induced lung injury: volutrauma from overdistension and atelectrauma from repeated alveolar opening/closing, worsened by exceeding lung-protective limits.
- Prognosis
Severity of mechanical impairment tracks with outcome: percent-predicted FEV1 and TLC drive staging systems (GOLD, the BODE index) and correlate with mortality.
- Prevention
Use lung-protective ventilation from the first breath in any intubated patient at risk for ARDS: tidal volume 4-8 mL/kg predicted body weight, plateau pressure below 30 cm H2O, not only once ARDS is established.
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Related articles
- Pulmonary function testing
Pulmonary function testing uses spirometry, lung volumes, and DLCO to classify abnormal lung physiology as obstructive, restrictive, or mixed and to grade severity against percent-predicted values. A reduced FEV1/FVC ratio defines obstruction, a reduced total lung capacity confirms restriction, and DLCO distinguishes the underlying cause and guides whether bronchodilator or bronchial challenge testing is added.
- Mechanical ventilation
Invasive and noninvasive positive pressure ventilation support gas exchange and reduce work of breathing in hypoxemic or hypercapnic respiratory failure, upper airway impairment, or inability to protect the airway.
- Acute respiratory distress syndrome (ARDS)
ARDS is acute (within 1 week), bilateral, non-cardiogenic hypoxemic respiratory failure from diffuse alveolar damage, diagnosed by the 2012 Berlin criteria and graded by the Pao2/Fio2 ratio. Management is lung-protective ventilation, PEEP titration, early prone positioning for severe disease, conservative fluid strategy, and treatment of the inciting insult.
- Acute exacerbation of COPD
An acute exacerbation of COPD is a change in baseline dyspnea, cough, or sputum that requires a change in therapy, most often triggered by viral or bacterial respiratory infection. Severity is graded by the treatment required, and management is a stepwise sequence of bronchodilators, glucocorticoids, targeted oxygen, and, when hypercapnic acidosis is present, noninvasive ventilation.
- Asthma
Asthma is a chronic inflammatory airway disorder causing episodic, reversible airflow obstruction confirmed by spirometry or bronchial challenge testing. Management is a stepwise ladder led by inhaled glucocorticoids plus as-needed SABA or ICS-formoterol, escalating to biologics for severe type 2-high disease, with an oral glucocorticoid burst and frequent SABA for exacerbations.
- Interstitial lung disease
Interstitial lung disease (ILD), also called diffuse parenchymal lung disease, is a diverse group of nonmalignant disorders causing inflammation and scarring of the alveolar interstitium, presenting as subacute to chronic exertional dyspnea and nonproductive cough. Diagnosis rests on pulmonary function testing plus high-resolution CT pattern recognition; management is exposure removal, glucocorticoids for inflammatory disease, antifibrotic therapy for progressive fibrotic disease, oxygen for hypoxemia, and early transplant referral.
- Acute hypercapnic respiratory failure
Acute hypercapnic (type II, ventilatory) respiratory failure is alveolar hypoventilation with arterial PCO2 above 45 mm Hg and acidemia, from pump failure or dead-space ventilation. Confirm with an arterial blood gas in every suspected patient, then treat the precipitant while titrating noninvasive or invasive ventilation to reverse the acidosis.