What is the mechanism of increased Functional Residual Capacity (FRC) in patients, particularly those with pre-existing lung disease such as Chronic Obstructive Pulmonary Disease (COPD) or asthma, during a bronchoscopy procedure?

Mechanism of Increased FRC During Bronchoscopy

The increase in Functional Residual Capacity (FRC) during bronchoscopy occurs primarily through mechanical airway obstruction by the bronchoscope itself, which creates expiratory flow limitation and prevents complete lung emptying, leading to dynamic hyperinflation—particularly problematic in patients with pre-existing obstructive lung disease like COPD or asthma. 1

Primary Mechanisms

Physical Airway Obstruction

• The bronchoscope physically occupies airway space, creating a fixed obstruction that increases airway resistance and limits expiratory flow. 1
• In spontaneously breathing patients, insertion of either an endotracheal tube (ETT) alone or an ETT-bronchoscope combination causes FRC to increase by approximately 30% in intubated patients and 17% in transnasal bronchoscopy patients. 1
• The bronchoscope acts as a one-way valve effect, allowing air in during inspiration but impeding expiratory flow, which traps gas in the lungs. 1

Dynamic Hyperinflation Mechanism

• When expiratory flow is limited by the bronchoscope, the interval between inspiratory efforts does not allow expiration to the relaxation volume of the respiratory system, leading to dynamic pulmonary hyperinflation. 2
• This creates intrinsic positive end-expiratory pressure (PEEPi), which is an inspiratory threshold load that must be overcome by inspiratory muscles to initiate the next breath. 2
• The rate of lung emptying is slowed by the physical obstruction, and incomplete exhalation causes progressive air trapping with each breath. 2

Exacerbated Effects in COPD Patients

Baseline Pathophysiology

• COPD patients already have increased FRC at baseline due to loss of lung elastic recoil (static factor) and expiratory flow limitation during tidal breathing (dynamic factor). 2
• These patients have very high V'/Q' units representing emphysematous regions with alveolar destruction and loss of pulmonary vasculature, making them particularly vulnerable to further hyperinflation. 2

Compounded Risk During Bronchoscopy

• The addition of bronchoscope-induced airway obstruction to pre-existing expiratory flow limitation in COPD creates a compounding effect that substantially increases end-expiratory lung volume and PEEPi. 2, 1
• Airway resistance, end-expiratory lung volume, and PEEPi increase substantially during procedures in COPD patients, with the increase in elastic load potentially exceeding the increase in resistive load. 2
• Patients with severe COPD (FEV1/FVC <50% or FEV1 <1 liter) have a 5% complication rate during bronchoscopy compared to 0.6% in those with normal lung function, largely due to these hyperinflation effects. 2

Effects in Asthmatic Patients

• Asthmatics experience a more pronounced fall in FEV1 during bronchoscopy compared to normal subjects, inversely correlated with baseline airway hyperreactivity. 2
• Lignocaine used during the procedure may paradoxically produce bronchoconstriction in asthmatic patients, further worsening expiratory flow limitation and contributing to FRC elevation. 2
• The combination of procedure-induced bronchospasm and mechanical obstruction by the bronchoscope creates a dual mechanism for air trapping in this population. 2

Ventilator-Dependent Patients

Pressure vs. Volume Control

• In mechanically ventilated patients, the bronchoscope increases airway resistance such that the ventilator cannot deliver adequate volume to maintain baseline end-expiratory lung volume, paradoxically sometimes causing FRC to decrease rather than increase. 3
• During flexible bronchoscopy with peak pressure limiting ventilation, 64% of intubated patients showed decreased end-expiratory lung volume (-325±371 mL) while 32% showed increases (65±59 mL), depending on ventilator mode and settings. 3

Suctioning Effects

• Bronchoscopic suctioning creates additional complexity—suction flow through a 16 Fr bronchoscope at commonly used vacuum levels (-20 to -80 kPa) ranges from 5-17 L/min, which exceeds minute ventilation in most patients and causes pronounced derecruitment with FRC decreasing by approximately 479 mL. 4
• The ventilator becomes unable to deliver enough volume in either volume-controlled or pressure-controlled modes to maintain FRC, and tracheal pressure can decrease below atmospheric pressure during suctioning. 4

Clinical Implications

Monitoring Requirements

• Patients with suspected COPD should have spirometric parameters checked before bronchoscopy, and those with severe COPD (FEV1 <40% predicted and/or SaO2 <93%) should also have arterial blood gas tensions measured. 2
• The increase in FRC can impair inspiratory muscle function and coordination, although diaphragm contractility when normalized for lung volume appears preserved. 2

Oxygen and Sedation Risks

• Oxygen supplementation and/or intravenous sedation may lead to increased arterial CO2 levels in COPD patients due to worsened V'/Q' mismatch from hyperinflation, so sedation should be avoided when pre-bronchoscopy arterial CO2 is elevated. 2
• The increased mechanical workload from hyperinflation means energy consumption of inspiratory muscles at any given minute ventilation is greater than in normal subjects. 2