Cystic Fibrosis

Overview

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the CFTR gene (cystic fibrosis transmembrane conductance regulator) located on chromosome 7. The defective protein impairs chloride and bicarbonate transport across epithelial cell membranes, producing thick, dehydrated secretions that obstruct the lungs, pancreas, sweat glands, reproductive tract, and other organs. Progressive obstructive lung disease is responsible for most of the morbidity and mortality in CF, and the respiratory therapist is at the center of its day-to-day management. With the advent of CFTR modulator therapy, life expectancy has improved dramatically, making long-term maintenance and infection prevention even more critical.

Key Concepts

Genetics. CF follows classic autosomal recessive inheritance: a patient must inherit two defective copies of CFTR, one from each parent. The most common disease-causing variant is the deletion of phenylalanine at position 508, designated F508del (also called ΔF508). More than 2,000 CFTR variants have been identified, though F508del accounts for approximately 70% of alleles in people of Northern European descent. Carriers of one mutation are unaffected.

Pathophysiology. Defective CFTR channels reduce the volume of airway surface liquid (ASL), collapsing the periciliary layer and impairing mucociliary clearance. Thick mucus accumulates in bronchi and bronchioles, creating an anaerobic, nutrient-rich environment ideal for bacterial colonization. Staphylococcus aureus is typically the first colonizer in childhood. Over time, Pseudomonas aeruginosa establishes chronic airway infection; once it adopts a mucoid (alginate-producing) phenotype, eradication becomes nearly impossible. The neutrophilic inflammatory response to chronic infection releases vast quantities of DNA and proteases, further thickening secretions and damaging airway walls. The end result is progressive bronchiectasis, air trapping, and ultimately respiratory failure.

Burkholderia cepaciacomplex (BCC) deserves special mention: acquisition is associated with rapid lung function decline, occasional “cepacia syndrome” (fulminant pneumonia), and poor post-transplant outcomes. BCC is transmissible between CF patients, making strict infection control mandatory.

Multisystem involvement. Pancreatic duct obstruction leads to exocrine insufficiency, malabsorption, and fat-soluble vitamin deficiency; approximately 85% of CF patients require pancreatic enzyme replacement therapy. CF-related diabetes (CFRD) is the most common CF comorbidity in adults, stemming from progressive loss of pancreatic islet cells. Chronic sinusitis, nasal polyps, congenital bilateral absence of the vas deferens (CBAVD) causing male infertility, and meconium ileus at birth round out the clinical picture.

Assessment & Findings

Diagnosis.The sweat chloride test remains the diagnostic gold standard. A concentration of 60 mmol/L or higher is diagnostic of CF; 30–59 mmol/L is intermediate and triggers genetic confirmation; below 30 mmol/L makes CF unlikely. Newborn screening programs use immunoreactive trypsinogen (IRT) measured in dried blood spots; an elevated IRT prompts reflex CFTR genotyping and confirmatory sweat testing. Identification of two disease-causing CFTR mutations on genetic testing confirms the diagnosis.

Pulmonary function.Spirometry typically reveals an obstructive pattern (reduced FEV₁/FVC ratio) that evolves into a mixed obstructive-restrictive picture as bronchiectasis and hyperinflation progress. FEV₁ is the primary metric for tracking disease severity over time and guides decisions about CFTR modulator eligibility, transplant listing, and exacerbation management.

Microbiology surveillance. Quarterly sputum cultures (or oropharyngeal swabs in young children) track colonization patterns—specifically acquisition of P. aeruginosa, mucoid conversion, BCC, and methicillin-resistant S. aureus (MRSA). Culture results directly guide antibiotic selection during exacerbations.

Pulmonary exacerbation.Exacerbations are defined clinically and include: increased cough frequency or sputum production, change in sputum character, declining FEV₁ (typically ≥10% below baseline), increased dyspnea, unintentional weight loss, and new or worsened infiltrates on chest imaging. Prompt recognition and treatment are essential because incomplete FEV₁ recovery after exacerbations accelerates long-term decline.

RT Priorities / Interventions

Airway clearance—daily for life. No single technique is superior; the best regimen is the one the patient performs consistently. Options include:

• Chest physiotherapy (CPT) with postural drainage and percussion—effective but requires a trained caregiver.
• Positive expiratory pressure (PEP) and oscillatory PEP (e.g., Flutter, Acapella)—self-administered; oscillatory PEP adds vibratory shear forces to loosen secretions.
• High-frequency chest-wall oscillation (HFCWO / “the vest”)—inflatable vest delivers rapid chest compressions, promoting secretion mobilization without caregiver assistance.
• Autogenic drainage (AD)—a breathing technique using tidal volume manipulation at three lung levels to progressively move secretions from peripheral to central airways.
• Active cycle of breathing technique (ACBT)—combines breathing control, thoracic expansion exercises, and the forced expiration technique (huff coughing).

See the airway-clearance guide for detailed technique descriptions and patient teaching points.

Inhaled therapy sequence—order is non-negotiable. Each step prepares the airway for the next:

1. Bronchodilator (e.g., albuterol) — opens airways and prevents bronchospasm triggered by hypertonic saline.
2. Hypertonic saline (7%) — osmotically rehydrates airway surface liquid, restoring cilia function and loosening mucus.
3. Dornase alfa (Pulmozyme) — cleaves extracellular DNA from degraded neutrophils, dramatically reducing sputum viscosity.
4. Airway clearance technique — mobilizes the now-thinned secretions toward the central airways for expectoration.
5. Inhaled antibiotic (tobramycin inhalation solution, aztreonam lysine) — administered last so it penetrates cleared, open airways for maximum deposition.

Understanding dornase alfa. Dornase alfa is recombinant human DNase I (rhDNase). CF sputum contains massive quantities of DNA released by the neutrophils dying in the airways; this DNA forms a viscous gel independent of mucin content. Dornase alfa specifically cleaves these long DNA strands, reducing viscosity. Crucially, it does notbreak disulfide bonds the way N-acetylcysteine does—its mechanism is entirely DNA-specific.

CFTR modulator therapy. Modulators target the dysfunctional protein itself rather than its downstream effects:

• Ivacaftor (Kalydeco)—a potentiator that increases the open-probability of CFTR channels already present at the cell surface; most effective for gating mutations (e.g., G551D).
• Elexacaftor/tezacaftor/ivacaftor (Trikafta)—triple combination approved for patients ≥2 years with at least one F508del allele. The elexacaftor and tezacaftor components correct the protein-folding defect and improve trafficking to the cell surface; ivacaftor then potentiates channel opening. Clinical trials demonstrated FEV₁ improvements of ~14 percentage points and a 63% reduction in exacerbations.

Pulmonary exacerbation management.Hospitalization typically involves intensified airway clearance (up to four sessions daily), IV antibiotics targeted to prior culture results (anti-pseudomonal coverage is standard in chronically colonized patients), aggressive nutritional support, and close spirometric monitoring. The goal is full FEV₁ recovery to pre-exacerbation baseline.

Infection control. CF patients must not share waiting rooms, hallways, or nebulizer equipment. Cohorting CF patients together (once a common inpatient practice) is now discouraged because of cross-transmission risk. Single-patient rooms, dedicated equipment, gown-and-glove precautions, and patient education about avoiding contact with other CF patients are all standard.

Common Pitfalls

• Wrong inhaled therapy sequence. Giving dornase alfa or an inhaled antibiotic before airway clearance wastes drug in mucus-plugged airways. The antibiotic must always be last.
• Omitting the pre-treatment bronchodilator. Hypertonic saline is a potent osmotic stimulus that can trigger severe bronchospasm, particularly in patients with airway hyperreactivity. Albuterol pre-treatment is standard practice, not optional.
• Inadequate infection-control precautions. Allowing CF patients to share clinic waiting areas, equipment, or inpatient rooms creates risk of transmitting BCC and mucoid Pseudomonas—organisms that, once acquired, cannot be eradicated.
• Treating CF lung disease as asthma or COPD. CF is a distinct pathophysiology requiring CF-specific therapies (dornase alfa, hypertonic saline, CFTR modulators, anti-pseudomonal antibiotics). Standard COPD or asthma protocols miss critical steps.
• Conflating dornase alfa with traditional mucolytics. Students and clinicians sometimes describe dornase alfa as “breaking mucus bonds.” Its mechanism is DNA cleavage, not disulfide-bond reduction—an important distinction for board exams and clinical dosing rationale.

Board Exam Pearls

• Sweat chloride ≥60 mmol/L = diagnostic for CF; 30–59 mmol/L = intermediate; <30 mmol/L = CF unlikely.
• CF is autosomal recessive; F508del (delta-F508) is the most common CFTR mutation.
• Mucoid Pseudomonas aeruginosa is the hallmark chronic colonizer in CF; once mucoid, eradication is essentially impossible.
• Inhaled therapy order: bronchodilator → hypertonic saline → dornase alfa → airway clearance → inhaled antibiotic.
• Dornase alfa (rhDNase) reduces sputum viscosity by cleaving extracellular DNA—not by breaking disulfide bonds.
• Elexacaftor/tezacaftor/ivacaftor (Trikafta) targets patients with ≥1 F508del allele and is the most broadly applicable CFTR modulator.
• Burkholderia cepacia complex acquisition in CF carries a poor prognosis and can be transmitted between patients—strict segregation is required.
• Newborn CF screening uses immunoreactive trypsinogen (IRT); confirmatory sweat testing and CFTR genotyping follow a positive screen.

FAQ

Related Resources

Sources

1. Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021.
2. Mogayzel PJ Jr, Naureckas ET, Robinson KA, et al. Cystic fibrosis pulmonary guidelines: chronic medications for maintenance of lung health. Am J Respir Crit Care Med. 2013;187(7):680-689.
3. Gardenhire DS. Rau's Respiratory Care Pharmacology. 11th ed. Elsevier; 2023.