30.2 Acute Bronchitis & Lower Respiratory Tract Infections

Acute Bronchitis: Pathophysiology, Natural History & Etiology

Acute bronchitis is one of the most frequent reasons for outpatient ambulatory visits in primary care, accounting for over 10 million office evaluations annually in the United States. It is defined as a self-limiting inflammation of the large conducting airways (tracheobronchial tree) presenting as an acute cough with or without sputum production in a patient without underlying chronic lung disease (such as COPD, asthma, bronchiectasis, or cystic fibrosis).

Pathophysiology & Natural History

• Mechanisms: Inoculation of the respiratory tract leads to hyperemic bronchial mucosa, desquamation of ciliated columnar epithelial cells, lymphocytic infiltration of the submucosa, excessive mucous secretion, and transient bronchial hyperresponsiveness.
• The Expected 3-Week Timeline: Clinical studies demonstrate that the natural history of acute bronchitis features a cough that averages 18 days (range 1 to 3 weeks), with up to 25% of patients continuing to cough beyond 3 to 4 weeks. Clinician education regarding this 18-day average is paramount: patients who expect coughs to resolve within 5 to 7 days frequently request antibiotics under the false premise of an abnormally prolonged illness.

Microbial Etiology

• Viral Pathogens (>90%): The vast majority of acute bronchitis presentations are viral, including:
  • Rhinovirus and Enterovirus (most prevalent);
  • Coronaviruses (common human strains);
  • Influenza A and Influenza B viruses;
  • Respiratory Syncytial Virus (RSV);
  • Parainfluenza viruses;
  • Human Metapneumovirus (hMPV).
• Atypical Bacterial Pathogens (<10%): Uncommon causes include Bordetella pertussis, Mycoplasma pneumoniae, and Chlamydophila pneumoniae.
• The Critical Negative Rule: Typical pyogenic bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis) do NOT cause acute bronchitis in ambulatory patients with normal baseline lung architecture! These organisms cause pneumonia when alveolar spaces are infected, or acute exacerbations in patients with pre-existing COPD, but they are not primary pathogens in uncomplicated acute bronchitis.

---

The Sputum Color Fallacy: Debunking Purulence

A persistent clinical myth in ambulatory medicine is that yellow or green (purulent) sputum signifies bacterial infection requiring antimicrobial therapy.

• The Cellular Reality: Sputum purulence is driven by the release of myeloperoxidase, a green heme-containing enzyme released from the cytoplasmic granules of activated polymorphonuclear neutrophils (PMNs), accompanied by cellular debris from desquamated bronchial epithelium.
• Evidence-Based Rule: Multiple randomized trials have demonstrated that the presence of purulent, discolored, or thick green sputum correlates with intense mucosal inflammation, NOT the presence of bacterial pathogens. Purulent sputum does not predict bacterial etiology, nor does it predict clinical response to antibacterial therapy.

---

Choosing Wisely & Antibiotic Stewardship in Acute Bronchitis

Despite clear clinical guidelines, acute bronchitis remains one of the leading drivers of inappropriate antibiotic prescribing in adult ambulatory medicine, with antibiotics historically prescribed in 60% to 80% of clinical visits.

Evidence Against Antibiotic Prescribing

• Negligible Clinical Efficacy: A landmark Cochrane Systematic Review of 17 randomized controlled trials (over 5,000 patients) demonstrated that routine antibiotic prescribing reduced the total duration of cough by approximately 0.5 days (approx. 12 hours) over a 2-to-3-week illness—a clinically imperceptible benefit.
• Significant Adverse Harms: Approximately 1 in 5 to 1 in 10 patients prescribed antibiotics experience adverse drug events, including:
  • Gastrointestinal distress (nausea, severe vomiting, abdominal cramping, severe diarrhea);
  • Vulvovaginal candidiasis;
  • Allergic cutaneous exanthems, urticaria, and life-threatening anaphylaxis;
  • Life-threatening Clostridioides difficile colitis;
  • Escalation of individual and community antimicrobial resistance.
• Guidelines Consensus: The American College of Physicians (ACP), Centers for Disease Control and Prevention (CDC), and American Academy of Family Physicians (AAFP) Choosing Wisely campaign explicitly recommend:

  "Do not prescribe antibiotics for uncomplicated acute bronchitis in otherwise healthy patients without chronic lung disease."

Communication Strategies to Avoid Antibiotic Overuse

Clinicians often prescribe antibiotics due to perceived patient pressure or fear of patient dissatisfaction. Evidence demonstrates that patient satisfaction correlates with adequate communication and validation, not antibiotic prescriptions:

1. Validate Distress: Acknowledge the disruptive nature of the cough ("I can see how exhausted you are from coughing through the night").
2. Avoid Ambiguous Labels: Avoid terms like "chest cold" or "bronchial infection" without explaining they are viral. Explain: "You have acute viral bronchitis, which is an inflammation of the airways caused by a virus. Antibiotics kill bacteria, not viruses, and taking them will not make your cough resolve any faster while exposing you to side effects like diarrhea and rash."
3. Set Realistic Expectations: Emphasize that post-viral airway hyperreactivity takes an average of 18 days to resolve.
4. Provide a Written Symptom-Relief Plan: Outline non-pharmacologic and pharmacologic antitussive strategies.
5. Safety-Netting & Delayed Prescribing: Instruct the patient to contact the clinic if they develop new high fevers, shortness of breath, hemoptysis, or if symptoms worsen after initial improvement.

---

Clinical Rule-Out of Pneumonia: The Vital Signs & Exam Rule

The primary clinical challenge in evaluating acute cough is differentiating self-limited acute bronchitis from potentially life-threatening community-acquired pneumonia (CAP) without ordering unnecessary chest radiographs on every patient.

                  ACUTE COUGH EVALUATION IN AMBULATORY ADULTS

                       Acute Cough Illness (<=3 Weeks)
                                      │
                                      ▼
                  Check Vital Signs & Perform Lung Exam
                                      │
           ┌──────────────────────────┴──────────────────────────┐
           ▼                                                     ▼
   NORMAL VITAL SIGNS & EXAM                             ANY ABNORMAL FINDING
   • Heart Rate <100 bpm                                 • Heart Rate >=100 bpm
   • Respiratory Rate <20 breaths/min                    • Respiratory Rate >=20 bpm
   • Temperature <38.0°C (100.4°F)                       • Temperature >=38.0°C (100.4°F)
   • Lung Exam Normal (no crackles,                      • Focal lung crackles, egophony,
     bronchial breath sounds, dullness)                    or dullness to percussion
   • Normal mental status                                • SpO2 <95% on room air
           │                                             • Age >=65 with acute confusion
           ▼                                                     │
   =================================                             ▼
   NEGATIVE PREDICTIVE VALUE >99%                    OBTAIN CHEST RADIOGRAPH
   FOR PNEUMONIA                                     (PA and Lateral Views)
   • Chest Radiograph NOT Indicated                              │
   • Diagnosis: ACUTE BRONCHITIS                     ┌───────────┴───────────┐
   • Strict Antibiotic Stewardship                   ▼                       ▼
   • Reassurance & Symptomatic Relief           Infiltrate Present      Normal CXR
                                                Diagnose CAP            Acute Bronchitis

The Evidence-Based Vital Signs Decision Rule

A patient presenting with acute cough who meets ALL four of the following criteria:

1. Heart rate < 100 beats/min;
2. Respiratory rate < 20 breaths/min;
3. Oral body temperature < 38.0°C (100.4°F); AND
4. Normal chest examination (absence of focal crackles, egophony, bronchial breath sounds, or dullness to percussion);

Has a negative predictive value (NPV) exceeding 99% for pneumonia. In these patients, posteroanterior (PA) and lateral chest radiography is unnecessary, unhelpful, and not cost-effective.

Indications for Chest Radiography (PA and Lateral)

• Any abnormal vital sign: Tachycardia (HR ≥100), tachypnea (RR ≥20), or documented fever (T ≥38.0°C);
• Any focal auscultatory abnormality: Localized crackles, bronchial breath sounds, whispered pectoriloquy, egophony, or asymmetric dullness to percussion;
• Pulse oximetry SpO2 <95% on ambient room air;
• Age ≥65 years presenting with altered mental status, delirium, acute confusion, or unexplained functional decline (frequently lack fever and cough in pneumonia);
• Presence of red-flag symptoms: Frank hemoptysis, severe pleuritic chest pain, or unremitting high fevers.

---

Symptomatic & Supportive Pharmacotherapy

---

Pertussis ("Whooping Cough", Bordetella pertussis)

Pertussis is an acute, highly contagious respiratory tract infection caused by the fastidious Gram-negative coccobacillus Bordetella pertussis. Despite childhood immunization programs, pertussis incidence has surged in adolescents and adults due to waning vaccine-induced immunity following acellular pertussis (DTaP/Tdap) vaccines.

Clinical Staging & Natural History

                         CLINICAL STAGES OF PERTUSSIS

  Catarrhal Stage (1-2 Weeks)    Paroxysmal Stage (2-6+ Weeks)     Convalescent Stage (Months)
  ┌───────────────────────────┐  ┌───────────────────────────────┐ ┌───────────────────────────┐
  │ • Rhinorrhea, low fever   │  │ • Staccato coughing bursts    │ │ • 'Cough of 100 days'     │
  │ • Indistinguishable from  │  │ • Inspiratory 'whoop'         │ │ • Gradual decrease in     │
  │   viral upper resp. inf.  │  │ • Post-tussive emesis         │ │   cough paroxysms         │
  │ • MOST CONTAGIOUS PHASE   │  │ • Absolute lymphocytosis      │ │ • Paroxysms can recur     │
  │ • Antibiotics alter course│  │ • Subconjunctival hemorrhages │ │   with subsequent viral   │
  │   if started here         │  │ • Antibiotics halt spread     │ │   respiratory infections  │
  └───────────────────────────┘  └───────────────────────────────┘ └───────────────────────────┘

1. Catarrhal Stage (1 to 2 Weeks)

• Manifests as an insidious, mild upper respiratory illness: low-grade fever, mild rhinorrhea, sneezing, lacrimation, and an occasional non-productive hack.
• The Contagion Peak: This is the period of maximal communicability and bacterial shedding. Because symptoms are indistinguishable from a common cold, pertussis is rarely suspected or diagnosed during this window. Antimicrobial therapy initiated during this stage effectively abbreviates the clinical illness.

2. Paroxysmal Stage (2 to 6 Weeks, up to 10 Weeks)

• Hallmarked by agonizing paroxysms of rapid, staccato, expiratory coughing bursts (often 5 to 15 coughs in a single exhalation), during which the patient is unable to breathe.
• The paroxysm terminates with a forceful, massive inspiratory gasp against a partially closed, edematous glottis, producing the characteristic high-pitched inspiratory "whoop".
• Cardinal Diagnostic Signs:
  • Post-tussive emesis (highly specific for pertussis in adults; positive likelihood ratio ~4.0);
  • Post-tussive exhaustion, syncope, or apnea;
  • Bilateral subconjunctival hemorrhages, facial petechiae, and epistaxis from elevated intrathoracic and venous pressures;
  • Cough-induced rib fractures and inguinal hernias.
• Atypical Presentation in Adults & Adolescents: In previously vaccinated adults with waning immunity, the classic inspiratory "whoop" is absent in over 50% of cases. The condition often manifests simply as an intractable, exhausting nocturnal cough persisting beyond 2 to 3 weeks with post-tussive retching.
• Infants <6 Months: Do NOT develop the classic "whoop"; they present with life-threatening apneic spells, cyanosis, bradycardia, seizures, and cardiogenic collapse.

3. Convalescent Stage (Weeks to Months)

• Gradual resolution of coughing paroxysms over several months (historically known as the "cough of 100 days"). Cough paroxysms can temporarily re-emerge during subsequent viral respiratory tract infections.

Diagnostic Testing & Laboratory Evaluation

• Complete Blood Count (CBC): Marked absolute lymphocytosis (white blood cell count often 20,000 to 50,000/µL with >60% to 70% mature lymphocytes). Pertussis toxin blocks the extravasation of lymphocytes from the intravascular space into secondary lymphoid tissues.
• Nasopharyngeal Swab for Pertussis PCR: The gold-standard diagnostic modality of choice during the first 3 to 4 weeks of cough (sensitivity >90%, specificity >95%). Swabs must use synthetic polyester or Dacron tips (calcium alginate or cotton tips inhibit PCR assays).
• Nasopharyngeal Culture: Inoculated onto specialized selective media (Regan-Lowe or Bordet-Gengou charcoal agar). Highly specific (100%), but sensitive only during the initial 2 weeks of illness and requires 7 to 10 days of incubation.
• Pertussis Serology (IgG Anti-Pertussis Toxin): Preferred diagnostic modality when the patient presents after 3 to 4 weeks of cough, when bacterial shedding has ceased and PCR sensitivity drops precipitously.

Antimicrobial Treatment Protocols

                      PERTUSSIS ANTIMICROBIAL REGIMENS

        First-Line (All Ages): MACROLIDES
        ─────────────────────────────────────────────────────────────
        • AZITHROMYCIN (Preferred):
          - Adults: 500 mg PO on Day 1, then 250 mg PO daily on Days 2-5
          - Infants/Children: 10 mg/kg Day 1, then 5 mg/kg daily Days 2-5
          - Preferred in neonates (lowest risk of pyloric stenosis)
        • CLARITHROMYCIN:
          - Adults: 500 mg PO BID for 7 days
        • ERYTHROMYCIN:
          - Adults: 500 mg PO QID for 14 days (GI toxicity, pyloric stenosis risk)
        ─────────────────────────────────────────────────────────────
        Alternative (Macrolide Allergy / Intolerance):
        • TRIMETHOPRIM-SULFAMETHOXAZOLE (TMP-SMX):
          - Adults: 1 DS tablet (160/800 mg) PO BID for 14 days
          - CONTRAINDICATIONS: Infants <2 months (kernicterus) & term pregnancy

The Clinical Timing Paradox

• Initiation ≤3 Weeks of Cough: Macrolide therapy started in the catarrhal or early paroxysmal stage significantly reduces symptom severity and shortens illness duration.
• Initiation >3 Weeks of Cough: Initiating antibiotics after 3 to 4 weeks of cough DOES NOT alter or reduce clinical symptoms, because the tracheobronchial ciliated epithelium has already suffered extensive cytotoxic damage by pertussis toxin and tracheal cytotoxin. However, antibiotics are still universally indicated to eliminate viable nasopharyngeal carriage and halt secondary transmission to susceptible contacts (especially unimmunized infants).

Post-Exposure Antimicrobial Prophylaxis (PEP)

• Universal Household Recommendation: PEP is recommended for all household and close contacts of a confirmed pertussis case, regardless of their personal immunization status, within 21 days of exposure.
• High-Risk Prioritization: Prophylaxis is mandatory if the contact setting includes:
  • Infants aged <1 year (highest mortality risk);
  • Pregnant women in their third trimester (to prevent transmission to the newborn);
  • Individuals with pre-existing chronic cardiopulmonary disease or severe immunocompromise;
  • Daycare attendees and staff.
• Prophylactic Regimen: Identical dosing regimens and durations as active treatment (e.g., Azithromycin 5-day course).

Immunization & Prevention Across the Lifespan

• Adolescent & Adult Booster: Single lifetime dose of Tdap for all adults aged 19 to 64 years who have not previously received it, followed by routine Td or Tdap decennial boosters every 10 years.
• Pregnancy Protocol: A single dose of Tdap is recommended during EVERY pregnancy, ideally between 27 and 36 weeks of gestation, regardless of the mother's prior vaccination history. This timing optimizes maternal anti-pertussis IgG production and maximizes active transplacental transfer to the fetus, conferring vital passive immunity during the vulnerable initial 2 months of life prior to the infant's primary DTaP immunization series.