4.3 Pulmonary Pharmacologic Agents

Inhaled Bronchodilators: SABAs and SAMAs

In the intensive care unit, inhaled bronchodilators are the primary treatment for acute bronchospasm associated with asthma, COPD, and anaphylaxis.

Pharmacological Mechanisms

• Albuterol (Short-Acting Beta-2 Agonist - SABA): Albuterol stimulates beta-2 adrenergic receptors on airway smooth muscle. This activates the enzyme adenyl cyclase, which converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP). Increased intracellular cAMP levels lead to relaxation of bronchial smooth muscle and bronchodilation. The standard ICU nebulizer dose is 2.5 mg in 3 mL of normal saline, or 4 to 8 puffs via metered-dose inhaler (MDI) with an in-line spacer.
• Ipratropium Bromide (Short-Acting Muscarinic Antagonist - SAMA): Ipratropium blocks post-ganglionic M3 muscarinic acetylcholine receptors on bronchial smooth muscle. This inhibits the production of cyclic guanosine monophosphate (cGMP), preventing acetylcholine-mediated bronchoconstriction and mucus secretion. The standard dose is 0.5 mg nebulized or 4 to 8 puffs via MDI.
• Synergistic Effect: Combining albuterol and ipratropium (DuoNeb) provides dual-pathway bronchodilation, targeting both the sympathetic and parasympathetic systems.

Aerosol Delivery Optimization in Mechanical Ventilation

Aerosol deposition is highly inefficient in mechanically ventilated patients, with typically less than 10% of the drug reaching the lungs due to deposition in the endotracheal tube and circuit. To optimize delivery:

1. Nebulizer Choice: Vibrating mesh nebulizers (e.g., Aerogen) are preferred over jet nebulizers. Jet nebulizers require an external gas flow (6-8 L/min) which dilutes the tidal volume, alters trigger sensitivity, and interferes with volume measurements. Vibrating mesh nebulizers do not introduce external gas flow.
2. Placement: Place the nebulizer in the inspiratory limb, 15 to 30 cm back from the patient Y-piece. This allows the circuit tubing to act as a spacer, letting the aerosol cloud accumulate during expiration so it can be delivered in a bolus during the next inspiration.
3. Ventilator Settings: If tolerated, utilize a decelerating flow pattern, set a larger tidal volume, and incorporate an inspiratory pause to facilitate particle sedimentation in the distal airways.

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Mucolytic Therapies

Mucolytics are indicated for patients with thick, tenacious secretions that impair airway clearance and worsen atelectasis.

N-acetylcysteine (Mucomyst)

N-acetylcysteine is a sulfhydryl-containing compound that acts directly on the molecular structure of mucus.

• Mechanism: The free sulfhydryl groups open the disulfide bonds of mucoproteins, breaking the mucus gel into smaller, less viscous subunits. It is administered as a 3 to 5 mL dose of a 10% or 20% solution via nebulization.
• Critical Risk (Bronchospasm): N-acetylcysteine is highly irritating to the airway mucosa and can trigger severe, life-threatening bronchospasm. Protocol: It must never be administered as monotherapy. It must always be combined with, or immediately preceded by, an inhaled bronchodilator (e.g., albuterol).
• Practical Concerns: The drug has a strong sulfur odor (rotten eggs) that can cause patient nausea, and it can react with and corrode certain metals in delivery equipment.

Dornase Alfa (Pulmozyme)

Dornase alfa is a recombinant human deoxyribonuclease (rhDNase).

• Mechanism: In patients with Cystic Fibrosis (CF), chronic airway infections cause neutrophils to accumulate and die in the lungs. As they degenerate, they release high-molecular-weight extracellular DNA, which makes the sputum thick and purulent. Dornase alfa selectively cleaves this extracellular DNA, rapidly liquefying the secretions.
• Dose: The standard dose is 2.5 mg nebulized once or twice daily.
• Indication Limitation: Dornase alfa is only indicated for Cystic Fibrosis. It is not effective in standard COPD or pneumonia because their secretions do not contain the same high concentration of neutrophil DNA.

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Neuromuscular Blocking Agents (NMBAs)

Chemical paralysis is indicated in patients with early, severe ARDS (defined as a $PaO_2/FiO_2 < 150$) to eliminate patient-ventilator dyssynchrony, guarantee lung-protective ventilation, and decrease systemic oxygen consumption.

Cisatracurium (Nimbex)

Cisatracurium is the preferred NMBA for continuous infusion in the ICU.

• Hofmann Elimination: Unlike other paralytics (such as vecuronium or pancuronium) that rely on hepatic metabolism and renal excretion, cisatracurium undergoes Hofmann elimination. This is a temperature- and pH-dependent chemical degradation that occurs spontaneously in the bloodstream. Because it does not accumulate in patients with renal or hepatic failure, it is the safest choice for critically ill patients with multi-organ dysfunction.
• Dosing: Initiated with an IV bolus of 0.1 to 0.2 mg/kg, followed by a continuous infusion of 1 to 3 mcg/kg/minute.

Monitoring and Safety

• Train-of-Four (TOF): Depth of paralysis is monitored using a peripheral nerve stimulator. Electrodes are placed over a nerve (typically the ulnar nerve), and four electrical pulses are delivered. The clinician counts the number of muscle twitches (e.g., thumb adductions).
  • Target: 1 to 2 twitches out of 4 is the therapeutic target for ARDS.
  • Under-paralysis (3-4 twitches): Risks dyssynchrony and lung injury.
  • Over-paralysis (0 twitches): Increases the risk of prolonged post-paralysis diaphragmatic atrophy and critical illness myopathy.
• Absolute Sedation Requirement: NMBAs possess no sedative or analgesic properties. A paralyzed patient is fully conscious and can feel pain but cannot move or breathe. It is a critical safety rule that deep sedation (RASS -5) and continuous analgesia must be established and verified before initiating a paralytic.