Effects of Anesthesia on the Respiratory System

Study Notes

Effects of Anesthesia on the Respiratory System

Anesthesia can affect both ventilation and gas exchange in the respiratory system.
Ventilation, the movement of air into and out of the lungs, can be impaired by airway obstruction, reduced ventilation, and changes in functional residual capacity (FRC).
Gas exchange, the transfer of oxygen into the blood and carbon dioxide removal, is affected by changes in FRC, ventilation-perfusion mismatch, and hypoxic pulmonary vasoconstriction (HPV).
Airway obstruction during anesthesia is caused by relaxation of pharyngeal muscles, posterior displacement of the tongue, loss of ability to manage secretions, and loss of the cough reflex.
All anesthetic drugs except ketamine, ether, and nitrous oxide cause a dose-dependent reduction in minute ventilation, leading to hypercapnia.
Anesthetic drugs also reduce the ventilatory response to carbon dioxide.
FRC (Functional Residual Capacity): The amount of air left in the lungs after a normal exhalation. Anesthesia can reduce FRC, increasing the risk of atelectasis (collapse of alveoli).
Ventilation-perfusion mismatch: An imbalance between the amount of air reaching the alveoli and the blood flow through the capillaries surrounding them. This can reduce gas exchange efficiency.
Hypoxic Pulmonary Vasoconstriction (HPV): A mechanism that helps redirect blood flow from poorly ventilated areas of the lung to better ventilated areas, but can be impaired by anesthesia.
Mechanical ventilation: Can cause lung damage, particularly by large tidal volumes, leading to alveolar shearing stress and the release of inflammatory mediators. This can result in ARDS (Acute Respiratory Distress Syndrome), a life-threatening condition characterized by lung inflammation and fluid buildup.
Pneumothorax can occur due to high inspiratory pressures or large tidal volumes, especially in lungs that are stiff or noncompliant. This can lead to lung collapse.
Ketamine is a unique anesthetic that preserves laryngeal reflexes, maintains airway patency, and causes less respiratory depression.
Pre-operative management: Positioning patients at a 45-degree angle, pre-oxygenation, and administration of antimuscarinic drugs can help reduce the risk of adverse respiratory effects during anesthesia.
Intra-operative management: Mechanical ventilation with PEEP (Positive End-Expiratory Pressure), CPAP (Continuous Positive Airway Pressure), and recruitment maneuvers can help maintain alveolar patency and improve oxygenation.
Post-operative management: Oxygen therapy, head-up tilt, CPAP or bi-level noninvasive ventilation, and adequate analgesia can help prevent hypoxemia and atelectasis.
ARDS is a severe lung injury characterized by widespread inflammation and fluid buildup in the alveoli. Mortality rates can be up to 40%.

Pharmacological Effects of Anesthetic Drugs

Inhalational Agents:
- Isoflurane: Reduces minute ventilation (MV), depresses response to hypoxemia and hypercapnia, pungent causing coughing, and increases secretions.
- Sevoflurane: Maintains MV stable, bronchodilates, non-irritant, but can cause hypercapnia and depressed response to carbon dioxide.
- Halothane: Reduces MV, blunted response to hypoxemia and hypercapnia, bronchodilation, non-irritant, and reduced bronchial secretions.
Intravenous Induction Agents:
- Thiopentone: Causes dose-dependent respiratory depression, increases bronchial smooth muscle tone with increased bronchospasm and laryngospasm.
- Propofol: Laryngeal relaxation, ease of LMA insertion, bronchodilation, respiratory depression, and reduced response to hypoxemia and hypercapnia.
- Ketamine: Preserves laryngeal reflexes, maintains patent airway, less respiratory depression, reduction in bronchial smooth muscle tone, but increases saliva and mucus production.
- Opiates: Exhibit anti-tussive properties, but cause respiratory depression, chest wall rigidity, and bronchospasm.