Anesthesia Effects on Respiratory System PDF

Anesthesia effects on the respiratory system The primary functions of the respiratory system are: 1. Ventilation; the movement of air into and out of the lungs. 2. Gas exchange; is the transfer of oxygen into the blood and carbon dioxide removal. General anesthesia h...

Anesthesia effects on the respiratory system The primary functions of the respiratory system are: 1. Ventilation; the movement of air into and out of the lungs. 2. Gas exchange; is the transfer of oxygen into the blood and carbon dioxide removal. General anesthesia has several effects on both of these key functions. The passage of gas into the lungs may be impaired by obstruction of the airway; the distribution of gas within the lungs may change and the transfer of oxygen (and anesthetic gases) into the blood may be impaired. Most of these adverse effects can be seen during anesthesia and in many patients, these extend into the postoperative period. HOW ANAESTHESIA AFFECTS VENTILATION 1. Airway obstruction General anesthesia, with or without the use of neuromuscular blocking drugs, results in the loss of airway patency due to the relaxation of the pharyngeal muscles and posterior displacement of the tongue. The ability to manage secretions is lost, and saliva and mucous can obstruct the oropharynx. The loss of the cough reflex allows secretions (or refluxed gastric contents) onto the vocal cords, causing laryngospasm, or to enter the trachea and lungs causing bronchospasm. These effects result in airway obstruction and prevent the passage of gases into and out of the lungs resulting in hypoxia and hypercapnia. 2. Reduced ventilation All anesthetic drugs (except ketamine, ether, and nitrous oxide) cause a dose-dependent reduction in minute ventilation. This can be due to either a reduction in the respiratory rate (e.g., opioids), a 1 reduction in the tidal volume (e.g., volatile anesthetics), or both (e.g., propofol). The ventilatory response to carbon dioxide is reduced by all anesthetic drugs, as a result, anesthetized patients become hypercapnic. HOW ANAESTHESIA AFFECTS GAS EXCHANGE Oxygen is the gas of primary importance, but the exchange of anesthetic gases will also be influenced. Oxygenation is dependent upon 1. The inspired oxygen content. 2. The presence of a patent airway. 3. Adequate alveolar ventilation. 4. Appropriate matching between ventilation and perfusion in the lung. 5. The transfer of oxygen across the alveolar and endothelial membranes. Anesthesia affects gas exchange by 1. Changes in functional residual capacity (FRC) 2. Changes in ventilation and perfusion. 3. Hypoxic pulmonary vasoconstriction (HPV) HOW MECHANICAL VENTILATION CAN DAMAGE LUNG TISSUE 1. Acute Respiratory Distress Syndrome ARDS Mechanical ventilation can directly damage lung parenchyma. Large tidal volumes (>12ml/kg) cause alveoli shearing stress, releasing inflammatory substances. These inflammatory mediators result in edema in the interstitial wall of the alveoli, which reduces lung compliance and gas transfer causing hypoxia. This is termed acute respiratory distress syndrome (ARDS) and results in prolonged stays in the intensive care unit (ICU) with a mortality of up to 40%. 2  2. Pneumothorax (Barotrauma) Pneumothorax occurs when air is trapped between the two pleural layers of the lungs with a loss of negative pressure that causes the lung to collapse. High inspiratory pressures or large tidal volumes can cause pneumothorax, which is more likely in the stiff, noncompliant lungs caused by ARDS or in the non-elastic lungs of chronic obstructive airway disease. The pressure required to keep one group of alveoli open may rupture another group causing a pneumothorax. THE PHARMACOLOGICAL EFFECTS OF ANAESTHESIA Table 1. Summary of the effects of the main anesthetic drugs on the respiratory system. Inhalational Positive effects Negative effects agents Isoflurane Bronchodilation Reduced MV Reduced response to hypoxia & hypercarbia Pungent – causes coughing Increased secretions Sevoflurane MV stable Hypercarbia Bronchodilation Depressed response to CO2 Non-irritant 3  Halothane Bronchodilation Reduced MV, Blunted response Non- irritant to hypoxia and hypercarbia Reduced bronchial secretions Iv Induction Positive effects Negative effects agents Thiopentone Dose dependent respiratory depression Increased bronchial smooth muscle tone with increased bronchospasm & laryngospasm Propofol Laryngeal relaxation – ease Respiratory depression of Reduced response to hypoxia & LMA insertion hypercarbia Bronchodilation Ketamine Preserved laryngeal Increased saliva and mucous reflexes production Maintain patent airway Less respiratory depression Reduction in bronchial smooth muscle tone Opiates Anti-tussive Respiratory depression Chest wall rigidity Bronchospasm MV = Minute Ventilation TV = Tidal Volume RR = Respiratory Rate HPV = Hypoxic Pulmonary Vasoconstriction LMA = Laryngeal Mask Airway MANAGING THE EFFECTS OF ANAESTHESIA ON THE RESPIRATORY SYSTEM I. Pre-operatively a. Positioning patients at a 45° angle before induction helps to reduce the fall in the FRC. b. Pre-oxygenation to maximize the oxygen content of the FRC can significantly increase the time from apnea to hypoxia. c. Antimuscarinic drugs (atropine, glycopyrrolate) given before induction reduce the quantity of saliva in the airway. II. Intra-operatively 4  a. Mechanical ventilation, in particular for obese patients, reduces airway collapse and atelectasis. Positive end- expiratory pressure (PEEP) helps to maintain alveolar patency and prevent hypoxia. If the patient is breathing spontaneously, continuous positive airway pressure (CPAP) will have the same effect. b. PEEP and recruitment maneuvers can be used to open collapsed portions of the lung. Recruitment is achieved by prolonged periods of high PEEP, Lung protective strategies that are used to treat ARDS can also be used safely for obese patients, bariatric and laparoscopic surgery, and the elderly during anesthesia to reduce atelectasis and increase oxygenation. This effect is not continued after extubating. III. Post-operatively Oxygen can be continued into the postoperative period in patients at risk of hypoxia. Head-up tilt increases the FRC and helps prevent atelectasis. In obese patients, extubation onto a CPAP mask may help prevent airway collapse and atelectasis and maintain arterial oxygenation. Similarly, extubating ICU patients onto bi-level noninvasive ventilation has been shown to reduce the rate of re-intubation. Make sure the patient has good postoperative analgesia. Patients should be able to take deep breaths and cough. MCQ test 1- All the following are positive effects of ketamine on respiratory system except one a) Reduction in bronchial smooth muscle tone b) Maintain patent airway. c) Less respiratory depression d) Anti-tussive. e) Preserved laryngeal reflexes. 5 2- All the following methods used to decrease the effect of anesthesia on respiratory system except one a) Oxygen can be continued into the postoperative period in patients at risk of hypoxia. b) atropine, glycopyrrolate) given before induction reduce the quantity of saliva in the airway. c) Head down position before induction helps to reduce the fall in the FRC. d) Positive end-expiratory pressure (PEEP) helps to maintain alveolar patency and prevent hypoxia. e) In obese patients, extubation onto a CPAP mask may help prevent airway collapse and atelectasis. 3- Negative effects of Isoflurane on respiratory system (all true except one a) Increased secretions b) Reduced minute ventilation. c) Sweet odor. d) Reduced response to hypoxia. e) Reduced response to hypercarbia. 4- All the following anesthetic drugs cause a dose-dependent reduction in minute ventilation except one a) Ketamine. b) Propofol c) Thiopental d) etomidate. e) Isoflurane. 5- Anesthesia affects gas exchange by (all true except one) a) Change in FRC, b) Change in ventilation. c) Change in perfusion. d) Hypoxic pulmonary vasoconstriction. e) Change in tidal volume. 6