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Mechanical ventilation


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Mechanical ventilation

  2. 2. MECHANICAL VENTILATION - Mechanical ventilation (MV) is used to assist or replace spontaneous breathing. - It is implemented with special devices that can support ventilatory function and improve oxygenation through the application of high-oxygen-content gas and positive pressure.
  3. 3. INDICATIONS: • The primary indication for initiation of MV is respiratory failure, of which there are two basic types: • (1) hypoxemic, which is present when arterial O2 saturation (Sao2) <90% occurs despite an increased inspired O2 fraction and usually results from ventilation-perfusion mismatch or shunt; and • (2) hypercarbic, which is characterized by elevated arterial carbon dioxide partial pressure (PCO2) values (usually >50 mmHg) resulting from conditions that decrease minute ventilation or increase physiologic dead space such that alveolar ventilation is inadequate to meet metabolic demands.
  4. 4. INDICATIONS: -Acute respiratory failure with hypoxemia (acute respiratory distress syndrome, heart failure with pulmonary edema, pneumonia, sepsis, complications of surgery and trauma), which accounts for ~65% of all ventilated cases. -Hypercarbic ventilatory failure—e.g., due to coma(15%), exacerbations of chronic obstructive pulmonary disease (COPD; 13%), and neuromuscular diseases (5%) -Used to reduce cerebral blood flow in patients with increased intracranial pressure . -Used frequently in conjunction with endotracheal intubation for airway protection to prevent aspiration of gastric contents in otherwise unstable patients during gastric lavage for suspected drug overdose or during gastrointestinal endoscopy. -In critically ill patients, intubation and MV may be indicated before the performance of essential diagnostic or therapeutic studies if it appears that respiratory failure may occur during those maneuvers.
  5. 5. Contraindications for Noninvasive Ventilation :
  6. 6. TYPES OF MECHANICAL VENTILATION • noninvasive ventilation (NIV) • invasive (or conventional mechanical) ventilation (MV) NIV MIV
  7. 7. INVASIVE MECHANICAL VENTILATION • Conventional MV is implemented once a cuffed tube is inserted into the trachea to allow conditioned gas (warmed, oxygenated, and humidified) to be delivered to the airways and lungs at pressures above atmospheric pressure • avoid brain-damaging hypoxia • administration of mild sedation may facilitate the procedure. Opiates and benzodiazepines are good choices (S/E ON HEMODYNAMICS: DEPRESSED CARDIAC FUNCTION AND LOW SVR) • Morphine can promote histamine release from tissue mast cells and may worsen bronchospasm in patients with asthma : • fentanyl, sufentanil, and alfentanil are acceptable alternatives • Ketamine increases systemic arterial pressure and cause hallucinatory responses • shorter-acting agents—etomidate and propofol—have been used for both induction and maintenance; have fewer adverse hemodynamic effects; but are more expensive.
  8. 8. PRINCIPLES OF MV • Once the patient has been intubated, the basic goals of MV are to optimize oxygenation while avoiding ventilator-induced lung injury due to overstretch and collapse/re-recruitment. • K.a. “protective ventilatory strategy” • To prevent: high airway pressures and volumes and overstretching of the lung as well as collapse/re-recruitment which leads to poor clinical outcomes (barotrauma and volume trauma).
  9. 9. MODES OF VENTILATION • Mode refers to the manner in which ventilator breaths are triggered, cycled, and limited . • Trigger, either an inspiratory effort or a time-based signal, defines what the ventilator senses to initiate an assisted breath. • Cycle refers to the factors that determine the end of inspiration. For example, in volume-cycled ventilation, inspiration ends when a specific tidal volume is delivered. Other types of cycling include pressure cycling and time cycling. • Limiting factors are operator-specified values, such as airway pressure, that are monitored by transducers internal to the ventilator circuit throughout the respiratory cycle ; if the specified values are exceeded, inspiratory flow is terminated, and the ventilator circuit is vented to atmospheric pressure or the specified pressure at the end of expiration (positive end-expiratory pressure, or PEEP)
  10. 10. Assist-Control Mechanical Ventilation • ACMV is the most widely used mode of ventilation • an inspiratory cycle is initiated either by the patient’s inspiratory effort or, if none is detected within a specified time window, by a timer signal within the ventilator. • operator-specified tidal volume • ACMV is commonly used for initiation of MV because it ensures a backup minute ventilation in the absence of an intact respiratory drive and allows for synchronization of the ventilator cycle with the patient’s inspiratory effort. • PROBLEMS: Respiratory alkalemia , myoclonus or seizures • Dynamic hyperinflation leading to increased intrathoracic pressures (so-called auto-PEEP) if the patients respiratory mechanics are such that: • inadequate time is available for complete exhalation between inspiratory cycles. • Auto-PEEP can limit venous return, decrease cardiac output, and increase airway pressures, predisposing to barotrauma.
  11. 11. Intermittent Mandatory Ventilation • the operator sets the number of mandatory breaths of fixed volume to be delivered by the ventilator; between those breaths, the patient can breathe spontaneously • In synchronized mode (SIMV): • mandatory breaths are delivered in synchrony with the patient’s inspiratory efforts at a frequency determined by the operator. • If the patient fails to initiate a breath, the ventilator delivers a fixed-tidal-volume breath and resets the internal timer for the next inspiratory cycle. • SIMV differs from ACMV in that only a preset number of breaths are ventilator-assisted.
  12. 12. SIMV: • allows patients with an intact respiratory drive to exercise inspiratory muscles between assisted breaths. • useful for both supporting and weaning intubated patients. • may be difficult to use in patients with tachypnea AS they may attempt to exhale during the ventilator- programmed inspiratory cycle. • Then, the airway pressure may exceed the inspiratory pressure limit, and the ventilator-assisted breath will be aborted, and minute volume may drop below that programmed by the operator.
  13. 13. Pressure-Support Ventilation • Its patient-triggered, flow-cycled, and pressure-limited . • It provides graded assistance and differs from the other two modes in that the operator sets the pressure level (rather than the volume) to augment every spontaneous respiratory effort. • level of pressure is adjusted by observing the patient’s respiratory frequency. • During PSV, the inspiration is terminated when inspiratory airflow falls below a certain level. • With PSV, patients receive ventilator assistance only when the ventilator detects an inspiratory effort . • PSV is often used in combination with SIMV to ensure volume-cycled backup for patients whose respiratory drive is depressed. • PSV is well tolerated by most patients who are being weaned from MV;.
  14. 14. PRESSURE-CONTROL VENTILATION (PCV) • Its time-triggered, time-cycled, and pressure-limited • A specified pressure is imposed at the airway opening throughout inspiration. • tidal volume and inspiratory flow rate are dependent, rather than independent • preferred mode of ventilation for patients in whom it is desirable to regulate peak airway pressures, such as those with: - preexisting barotrauma, - post– thoracic surgery patients, in whom the shear forces across a fresh suture line should be limited
  15. 15. INVERSE-RATIO VENTILATION (IRV) • Its a variant of PCV that incorporates the use of a prolonged inspiratory time with the appropriate shortening of the expiratory time. • IRV has been used in patients with severe hypoxemic respiratory failure. • This approach increases mean distending pressures without increasing peak airway pressures.
  16. 16. Nonconventional Ventilatory Strategies
  17. 17. • Several nonconventional strategies have been evaluated for their ability to improve gas exchange and survival rates in severe hypoxemic respiratory failure. • These strategies include: • High-frequency oscillatory ventilation (HFOV), • Airway pressure release ventilation (APRV), • Partial liquid ventilation (PLV) using perfluorocarbons and the administration of nitric oxide gas delivered through the airways. • “Salvage” techniques like: Extracorporeal membrane oxygenation (ECMO) • ECMO to be considered in patients with severe respiratory failure refractory to conventional therapy. • Others include: • Proportional assist ventilation (PAV)[p,v,t and resp. resistance and compliance] • Neurally adjusted ventilatory-assist ventilation (NAV) [neural activation of diaphragm] • PAV/NAV : new modes to enhance patient ventilator synchrony.
  18. 18. PROTECTIVE VENTILATORY STRATEGY • Set a target tidal volume close to 6 mL/kg of ideal body weight. • Prevent plateau pressure (static pressure in the airway at the end of inspiration) exceeding 30 cm H2O. • Use the lowest possible fraction of inspired oxygen (Fio2) to keep the Sao2 at ≥90%. • Adjust the PEEP to maintain alveolar patency while preventing overdistention and closure/reopening.
  19. 19. • The SBT involves an integrated patient assessment during spontaneous breathing with little or no ventilatory support. • The SBT is usually implemented with a T-piece using 1–5 cmH2O CPAP with 5–7 cmH2O or PSV from the ventilator to offset resistance from the endotracheal tube. • Once it is determined that the patient can breathe spontaneously, a decision must be made about the removal of the artificial airway, which should be undertaken only when it is concluded that the patient has the ability to protect the airway, is able to cough and clear secretions, and is alert enough to follow commands.