Overview
Definition:
Mechanical ventilation is a life-support intervention that uses a machine (ventilator) to move air into and out of the lungs, providing breaths for patients who are unable to breathe adequately on their own
It is a crucial component of critical care management, particularly in surgical patients facing respiratory failure due to trauma, sepsis, major surgery, or underlying pulmonary conditions.
Epidemiology:
The need for mechanical ventilation is common in surgical intensive care units (SICUs)
Incidence varies widely based on surgical specialty and patient population, with higher rates in thoracic, trauma, and cardiothoracic surgery
Postoperative respiratory failure remains a significant cause of morbidity and mortality in surgical patients.
Clinical Significance:
Effective use of mechanical ventilation can be life-saving by ensuring adequate oxygenation and ventilation, reducing the work of breathing, and facilitating recovery
However, improper management can lead to ventilator-associated pneumonia (VAP), lung injury (VILI), barotrauma, volutrauma, and prolonged ventilator dependence, increasing patient morbidity and healthcare costs
Surgical residents must possess a fundamental understanding of ventilator principles to optimize patient outcomes.
Indications For Mechanical Ventilation
Absolute Indications:
Apnea or impending respiratory arrest
Severe hypoxemia (PaO2 < 50 mmHg on room air despite supplemental oxygen)
Severe hypercapnia with respiratory acidosis (pH < 7.25)
Respiratory muscle fatigue
Severe hypoventilation leading to impaired gas exchange.
Relative Indications:
Prophylaxis against respiratory failure in high-risk patients (e.g., thoracic surgery, severe sepsis)
Neuromuscular disorders affecting respiratory muscles
Management of increased intracranial pressure
Severe metabolic acidosis requiring controlled ventilation to support respiratory compensation.
Surgical Patient Specific:
Postoperative pulmonary edema
Flail chest
Severe chest wall trauma
Sepsis-induced acute respiratory distress syndrome (ARDS)
Massive hemorrhage requiring controlled ventilation
Airway protection in obtunded patients.
Modes Of Ventilation
Assist Control Ventilation Ac:
The ventilator delivers a set tidal volume or pressure with each breath, triggered by the patient or by the machine's set rate
Ensures that each breath meets the set volume/pressure and rate
Can be volume-controlled (VC-AC) or pressure-controlled (PC-AC).
Synchronized Intermittent Mandatory Ventilation Simv:
The ventilator delivers a set number of mandatory breaths (timed or patient-triggered) interspersed with spontaneous breaths
Allows the patient to breathe spontaneously between mandatory breaths, reducing the work of breathing and potentially facilitating weaning.
Pressure Support Ventilation Psv):
A mode where the patient triggers all breaths, and the ventilator provides a set level of positive pressure during inspiration to augment the patient's effort
Primarily used for weaning, it reduces the work of breathing and synchronizes with the patient's respiratory pattern.
High Frequency Oscillatory Ventilation Hfov:
Delivers very small tidal volumes at very high rates
Used in severe ARDS and pediatric patients where conventional ventilation is insufficient, aiming to improve gas exchange while minimizing lung injury.
Ventilator Settings And Optimization
Initial Settings For Adults:
Tidal Volume (Vt): 6-8 mL/kg ideal body weight
Respiratory Rate (RR): 12-20 breaths/min
Positive End-Expiratory Pressure (PEEP): 5-10 cmH2O
FiO2: Start at 0.4-0.6, titrate to maintain SpO2 88-95% or PaO2 55-80 mmHg
Trigger: Flow or pressure, set to patient's comfort (e.g., 1-2 L/min flow trigger).
Volume Controlled Ventilation Vc Ac:
Focus on setting Vt and RR
Monitor peak inspiratory pressure (PIP) for potential barotrauma
Adjust Vt to achieve adequate minute ventilation and target PaCO2
Adjust PEEP for oxygenation.
Pressure Controlled Ventilation Pc Ac:
Focus on setting inspiratory pressure and RR
Allows for more consistent distribution of ventilation across heterogeneous lungs
Monitor delivered Vt
Adjust inspiratory time to achieve adequate minute ventilation and CO2 clearance.
Peep Optimization:
PEEP helps recruit alveoli, reduce shunt, and improve oxygenation
Higher PEEP may be needed in ARDS but must be balanced against potential hemodynamic compromise and barotrauma
Optimal PEEP is often determined by titration to improve oxygenation or by recruited lung maneuvers.
FiO2 Titration:
Titrate FiO2 to maintain adequate oxygen saturation while avoiding oxygen toxicity
Target SpO2 88-95% in most patients, or 90-95% in some specific conditions
Monitor for signs of retinopathy of prematurity in neonates if applicable.
Monitoring And Assessment
Ventilator Waveforms:
Understanding flow, volume, and pressure waveforms (rectangular, decelerating, sine) provides crucial insights into patient-ventilator synchrony and potential issues like bronchospasm, air leaks, or circuit disconnections.
Arterial Blood Gases Abgs:
Essential for assessing oxygenation (PaO2, SaO2), ventilation (PaCO2), and acid-base status (pH, HCO3)
ABGs guide adjustments to ventilator settings (RR, Vt, PEEP, FiO2).
Oxygenation Parameters:
PaO2, SpO2, AaDO2 (Alveolar-arterial oxygen gradient), P/F ratio (PaO2/FiO2)
These help assess the severity of hypoxemia and the effectiveness of ventilatory support.
Ventilation Parameters:
PaCO2, ETCO2 (End-tidal CO2), minute ventilation
These assess the adequacy of CO2 removal
Changes in PaCO2 often reflect changes in alveolar ventilation.
Patient Ventilator Synchrony:
Observe for asynchrony (e.g., auto-triggering, ineffective efforts, double triggering) which can indicate patient discomfort, increased work of breathing, and potential complications
May require sedation, analgesia, or adjustment of ventilator settings.
Weaning From Mechanical Ventilation
Weaning Readiness Assessment:
Patient must be hemodynamically stable, able to initiate spontaneous breaths, have adequate cough reflex, and resolution of the underlying cause for ventilation
Key parameters include a negative inspiratory force (NIF) of -20 to -30 cmH2O, vital capacity (VC) > 10-15 mL/kg, and rapid shallow breathing index (RSBI) < 105 breaths/min/L.
Weaning Strategies:
Spontaneous breathing trials (SBT) using T-piece or PSV are preferred
Gradual reduction of support (e.g., decreasing PSV levels) can also be employed
Duration of SBT should ideally be short (e.g., 30-120 minutes) to assess tolerance.
Extubation Criteria:
Successful SBT, adequate tidal volume, protective cough, ability to manage secretions, stable hemodynamics, and satisfactory mental status
Post-extubation monitoring is crucial for early detection of stridor or respiratory distress.
Prolonged Mechanical Ventilation Pmv:
Patients requiring ventilation > 3 weeks
Requires a multidisciplinary approach involving intensivists, pulmonologists, therapists, and surgeons to identify reversible causes and consider tracheostomy for airway protection and improved secretion management.
Complications Of Mechanical Ventilation
Ventilator Associated Pneumonia Vap:
A nosocomial infection occurring >48 hours after intubation
Prevention includes head-of-bed elevation (30-45 degrees), oral care with antiseptic, timely discontinuation of ventilation, and judicious use of sedation.
Ventilator Induced Lung Injury Vili:
Lung injury caused or exacerbated by mechanical ventilation
Includes barotrauma (pneumothorax due to high pressures), volutrauma (lung overdistension due to large tidal volumes), and atelectrauma (repeated alveolar opening and closing)
Lung-protective ventilation strategies (low Vt, appropriate PEEP) are key to prevention.
Hemodynamic Compromise:
Positive intrathoracic pressure from mechanical ventilation can reduce venous return, leading to decreased cardiac output and hypotension, particularly in hypovolemic patients or those with cardiac dysfunction
PEEP titration is important.
Patient Ventilator Asynchrony:
Ineffective efforts, auto-triggering, or double triggering can lead to patient discomfort, increased oxygen consumption, and prolonged ventilation
Requires prompt identification and correction.
Tracheal Injury:
Can include vocal cord paralysis, tracheomalacia, tracheoesophageal fistula, and tracheal stenosis, especially with prolonged intubation or poorly fitting endotracheal tubes
Careful tube management and early tracheostomy consideration are vital.
Key Points
Exam Focus:
Understand the pressure-volume loops and their interpretation
Know the indications for mechanical ventilation in common surgical emergencies (e.g., ARDS in sepsis, flail chest)
Recall lung-protective ventilation strategies: low tidal volume (6-8 mL/kg IBW) and appropriate PEEP.
Clinical Pearls:
Always assess for patient-ventilator synchrony
Think "if it's not broken, don't fix it" with ventilation settings once optimal
Consider the underlying pathology driving respiratory failure
Aggressive weaning attempts are generally beneficial once criteria are met.
Common Mistakes:
Setting tidal volumes too high in obese patients without using ideal body weight
Inadequate PEEP in ARDS leading to poor oxygenation
Over-sedation hindering weaning efforts
Failing to recognize and manage patient-ventilator asynchrony
Not considering non-pulmonary causes of hypoxemia (e.g., cardiac, shunt).