MECHANICAL VENTILATION • Things “I” wish I knew when I was an Intern Amit Gupta, MD Internal Medicine North Mississippi Medical Center
Mechanical Ventilation • Indications for Intubation and Ventilation • Principles of Mechanical Ventilation • Patterns of Assisted Ventilation • Ventilator Dependence: Complications • Liberation from Mechanical Ventilation: Weaning • Troubleshooting • Arterial Blood Gases
Indications for Mechanical Ventilation • “….An opening must be attempted in the trunk of the trachea, into which a tube or cane should be put; You will then blow into this so that lung may rise again….And the heart becomes strong….” -Andreas Vesalius (1555)
Indications for Mechanical Ventilation 1. “Thinking” of Intubation: elective v/s emergent 2. “Act of weakness?” 3. Endotracheal tubes are not a disease and ventilators are not an addiction 4. And the usual elective and emergent indications that you all know!
Objectives of Mechanical Ventilation Improve pulmonary gas exchange Reverse hypoxemia and Relieve acute respiratory acidosis Relieve respiratory Distress Decrease oxygen cost of breathing and reverse respiratory muscle fatigue Alter pressure-volume relations Prevent and reverse atelectasis Improve Compliance Prevent further injury Permit lung and airway healing Avoid complications
Monitoring Lung Mechanics Proximal Airway Pressures (end-inspiratory) 1. Peak Pressure Pk Function of: Inflation volume, recoil force of lungs and chest wall, airway resistance 2. Plateau Pressure Pl Occlude expiratory tubing at end-inspiration Function of elastance alone
Use of Airway Pressures Pk increased Pl unchanged: Tracheal tube obstruction Airway obstruction from secretions Acute bronchospasm Rx: Suctioning and Bronchodilators
Use of Airway Pressures Pk and Pl are both increased: Pneumothorax Lobar atelectasis Acute pulmonary edema Worsening pneumonia ARDS COPD with tachypnea and Auto-PEEP Increased abdominal pressure Asynchronous breathing
Use of Airway Pressures Decreased Pk: System air leak: Tubing disconnection, cuff leak Rx: Manual inflation, listen for leak Hyperventilation: Enough negative intrathoracic pressure to pull air into lungs may drop Pk.
Compliance Static Compliance (Cstat): Distensibility of Lungs and Chest wall Cstat = Vt/Pl Normal C stat: 50-80 ml/cm of water Provides objective measure of severity of illness in a pulmonary disorder Dynamic Compliance: Cdyn: Vt/Pk *Subtract PEEP from Pl or Pk for compliance measurement Use Exhaled tidal volume for calculations
Patterns of Assisted Ventilation • Assist Control • Intermittent Mandatory Ventilation • Pressure Controlled Ventilation • Pressure Support Ventilation • Positive end-expiratory ventilation • Continuous Positive Airway Pressure
Assist Control Ventilation Volume-cycled lung inflation Patient can initiate each mechanical breath or Ventilator provides machine breaths at a preselected rate Maintain I:E ratio to 1:2 to 1:4. An increase in Peak flow decreases the time for lung inflation and increases the I:E Ratio I:E ratio of <1:2 can cause hyperinflation by air trapping Diaphragmatic contraction continues during ACV and increases the work of breathing.
Assist Control Ventilation Adverse effects: In a tachypneic patient>>Lead to overventilation and severe respiratory alkalosis>> Hyperinflation and Auto-PEEP>> Lead to Electromechanical dissociation
Intermittent Mandatory Ventilation • Delivers volume cycled breaths at a preselected rate with spontaneous breathing between machine breaths • Less Alkalosis and Hyperinflation • Synchronized IMV
Intermittent Mandatory Ventilation Disadvantages: Increased work of Breathing: Spontaneous breathing through a high resistance circuit Solution: Add Pressure support Cardiac Output Changes: C O decreased by decreasing ventricular filling C O increased by reducing ventricular afterload More significant decrease in patients with LV dysfunction
IMV vs. ACV Switch to IMV for: Rapid breathers with alkalosis and over- Inflation Switch to ACV for: Patients with respiratory muscle weakness and LV dysfunction
Pressure Controlled Ventilation • Pressure cycled breathing, fully ventilator controlled • Inspiratory flow rate decreases exponentially during lung inflation • (+)Reduces peak airway pressure and improves gas exchange • (-)Inflation volume varies with changes in mechanical properties of the lungs. • Suited for patients with neuromuscular diseases and normal lung mechanics
Inverse ratio Ventilation • PCV combined with prolonged inflation time • Inspiratory flow rate is decreased • I:E ratio reversed to 2:1 • Helps prevent alveolar collapse • (-) Hyperinflation, Auto-PEEP and decreased cardiac output • Use: ARDS with refractory hypoxemia or hypercapnia ?mortality benefit
Pressure Support Ventilation • Pressure augmented breathing • Allows patient to determine the inflation volume and respiratory cycle duration • Uses: augment inflation during spontaneous breathing or overcome resistance of breathing through ventilator circuits (during weaning) • Popular an a non-invasive mode of ventilation via nasal or face masks
Positive end-expiratory pressure • Alveolar pressure at end-expiration is above atmospheric pressure : PEEP • Extrinsic PEEP • Auto PEEP
Positive end-expiratory pressure • EXTRINSIC PEEP • Applied by placing pressure limiting valve in the expiratory limb of ventilator circuit • Prevents end-expiratory alveolar collapse and recruits collapsed alveoli • This decreases intrapulmonary shunting, improves gas exchange and improves lung compliance, allowing the FiO2 to be reduced to less toxic levels
Positive end-expiratory pressure Cardiac Performance: Greater reduction in cardiac filling and cardiac output (Q), irrespective of level of PEEP! It is a function of PEEP induced increase in mean intrathoracic pressure Oxygen transport Do2: Do2 = Q X 1.3 X Hb X SaO2 Systemic O2 delivery may vary with the effect of PEEP on the Cardiac Output.
Positive end-expiratory pressure • Best PEEP: Monitor Cardiac Output • Another measure: Venous Oxygen Saturation • If VOS decreases after PEEP applied= Drop CO • Swan-Ganz catheter may be indicated in most patients on PEEP
Positive end-expiratory pressure • CLINICAL USES: • Reduce toxic levels of FiO2 (ARDS not pneumonia) • Low-volume ventilation • Obstructive lung disease (Extrinsic=Occult PEEP)
Positive end-expiratory pressure • CLINICAL MISUSES: • Reducing Lung Edema • Routine PEEP • Mediastinal Bleeding after CABG
Continuous positive Airway Pressure • Spontaneous breathing • Patient does not need to generate negative pressure to receive inhaled gas • CPAP replaced spontaneous PEEP • Use: Non-intubated patients (OSA, COPD)
Occult PEEP • Intrinsic or Auto-PEEP or Hyperinflation • Incomplete alveolar emptying during expiration • Ventilator Factors: High inflation volumes, rapid rate, low exhalation time • Disease factors: Asthma, COPD • Consequences: Decreased CO/EMD, Alveolar rupture, Underestimation of thoracic compliance, increased work of breathing. • If extrinsic PEEP does not increase Pk, then occult PEEP is present
Complications of Mechanical Ventilation • Toxic effects of Oxygen • Decreased cardiac output • Pneumonia and sepsis • Psychological problems • Ventilator dependence
Complications of Mechanical Ventilation • Purulent sinusitis • Laryngeal Damage • Aspiration :Value of routine tracheal suctioning • Tracheal Necrosis (pressure below 20mm water) • Alveolar rupture: Pneumothorax, pneumomediastinum, subQ emphysema, pneumoperitoneum • Basilar and sub-pulmonic air collections in the supine position, as seen on X-ray
Liberation from Mechanical Ventilation: Weaning • Weaning: Gradual withdrawal of mechanical ventilation • Misconceptions: Duration- longer duration, harder to wean Method of weaning determines ability to wean Diaphragm weakness is a common cause of failed weaning Aggressive nutrition support improves ability to wean Removal of ET tube reduces work of breathing
Maximal Inspiratory Pressure • Pmax: Excellent negative predictive value if less than –20 (in one study 100% failure to wean at this value) An acceptable Pmax however has a poor positive predictive value (40% failure to wean in this study with a Pmax more than –20)
Frequency/Volume ratio • Index of rapid and shallow breathing RR/Vt • Single study results: RR/Vt>105 95% wean attempts unsuccessful RR/Vt<105 80% successful • One of the most predictive bedside parameters.
T-Piece Weaning • On-off toggle switch that circulates between on and off the ventilator • Inhaled gas is delivered at a high flow rate • Varied protocols: like 30min-2hr on and off, or keep as long as possible and if tolerated for >2-4hr…. Deemed successsful (RR, TV, HR, diaphoresis, sat) • Failed T piece: Resume Vent support till comfortable, 24h vent Airflow with CPAP patient
T-Piece with Ventilator • Drawback: increased resistance due to vent tubing and actuator valve in circuit • Provide minimum pressure support (PSV) :Pmin • Pmin= PIFR X R • PIFR is during spontaneous breathing • R is airflow resistance during mech ventilation • R= Pk-Pl/Vinsp • (Vinsp:inspiratory flow rate delivered by the vent)
IMV Weaning • Gradual decrease in no of machine breaths in between the spontaneous breaths • False security: It does not adjust to patient’s ventilatory demands to maintain constant MV • End point in IMV weaning is the T-piece trial • Most important to recognize when a patient is capable of spontaneous unassisted breathing • T-piece more rapid than IMV
Complicating Factors • DYSPNEA • Anxiety and dyspnea are detrimental (low dose haloperidol or morphine) • CARDIAC OUTPUT • Increased LV afterload can reduce CO, impair diaphragm function, promote pulmonary edema • (Use Swan to monitor CO, may use dobutamine) • ELECTROLYTE DEPLETION • OVERFEEDING
The Problem Wean • RAPID BREATHING: Check TV • Low TV>> Resume vent support • TV not low…….. Check arterial pCO2 • Arterial pCO2 decreased>sedate (anxiety) • Arterial pCO2 not decreased> Resume vent
The Problem Wean • ABDOMINAL PARADOX • Inward displacement of the diaphragm during inspiration is a sign of diaphragmatic muscle fatigue • HYPOXEMIA • May be due to low CO and MVO2 • HYPERCAPNIA • Increase in PaCO2-PetCO2: increase dead space ventilation • Unchanged gradient: Respiratory muscle fatigue or enhanced CO2 production
Tracheal Decannulation • Successful weaning is not synonymous with tracheal decannulation • If weaned and not fully awake or unable to clear secretions, leave ETT in place • Contrary to popular belief, tracheal decannulation increases the work of breathing due to laryngeal edema and secretions • Do not perform tracheal decannulation to reduce work of breathing
Inspiratory Stridor • Post extubation inspiratory stridor is a sign of severe obstruction and should prompt reintubation • Laryngeal edema (post-ext) may respond to aerosolized epinephrine in children • Steroids have no role • Most need reintubation followed by tracheostomy
ARDS and Low Volume Ventilation • ARDS Network trial : NEJM May 4, 2000 p1301-08 • Traditional: TV 10-15ml/kg, keep plateau<50cm water • Low TV ventilation: TV 6ml/kg, keep plateau<30cm water • Need high RR in Low TV group to prevent acidosis • Permissive hypercapnia tolerated well, if needed, use IV bicarb to maintain pH • May add PEEP in addition to the low TV group to prevent atelectrauma (open-close alveoli>> alveolar fracture) • Results: Lower mortality in the Low TV group (31% v/s 39.8% p<0.007); Higher days without vent use and lower average plateau pressures in low TV group.