Mechanical ventilation Dr.Poddutoori PGY3
Introduction • Specially designed pumps that support Ventilatory functions of RS and improve oxygenation through high O2 and positive pressure.
Indications for MV • Hypercarbic resp failure: • Basically decreased Min Ventilation or increased Physiologic dead space. • Neuromuscular disease:M. Gravis, Ascending polyradiculopathy, Myopahty • Muscle fatigue diseases: Asthma, COPD, restrictive lung disease • Pco2 >50mm Hg and Ph <7.3 • Hypoxemic Resp Failure: • Basically a V/Q mismatch and shunt problem • Pneumonia, Pulm edema, ARDS, Pulm hemorrhage • Sao2 <90 despite >60% fio2.
Other indications for MV • Increased ICP – controlled Hyperventilation to reduce cerebral blood flow • Post Op pulmonary HTN: to improve pulmonary hemodynamics • CHF in presence of Myocardial ischemia: To reduce preload and afterload and to reduce work of breathing • Prevent aspiration of Gastric contents: unstable patients needing Lavage for drug overdose and EGD
Physiologic aspects of MV • Hypoxia : fio2 • Hypercarbia: TV x RR = Min Ventilation • PEEP helps maintain patency of alveoli – reverses hypoxemia and atelectasis by improving V/Q matching • Normal is 0-10. Can be increased if refractory hypoxemia needing Fio2 >0.6
Modes of ventilation • O2 by Nasal cannula, ventimask, Nonrebreather • Non Invasive Positive pressure ventilation: CPAP and BIPAP • Invasive ventilation: • Assist control • SIMV
Establishing and maintaining airway • Cuffed ETT is used to maintain Positive pressure and prevent aspiration • Medications used: • Sedatives – avoid long acting benzos. Use Propofol or etomidate • Analgesics – Avoids morphine – can worsen bronchospasm. Use fentanyl. • Paralyzing agents – Succinylcholine should be avoided in renal failure, Tumor lysis, Hyperkalemia.
Terminology • Modes:refers to the manner in which ventilator breaths are triggered,cycled and limited. SIMV, AC • Trigger: defines what the ventilator senses to initiate an assisted breath -an inspiratory effort or time based signal • Cycle: refers to factors that determine the end of inspiration – volume, pressure or flow or time cycled. • Limiting factors: are operator specific values: ex Airway pressure.
Assist control mode • Inspiratory cycle is initiated by inspiratory effort of pt or by timer signal ( back up rate) • Every breath delivered consists of operator specified TV. • Vent rate is determined by either by pt or Operator specified backup rate – which ever is higher
ACMV • Advantages: • Often used for initiation of MV – to ensure backup min Ventilation and synchronization of vent cycle with pts insp effort. Disadvantages: -Respiratory Alkalemia:leading to myoclonus and seizures -Dynamic hyperinflation – auto PEEP and barotrauma, decreased Cardiac output.
SIMV • Pt is allowed to breath spontaneously • Preferred in pts with intact resp drive • Fixed mandatory breaths are delivered in addition to pts RR. • Only the preset number of breaths are ventilator assisted. • Total respiratory rate is Pts RR + Preset rate
SIMV • Advantages: • Exercise respiratory muscles • Easy to wean Disadvantages: -difficult to use in tachypnea- as dysynchrony might occur -cannot be used paralyzed pts.
CPAP and BiPAP • Non invasive ventilation • Used early stages of respiratory distress • Pt should be alert and cooperative • CPAP can also be used to assess extubation potential in intubated pts.
General support in ventilated patients • Sedation – Propofol or etomidate • Analgesia - fentanyl is best • GI prophylaxis: H2 receptor blockers/ carafate/antacids – carafate is preferred as it doesn’t change gastric Ph – so less colonization by nosocomial organisms • Nutrition support: Early feeding is encouraged. • Delayed Gastric emptying: Consider Reglan
Complications of MV • Pulmonary: • Barotrauma ( if Pr >50cm H2O ) • Vent Associated Pneumonia –( If intubated >72 hrs) – G –ve rods, S.Aureus, Anaerobes. • O2 toxicity • Tracheal stenosis • Respiratory deconditioning • Hypotension – almost always responsive to Volume support • GI: • Stress ulceration– H2 receptor blockers / sucralfate • Cholestasis – Total bilirubin < 4 mg/dl
Weaning criteria - MV • Upper airway functioning should intact – stridor or aspiration. • Intact cough – mobilizes secretions • Alveolar ventilation: Ph 7.35 – 7.45, Sao2 >90 %,Fio2 <0.5, PEEP <5. • Respiratory drive and chest wall function should be assessed ( TV, Insp Pressure, RR, VC) • Weaning index: breathing frequency/ TV <105. • VC >10 ml/kg, Insp Pr> -30 cm H2O.
Methods of weaning • Short term ventilated pts: 5 min/ hr. Gradually increased every hour. • T Piece ( on O2) • CPAP • Long term ventilated pts: • SIMV – decrease mandatory backup rate 2-4 breaths/min and reasses each time( if >25 or worsening PH indicates difficulty in weaning) • PSV - Gradually reduce the pressure to below Peak insp pressures by 5cm H2O until the pressures Just equal to ET tube pressure (5-10 normally)