Mechanical ventilation & Respiratory support therapy DR.REZWANUL HOQUE BULBUL MBBS, MS, FCPS, FRCSG, FRCS Ed ASSOCIATE PROFESSOR DEPARTMENT OF CARDIAC SURGERY BSMMU, DHAKA, BANGLADESH

DEFINITION It refers to the gamut of artificial means used to support ventilation and oxygenation. They encompass all forms of Positive Pressure Ventilation and those modes used to increase airway pressure above atmospheric during spontaneous ventilation

GOALS OF VENTILATION Increase efficiency of breathing Increase oxygenation Improve ventilation/perfusion relationships Decrease work of breathing

Types of Systems Negative Pressure Ventilator “Iron lung” Allows long-term ventilation without artificial airway Maintains normal intrathoracic hemodynamics Uncomfortable, limits access to patient

Types of Systems Positive Pressure Ventilator Uses pressures above atmospheric pressure to push air into lungs Requires use of artificial airway Types Pressure cycled Time cycled Volume cycled

Positive Pressure Ventilators Pressure Cycled Terminates inspiration at preset pressure Small, portable, inexpensive Ventilation volume can vary with changes in airway resistance, pulmonary compliance Used for short-term support of patients with no pre-existing thoracic or pulmonary problems

Positive Pressure Ventilators Volume cycled Most widely used system Terminates inspiration at preset volume Delivers volume at whatever pressure is required up to specified peak pressure May produce dangerously high intrathoracic pressures

Positive Pressure Ventilators Time cycled Terminates inspiration at preset time Volume determined by Length of inspiratory time Pressure limit set Patient airway resistance Patient lung compliance Common in neonatal units

CLASSIFICATION / On the basis of three functions INITIATION CONTROL Initiation triggered by machine at a predetermined time set regardless of patient effort ASSIST Supported breath is triggered when patient initiates a breath that develops negative pressure below a threshold termed sensitivity LIMIT VOLUME Tidal volume is preset while peak airway pressure is variable PRESSURE Airway pressure is preset while tidal volume is variable CYCLE OFF Tidal volume delivered then converted to expiration TIME May be prolonged by inspiratory pause FLOW RATE Converted to expiration when flow rate declines to predetermined level

VENTILATOR MODES Defined by inspiratory events while expiration is treated as independent entity

CONTROLLED MECHANICAL VENTILATION(CMV) Initiation is time dependent with a fixed rate Tidal volume is delivered along with inspiratory pause Tidal volume is delivered regardless of airway pressure If pressure limit is set delivery will stop once the limit is reached Eliminates patients work of breathing Requires an anaesthetized and paralyzed patient Useful when inspiratory effort contraindicated (flail chest) Patient must be incapable of initiating breaths Rarely used

ASSISTED MECHANICAL VENTILATION Initiation is assisted when sensitivity is reached Otherwise similar to CMV Better tolerated in light sedation Cannot be used in sedated or paralyzed Work of breathing is greater Can predispose to alkalosis if resp. rate is high

Assist Mode Assist/Control (A/C) Patient triggers machine to deliver breaths but machine has preset backup rate Patient initiates breath--machine delivers tidal volume If patient does not breathe fast enough, machine takes over at preset rate Tachypnea patients may hyperventilate dangerously

A/C mode

INTERMITTENT MANDATORY VENTILATION (IMV) Assist Mode INTERMITTENT MANDATORY VENTILATION (IMV) Combination of CMV and Spontaneous modified circuit allows continuous flow that allows patient to breathe between machine delivered breath with minimal work of breathing Delivery is regardless of the stage of respiration Patient breathes on own Machine delivers breaths at preset intervals Patient determines tidal volume of spontaneous breaths Used to “wean” patients from ventilators patient –machine dyssynchrony may lead to lung over distension or fighting with the ventilator Patients with weak respiratory muscles may tire from breathing against machine’s resistance Largely abandoned in favor of SIMV

SYNCHRONISED INTERMITTENT MANDATORENTILATION (SIMV) Assist mode SYNCHRONISED INTERMITTENT MANDATORENTILATION (SIMV) IMV in assist mode Machine timed to delay ventilations until end of spontaneous patient breaths Avoids over-distension of lungs Decreases barotrauma risk However if patient does not breathe then IMV delivers Relative work of breathing is less Does not contribute to central hyperventilation syndrome Does not require sedation or paralysis In COPD patients, hypercarbia is preserved Popular mode for weaning of patients

SIMV mode

PRESSURE SUPPORT VENTILATION Applicable in spontaneous ventilation Initiation of respiration cause rapid flow of gas until selected pressure is crossed and continues until inspiration drive is stopped Advantage over SIMV being support is provided for each breath Requires spontaneous ventilation but spontaneous breath in SIMV mode are also supported Cannot be used in low pulmonary compliance Central hyperventilation syndrome along with resp.alkalosis may develop Monitoring of tidal volume is required

Pressure support ventilation

Volume control vs. pressure control There are, effectively, two ways of assisting inspiration – by using positive pressure to deliver a certain amount of volume (volume control ventilation), or by delivering a certain amount of pressure (pressure control ventilation). Volume control means volume constant and pressure variable. Pressure control means pressure constant (or limited) volume variable ventilation. The mode of ventilation, is the way in which the ventilator uses volume or pressure to bump the patient up the pressure-volume curve. Patients may be given mandatory breaths (controlled ventilation) or may have their spontaneous breaths assisted (assist control ventilation). An alternative mode (which is often used with controlled modes) is pressure support, which allows a patient breath spontaneously, start and finish breaths and determine the tidal volume.  Mechanical ventilation that is achieved regardless of the patient's spontaneous breathing, but that uses pressure as the major determining variable, along with rate and time, of how much air the patient receives.

High Frequency Ventilation (HFV) Small volumes, high rates Allows gas exchange at low peak pressures Mechanism not completely understood High frequency positive pressure ventilation--60-120 breaths/min High frequency jet ventilation--up to 400 breaths/min High frequency oscillation--up to 3000 breaths/min

High Frequency Ventilation (HFV) Useful in managing: Tracheobronchial or bronchopleural fistulas Severe obstructive airway disease Patients who develop barotrauma or decreased cardiac output with more conventional methods Patients with head trauma who develop increased ICP with conventional methods Patients under general anesthesia in whom ventilator movement would be undesirable

Alternative modes of ventilation Non-invasive positive pressure ventilation using specialized face or nasal mask Negative pressure ventilation using special apparatus in COPD patient Airway pressure release ventilation- Lung is kept inflated at constant pre-set pressure to achieve alveolar recruitment and inflation, intermittent release of pressure to allow exhalation Mandatory minute ventilation- Pre-set minute ventilation either by spontaneous or by supplemental breath by machine Inverse ratio ventilation- normal I:E ratio is reversed from 1:2/1:3 to 1:1-3:1 Partial liquid ventilation- Perfluorocarbon liquid instilled into lung followed by standard mechanical ventilation ECMO

POSITIVE END EXPIRATORY PRESSURE commonly kept at 3 – 5 cm of water as a substitute for physiological PEEP provided by closed glottis primary goals include: 1. increase functional Residual Capacity 2. distends patent alveoli 3. recruits previously collapsed alveoli 4. In pulmonary edema, may redistribute extra vascular lung water from alveolar capillary interstitium to peribronchial and perihilar interstitium

increases air trapping, CO2 retention and hypercapnia Disadvantages 1. Alveolar distention and pulmonary dead space increases air trapping, CO2 retention and hypercapnia 2. Not useful in diseased lung 3. Not tolerated in awake patients 4. Increased work of breathing 5. Increased chances of barotrauma Contraindications 1. Severe hemodynamic instability 2. Acute bronchospasm 3. Severe Emphysema 4. Pneumothorax Suspected or present

Continuous Positive Airway Pressure (CPAP) PEEP without preset ventilator rate or volume Physiologically similar to PEEP May be applied with or without use of a ventilator or artificial airway Requires patient to be breathing spontaneously Does not require a ventilator but can be performed with some ventilators

RATIONALE FOR MECHANICAL VENT. Provides the time to reverse the adverse effects on lung function induced by anaesthesia and surgery Allows aggressive pain control without concern for respiratory depression Reduces the work of breathing and saves energy at the critical period In patients who are hemodynamically unstable, ensures effective ventilation reducing the number of variables in management

INDICATION OF MECHANICAL VENTILATION PLANNED POST OPERATIVE Based on Surgical procedures 1. Cardiac Surgery 2. Major Vascular Surgeries 3. Procedures with major blood loss 4. High risk Surgical Incisions Planned- other diseases 1. Neuromuscular and Mechanical dysfunction 2. Parenchymal lung diseases 3. Acute lung injury 4. Multi System Organ Failure Unplanned 1. Depressed CNS response to hypoxia and hypercapnia upper abdominal reduces FRC by 60% thoracotomy reduces FRC by 40%

CARDIO PULMONARY COMPLICATION INDICATION (CONT’D CARDIO PULMONARY COMPLICATION Acute Myocardial Infarction Arrhythmias Pulmonary edema Severe bronchospasm Lobar Atelectasis SURGICAL COMPLICATIONS Hemorrhage Cardiac contusion following TEE, Phrenic Nerve injury OPERATIVE CATASTROPHE Cardiac arrest, Malignant hyperthermia, ABO mismatch Gastric Acid aspiration, Major anaphylactic reaction INADEQUATE REVERSAL OF ANAESTHESIA inadequate elimination of anaesthetic agents inadequate reversal of opioid analgesics inadequate reversal of muscle relaxants

Ventilator Settings Tidal volume--10 to 15ml/kg (std = 12 ml/kg) Respiratory rate--initially 10 to 16/minute FiO2--0.21 to 1.0 depending on disease process 100% causes oxygen toxicity and atelectasis in less than 24 hours 40% is safe indefinitely PEEP can be added to stay below 40% Goal is to achieve a PaO2 >60 I:E Ratio--1:2 is good starting point Obstructive disease requires longer expirations Restrictive disease requires longer inspirations Changes are made according to Arterial Blood Gas reports Maintain with ACV, use sedatives & muscle relaxants if needed Switch to Synchronized Intermittent Mandatory Ventilation (SIMV) mode, reduce respiratory rate gradually and finally switch to Spontaneous mode Extubate the patients when the criteria's are met.

Ventilator Settings Ancillary adjustments Inspiratory flow time Temperature adjustments Humidity Trigger sensitivity Peak airway pressure limits Sighs

Quick Guide to Setup Self check and/or Calibration as needed Check circuit and connections Set Mode: Usually “Assist/Control” Adjust “I” time: Usually 1 second Set tidal volume: 10-12 ml/kg is standard May need to set “Flow” based on “I” time Set ventilatory rate: Adult 12-16/min

WEANING FROM VENTILATOR Criteria 1. Mental Alertness 2. No active bleeding 3. Haemodynamic stability 4. Normothermic 5. Satisfactory Arterial Blood Gas report PaO2 > 70 mm of Hg on an FIO2 <50% PaCO2 < 45 mm of Hg pH 7.35 – 7.45 For Extubation Vital Capacity > 10 – 15ml / kg Negative Inspiratory Force > 25 cm water Spontaneous Respiratory Rate < 30 / min D(A-a)O2 <350 TORR ON 100% O2

Ventilator Complications Mechanical malfunction Keep all alarms activated at all times BVM must always be available If malfunction occurs, disconnect ventilator and ventilate manually

Ventilator Complications Airway malfunction Suction patient as needed Keep condensation build-up out of connecting tubes Auscultate chest frequently End tidal CO2 monitoring Maintain desired end-tidal CO2 Assess tube placement

Ventilator Complications Pulmonary barotrauma Avoid high-pressure settings for high-risk patients (COPD) Monitor for pneumothorax Anticipate need to decompress tension pneumothorax low chance with peak airway pressure < 45 cm of water high tidal volume may injure alveolar capillary membrane Oxygen toxicity incidence and rapidity of oxygen related injury increases with FIO2 > 60%

Ventilator Complications Renal malfunction Gastric hemorrhage Pulmonary atelectasis Infection Oxygen toxicity Loss of respiratory muscle tone increased intra cranial pressure

-Ventilator malfunction, improper setting Ventilator complications Acute Respiratory Insufficiency present when there is evidence of inadequate oxygenation (PaO2 <60 mm Hg on FIO2 0.5) or Ventilation (PCO2 >50 mm Hg) during mechanical ventilatory support. Causes: -Ventilator malfunction, improper setting - Endotracheal tube malfunction - Pulmonary Problems – Atelectasis, Pulmonary edema, Pneumonia, Pneumothorax, Haemothorax - Low cardiac output states - Aspiration Pneumonia

Management: - Examine the ventilator setting and function, ET Tube, ABGs, CXR - Hand ventilate with 100% O2, increase FIO2 in ventilator until problem is corrected - May require repositioning of ET tube, insertion of Intra thoracic tube - Asses and optimise hemodyanmics - Add Inverse Ratio Ventilation with PEEP increment - Consider sedation and Paralysis in patient Ventilator synchronicity - Treat identifiable causes

Chronic Respiratory insufficiency inability to wean from ventilator within 48-72 hours caused by problems that primarily impair oxygenation or produce primary Ventilatory insufficiency Causes: Hypoxia, ARDS, Sepsis, Metabolic Abnormalities, Phrenic nerve paralysis Management: Treat the primary cause Patients may even require Tracheostomy for further airway management

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