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  1. Volume preset time cycled: These delivers fixed present tidal volume at fixed interval
  2. Pressure preset flow cycled - no more used

Mechanical Ventilation

  1. Respiratory failure is caused by failure to ventilate. Treatment of respiratory failure is to increase the patient's alveolar ventilation, i.e., rate and depth of breathing, either by:-
    1. Reversing the cause
    2. Using mechanical ventilation
  2. Mechanical ventilation, as the name suggests, is to provide alveolar ventilation mechanically by the help of ventilators.
    1. Ventilators primarily assist for inspiration (inspiratory assistance), expiration is passive (depending on lung recoil pressure i.e., compliance and airway/circuit resistance). Ventilators generate gas flow by creating a pressure gradient between proximal airway and the alveoli flow occurs from high pressure to low pressure.
    2. In positive pressure ventilation, positive pressure is generated in proximal airways so that flow occurs from proximal airway (High pressure, i.e., positive pressure) to alveoli Air is pushed into the trachea.
    3. In negative pressure ventilation, negative pressure is generated inside lung, so that flow occurs from proximal airways to alveoli (low pressure, i.e., negative pressure) Air is sucked into the lung.
    4. Almost all the modem ventilators provide positive pressure ventilation.
Modes of mechanical ventilation
Mechanical ventilatory support can be:-
  1. Total ventilator support (Controlled mechanical ventilation [CMV]).
    Ventilator provides a mechanical breath and the patients own effort is nil ventilator is active and does all the work; patient is passive. It is also known as intermittent positive pressure ventilation (IPPV).
    Controlled mechanical ventilation may be:-
    1. Volume controlled ventilation (simply known as controlled mechanical ventilation [CMV]) Flow delivery to the patient is governed by a preset flow rate and volume.
    2. Pressure controlled ventilation Flow delivery to the patient is governed by a preset inspiratory pressure.
  2. Partial ventilator support (Assisted mechanical ventilation [AMY])
    As the name suggests, ventilator simply assists a breath initiated by the patient. AMV may be:-
    1. Assist - controlled mechanical ventilation (ACM).
    2. Intermittent mandatory ventilation (IMV).
    3. Synchronized intermittent mandatory ventilation (SIMV).
    4. Pressure support ventilation (PSV)
Setting of ventilator
Typical initial ventilator setting are:-
a. Tidal volume 10 ml/kg
b. Inspiration/Expiration ratio 1:2
c. Frequency 10 – 12 breaths/minute
d. Inspiratory flow rate 60 – 80 liter/minute
e. PEEP 3-5 CM H2O
f. Trigger sensitivity (for assist mode) - 1 to -2 cm H2O
g. FIO2 (delivery oxygen concentration) 0.5 (50%)
Weaning from ventilator
  1. Weaning means discontinuing the ventilatory support.
  2. Although weaning process vary from patient to patient and is possible to wean patient in any mode of ventilation except control mode ventilation (AIDS) because in controlled mode ventilation patients own effort is nil and only the ventilator is delivering the present tidal volume.
  3. Therefore this mode will play no role in weaning a patient from mechanical ventilation.
  4. Methods commonly used for weaning are:-
    1. Synchronized intermittent mandatory ventilation (SIMV)
    2. Intermittent mandatory ventilation
    3. Assist control ventilation
    4. Pressure support ventilation (PSV)
    5. Combination of SIMV and PSV
    6. T-piece
    7. CPAP
Complications of mechanical ventilation (including CMV)
  1. Barotrauma:- Barotrauma occurs as positive pressure can cause forceful leak of gases from the contained alveolar complex into cavities and interstitial tissues. This can cause pneumothorax, pneumomediastinum, bronchopleural fistula, pneumopericardium/cardiac temponade, pneumoperitoneum, systemic air embolism and pulmonary embolism.
  2. Hemodynamic complications:- Increased intrathoracic pressure causes decrease in venous return which results in decreased cardiac output.
  3. Nosocomial infections:- Pneumonia, UTI
  4. Acid-base disturbances:- Respiratory alkalosis (AIIMS 93) due to CO2 washout. Inappropriate ventilator setting can also cause hypercapnia and respiratory acidosis.
  5. Water retention:- Vasopressin released from anterior pituitary is increased due to reduction in Cardiac output and psychological stress.
  6. Liver and kidney dysfunction:- Due to decreased cardiac output.
  7. Respiratory muscle wasting:- Due disuse atrophy.
  8. GIT:- Stress ulcers, Paralytic ileus.
  9. Oxygen toxicity:- If higher concentration is used for Prolonged period.

Positive Airway Pressure Therapy

Positive airway pressure therapy means proving positive pressure during breathing. It may be :-
  1. Positive end-expiratory pressure (PEEP)
    PEEP is the positive pressure is applied at the end of expiration during mechanical ventilation. So, PEEP has two' features:-
    1. Positive pressure is applied during expiration (at the end).
    2. Used for mechanical; ventilation
  2. Continuous positive Airway pressure (CPAP)
    CPAP is application of positive-pressure:-
    1. During both inspiration and expiration.
    2. ln spontaneous breathing
Pulmonary effects of PEEP and CPAP
Positive pressure in PEEP & CPAP results in : -
  1. Re-expansion (recruitment) and stabilization of partially collapsed lung, which causes: -
    1. Increased FRC, tidal volume & lung compliance.
    2. Correction of ventilation/perfusion abnormalities Improved oxygenation.
  2. Redistribution of extravascular lung water from interstitial space between alveoli and endothelial cells towards peri-bronchial and perihilar areas As fluid moves away from the space between vessels & alveolar space, oxygen can diffuse more easily from alveoli into the blood Improved arterial oxygenation.
Adverse effects of PEEP/CPAP
  1. Pulmonary
    Controlled PEEP or CPAP improve lung compliance and correct ventilation/perfusion abnormalities. However, excessive PEEP or CPAP can overdistend alveoli & bronchi which results in:-
    1. Increased dead space ventilation
    2. Reduced lung compliance
    3. Increased pulmonary vascular resistance & Right ventricular afterload.
    4. Barotrauma (Pneumothorax, pneumopericardium, pneumoperitoneum, Subcutaneous emphysema).
  2. Extrapulmonary
    These adverse effects are primary circulatory and are related to transmission of elevated airway pressure to the chest :-
    1. Decreased cardiac output (due to increased intrathoracic pressure venous return is decreased).
    2. Decreased hepatic & renal blood flow (due to decreased cardiac output & elevated CVP)
    3. Increased ADH & angiotensin
    4. Decreased urinary output, GFR and free water clearance (Decrease Renal function).
    5. Increased ICT (due to increased CVP).
Indications of CPAP
If one or more of the following is present, CPAP is indicated:-
  1. Abnormal physical examination:- The presence of increased work of breathing as indicated by an increase in respiratory rate> 30% of normal, substernal & suprasternal retraction, grunting, nasal flaring, the presence of pale or cyanotic skin colour and agitation.
  2. Inadequate arterial blood gas values:- Inability to maintain a PO2 greater than 50 mm Hg with FiO2 ≤ 0.60 or with FiO2 of 0.40 in newborns. 
  3. Abnormal chest x-ray:- Presence of poorly expanded and/or infiltrated lung fields.
    One or more of the clinical presentation described above can be present in (indications for CPAP):-
    1. Very preterm infants (born at < 28 weeks gestation age, weight < 1kg) nasal CPAP + surfactant therapy.
    2. Apnea of prematurity (prolonged & recurrent attack).
    3. Respiratory distress syndrome (in both term and preterm infants).
    4. Weaning of chronically ventilator dependent infants.
    5. Recent extubation (following mechanical ventilation).
    6. Post operative thoracotomy care.
    7. Atelectasis & one lung ventilation.
    8. Pulmonary edema
    9. Tracheomalacia or other similar abnormalities of lower airway.
    10. Sleep apnea
    11. For administration of NO in spontaneously breathing infants.
Guidelines for CPAP use
CPAP is started at a pressure of 5 cm of H2O and FiO2 of 40-50%. If there is no improvement, first pressure is increased gradually by 1-2 cm H2O to a maximum 8-12 cm of water. If there is no improvement after pressure is increased to a maximum limit, FiO2 is increased gradually by 5% to maximum of 80%.

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