Indications

The major indication for mechanical ventilation is acute respiratory failure, of which there are two basic causes:

  1. Ventilatory (Hypercapnic respiratory failure)

  1. Inefficient Gas Exchange (Hypoxic respiratory failure)

 

Goals of Mechanical Ventilation

  1. Relieve respiratory distress
  2. Decrease work of breathing
  3. Improve pulmonary gas exchange
  4. Reverse respiratory muscle fatigue
  5. Permit lung healing
  6. Avoid complications

 

Minimizing the Work of Breathing (WOB)

Reduce Ventilatory Demand

  1. Decrease C02 production
  2. Decrease deadspace
  3. Decrease ventilatory drive

    1. Correct metabolic acidosis
    2. Reduce psychogenic stress
    3. Maintain adequate oxygenation
    4. Consider sedation

Improve Respiratory Impedance

  1. Decrease airflow resistance

    1. Improve secretion clearance
    2. Reverse bronchospasm
    3. Reduce circuit resistance

  2. Increase thoracic compliance

    1. Diuresis
    2. Apply PEEP/CPAP

  3. Synchronize machine output to patient demand

Improve Breathing Efficiency

  1. Decrease auto-PEEP
  2. Appropriate positioning

 

Negative Pressure Ventilators

  1. Iron lung
  2. Chest cuirasse

 

Positive Pressure Ventilators

Almost all modern ventilators employ the principle of intermittent positive-pressure ventilation (IPPV), which produces lung inflation by generating and applying positive pressure to the airways.

Pressure-cycled ventilators:

Gas is allowed to flow into the lungs until a present airway pressure limit is reached, at which time a valve opens allowing exhalation to ensue. The volume delivered by the ventilator varies with changes in airway resistance, lung compliance, and integrity of the ventilatory circuit.

Volume-cycled ventilators:

Gas flows to the patient until a preset volume is delivered to the ventilator circuit, even if this entails a very high airway pressure.

 

Ventilator Modes

Controlled Mechanical Ventilation

  1. The ventilator delivers a present number of breathes/min of a preset volume
  2. Additional breathes cannot be triggered by the patient, as in the case of ACV
  3. Used in patients who are paralyzed

Assist Control Ventilation

  1. Delivers a preset volume

  2. Clinician sets tidal volume, back-up rate, sensitivity, flow rate
  3. Set back-up rate about 4 breathes/min below patient's spontaneous rate
  4. Delivered volume may be markedly decreased in patients with airway obstruction or stiff lungs when volume becomes compressed in tubing (usually 2-4 ml/cm H20).

Intermittent Mandatory Ventilation

  1. Delivers a preset volume at a preset rate
  2. Permits spontaneous breathing (unlike AC)
  3. Although it produces a statistically significant decrease in the degree of respiratory alkalosis, the change is unlikely to be clinically significant
  4. Considerable respiratory work may result from demand valve
  5. As SIMV rate is decreased, the work of breathing and pressure-time product (a superior index of energy expenditure) increase for both the spontaneous and assisted breathes. At any SIMV rate, there is no difference in pressure-time product between spontaneous and assisted breathes.

  6. This indicates that patients display little breath-to-breath adaptation to machine assistance during SIMV.

Pressure Support Ventilation

  1. Fixed amount of pressure (set by clinician) augments each breath
  2. Pressure is maintained at preset level until patient's inspiratory flow falls to a certain level (e.g., 25% of peak flow)
  3. Patient has control over rate, inspiratory time, and inspiratory flow rate.
  4. Tidal volumes is determined by level of PSV, patient effort, and pulmonary mechanics.

Other Methods of Mechanical Ventilation

Inverse Ratio Ventilation

Rationale

  1. Sustained elevations in airway pressure may more effectively recruit collapsed alveoli.
  2. I:E ratio of > 1:1 may achieve higher mean airway pressure with lower peak alveolar pressure and lower PEEP than conventional mechanical ventilation (provided that excessive gas trapping does not occur).

Methods

  1. Pressure controlled (preset) ventilation (PCV) with prolonged inspiratory time
  2. Most common method
  3. Ventilation is a function of mechanics and intrinsic PEEP: the adequacy of the level of ventilation needs to be carefully monitored
  4. Volume controlled ventilation modified by

    1. Slow, constant inspiratory flow rate (IFR)
    2. Constant IRF, with end-inspiratory pause
    3. Decelerating IFR

Problems

  1. Marked increase in gas trapping (PEEPi)

    1. barotrauma, decreased cardiac output
    2. if I:E ratio <2:1, PEEPi is usually <10-15 cm H20)
  2. Decreased VT with increased PCO2
  3. Discomfort (need deep sedation + paralysis)
  4. Indications and methodology not well defined

Permissive Hypercapnia

Rationale:

This is based on the notion that high tidal volumes induce or aggravate lung injury.

Study of Hickling et al (Intensive Care Med 1990;16:372)

  1. Retrospective study of 50 patients with severe ARDS

  2. Aimed for PIP <30 cm H20 (always < 40 cm H20)

    1. VT as low as 5 ml/kg
    2. PaC02 62 torr (as high as 129 torr)
    3. pH 7.29 (range, 7.02-7.38)
    4. Mortality 16% (vs 40% predict)

Volutrauma in Experimental Animals

  1. Alveolar overdistension rapidly produces

    1. Detachment of endothelial cells from basement membrane
    2. Destruction of type I alveolar cells
    3. Increased endothelial and epithelial permeability

  2. Increased lung volume, not pressure, is responsible

Study of Dreyfus et al (ARRD 1988;137:1159)

  1. Extravascular lung water measurements in rats - Ventilator settings:

HiP-HiV: Peak airway pressure 45 cm H20
VT 40 ml/kg
LoP-HiV: Negative pressure (iron lung)
VT 44 ml
HiP-LoV: PIP 45 cm H20 (chest strapping)
VT 19 mVkg

  1. Lung water increased with both HiV conditions, whereas HiP-LoV was no different than findings in control animals

Ventilator Settings

  1. Ventilator mode
  2. Oxygen concentration
  3. Tidal volume
  4. Set rate
  5. Inspiratory flow rate
  6. Inspiration:expiration ratio
  7. PEEP

Inspired Oxygen Concentration

  1. Initially select high FI02 and adjust when obtain ABGs after 20 min
  2. Aim for lowest FI02 that will achieve Pa02 of 60-70 mm Hg (or Sp02 of 92%)
  3. PEEP may be required to achieve a decrease in FI02
  4. Oxygen toxicity
    1. Exposure to FI02 of 1.0 up to 24 hr does not result in a significant clinical risk, but beyond that time it is clearly toxic.
    2. An FI02 of 0.50 is generally considered safe for several weeks if required.
    3. For FI02 between 0.5 and 1.0, the duration of safe exposure prior to the onset of toxicity in humans is unknown.
    4. When managing hypoxemic patients, there is more to fear from severe hypoxemia than the potential threat of oxygen toxicity.

Tidal Volume

  1. Tidal volume during normal spontaneous breathing equals 5 ml/kg. Employment of this volume during mechanical ventilation results in atelectasis which can be avoided by using intermittent sighs.
  2. Large tidal volumes of 10-15 ml/kg may produce alveolar injury.
  3. Preferred tidal volume = 7-8 ml/kg
  4. Remember that some volume is lost (due to compression) in the circuit (2-3 ml/cm H20).

Ventilator Rate

  1. On bedside chart, record both the rate set on the ventilator and the patient's total respiratory rate

    1. IMV
        -Initially, set rate should be close to the patient's total rate
        -Slowly decrease set rate with patient's tolerance in order to wean

    2. AC
        -Back-up rate should be 2-4 breaths below spontaneous rate
        -Decreasing set rate does not decrease level of support

Inspiratory Flow Rate

  1. An inspiratory flow rate (IFR) OF 60 L/min achieves optimal gas exchange in most patients.
  2. An IFR of 100 L/min achieves better gas exchange in patients with COPD, probably because the decrease if I:E ratio (with prolongation of expiration) allows more complete emptying of gas-trapped regions.
  3. An inappropriately low IFR can markedly increase the active work of breathing by a patient.

Inspiration:Expiration Ratio

  1. Usually 1:2
  2. Inverse ratio ventilation (up to 4:1)

PEEP

Goals

  1. Improve oxygenation and minimize risk of oxygen toxicity of excess fluid within the lung. By recruiting collapsed alveoli, it permits inspiration to occur on the steep portion of the pressure-volume curve. These changes combined with possible redistribution of excess fluid within the lungs result in a decrease in shunt.
  2. Modify natural course of lung injury; however, prophylactic PEEP does not decrease the risk of ARDS in patients at risk.

Institution of PEEP

  1. Ensure that PEEP is the only variable being altered
  2. Use stepwise increments (3-5 cm H20)
  3. Minimize interval between change and evaluation

Optimal PEEP

  1. Subject of controversy
  2. Do not base Pa02 solely (as improvement in P02 may be accompanied by cardiovascular impairment)
  3. As a general rule, optimal PEEP is achieved by maximizing 02 delivery at lowest FI02 setting.
  4. Measurements of shunt fraction and thoracic compliance have been used as a guide to optimal PEEP; these are no longer commonly employed on routine basis.

Decreasing PEEP

  1. Abrupt reduction in PEEP may produce severe hypoxemia that takes days to reverse.
  2. Need stable patient with Pa02 > 80 mm Hg and FI02 < 0.40 before decreasing PEEP.
  3. Harborview Three-Minute Rule:

    1. Measure Pa02 and decrease PEEP by < 5 cm H20
    2. Obtain ABG after 3 min and immediately return PEEP to previous setting.
    3. If Pa02 falls by > 20%, maintain PEEP at previous setting
    4. If Pa02 falls by < 20%, there is a 90% likelihood that PEEP can be successfully decreased

Monitoring during Mechanical Ventilation

  1. Physical examination
  2. Respiratory rate (set, spontaneous)
  3. Delivered and spontaneous tidal volume
  4. Rib cage-abdominal motion
  5. Compliance (static, dynamic)
  6. Peak inspiratory pressure
  7. Auto PEEP
  8. Airway pressure waveform
  9. Work of breathing
  10. Gas exchange (ABG, Sp02)

Compliance

Measurement of delivered tidal volume, peak airway pressure, plateau pressure (during an end-inspiratory occlusion lasting up to 2 s) and PEEP permits the calculation of static and dynamic respiratory compliance.

Static compliance (Cst) = VT / (Plateau pressure - PEEP)
= 60-100 ml/cm H20
Decreased by pneumonia, edema, atelectasis, pneumothorax, or endobronchial intubation.
Dynamic characteristic (Cdyn) = VT / (Peak pressure - PEEP)
= 50-80 ml/cm H2)
Decreased by bronchospasm, mucus plugging, kinked tube, or decreased static compliance.

 

Airway Pressure Waveform

  1. During true passive inflation, the airway pressure tracing shows a smooth rise, it remains convex upward, and it is highly reproducible from breath to breath.
  2. In a patient who is receiving partial ventilator support (e.g., AC, IMV, PSV), the degree of deformation and scooping of the airway pressure provides a mean of monitoring the amount of effort expended by a patient

Rapid Shallow Breathing Index

f/VT ration  = 

Frequency (breaths min)
Tidal volume (liter)

If f/VT > 100 breaths per min/L, weaning failure is likely.

 

Complications

  1. Decreased cardiac output
  2. Barotrauma
  3. Endotracheal tube complications
  4. Infection
  5. Organ impairment (Renal, GI, CNS)
  6. Psychological impairment

Ventilator-Associated Pneumonia

  1. Reported incidence, 9-10% (usually, 30%)
  2. Mortality, 50-80% (vs 30% is comparable patients without pneumonia)
  3. Risk factors
    1. Underlying disease
    2. Impaired host defenses
    3. Depressed mucociliary transport
    4. Endotracheal/tracheostomy tube
    5. Aspiration
    6. Nebulizers

  4. Ventilator circuit

Craven (ARRD 1986;133:792)
Pneumonia rate 29% if tubing changed q 24 hr; 14% if tubing changed q 48 hr

Dreyfuss (ARRD 1991;p143)738)
Pneumonia rate (confirmed by bronchoscopy "brush") - same for circuit changes q 48 hr versus no change (average: 10 days)

  1. Clinical diagnosis

    1. very unreliable
    2. Fagon, Chastre, et al (ARRD 1988;138:110)
      • 147 ventilator patients with a new pulmonary infiltrate and purulent secretions, most of whom also had a fever and leukocytosis.
      • "Bronchoscopy brush" with quantitative cultures (> 103 colony-forming units per ml) were positive in only 31% of these patients.
      • No combination of 16 clinical variables was helpful (stepwise logistic regression).

 

Nonconventional Mechanical Ventilation

  1. Non-invasive nasal ventilation
  2. High-frequency ventilation
  3. Prone posture ventilation
  4. Pulmonary gas exchange devices
    1. Extravascular (ECMO, ECCO2 removal)
    2. Intravascular (IVOX)
  5. Tracheal insufflation of oxygen (TRIO)
  6. Constant flow ventilation (CFV)