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Mechanical Ventilation 

Mechanical Ventilation
Chapter:
Mechanical Ventilation
Author(s):

Michael P. Mendez

DOI:
10.1093/med/9780190862800.003.0026
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date: 18 September 2020

  1. A. Introduction

    1. a. Definition. A ventilator is a pump that delivers a set volume or pressure of mixed gas (usually oxygen and nitrogen).

    2. b. Phases of ventilatory support. The three main phases of ventilatory support are initiation, maintenance, and liberation.

      1. i. Initiation. Chapter 14 describes guidelines for making the decision to intubate.

      2. ii. Maintenance. The goal of maintenance is to avoid causing or worsening existing lung injury while achieving a balance between making the patient comfortable and allowing enough awareness for assessment of neurologic status and pain. Any of the modes of ventilation detailed in this chapter may be appropriate.

      3. iii. Liberation (also called weaning). Timing depends on improvement or reversal of the patient’s underlying disease, good mental status, and adequate respiratory mechanics and reserve.

  2. B. Modes of Positive Pressure Ventilation

    1. a. Volume-cycled modes

      1. i. Synchronized intermittent mandatory ventilation (SIMV). The ventilator delivers a defined tidal volume at a set rate but delivers no tidal volume for patient-initiated breaths above the set rate. Ventilator breaths are synchronized with patient-initiated breaths. This mode has fallen out of favor because of the disadvantages listed below.

        1. 1. Advantages. SIMV offers maximum control of ventilation and possibly greater patient-ventilator synchrony.

        2. 2. Disadvantages

          • a. SIMV is inappropriate for patients who require fully supported breaths due to severe underlying illness or high oxygen demand.

          • b. SIMV may prolong liberation from mechanical ventilation when used as a weaning mode. It is difficult for the patient to overcome resistance of the tubing, which may potentiate respiratory muscle fatigue, when breathing unassisted over the set rate.

      2. ii. Assist control (AC). The ventilator delivers a set tidal volume at a set rate and delivers the same tidal volume for patient-initiated breaths above the set rate. Ventilator breaths are synchronized with patient breaths, and pressures with each breath are allowed to vary.

        1. 1. Advantages. The patient receives assistance with every breath.

        2. 2. Disadvantages

          • a. The patient may develop respiratory alkalosis because every patient-initiated breath results in a full tidal volume.

          • b. High pressures may result from the delivery of a set tidal volume.

          • c. May be less comfortable than a pressure-cycled mode.

    2. b. Pressure-cycled modes. In the pressure control (PC) mode, the ventilator delivers a constant pressure at a set rate and delivers the same pressure for patient-initiated breaths above the set rate. Tidal volumes, however, vary.

      1. i. Advantages. This mode can be used when airway pressures are too high on SIMV or AC.

      2. ii. Disadvantages. The patient may not receive adequate tidal volumes, especially if the lungs are stiff or the patient has significant airway resistance (e.g., severe obstructive airway disease).

    3. c. Flow-cycled modes

      1. i. Pressure support (PS). The ventilator delivers a set pressure only when the patient initiates a breath. Pressure support ceases when flow decreases to 25% of maximum inspiratory flow.

        1. 1. Advantages

          • a. Because this method is more physiologic than many of the others, patient comfort is enhanced.

          • b. As this mode allows the patient to regulate respiratory rate and tidal volume, it is often used in preparation for liberation from mechanical ventilation.

        2. 2. Disadvantages

          • a. The patient must trigger every respiration. Thus, pressure support ventilation is not appropriate for patients who hypoventilate (i.e., “will not breathe”) or are not able to spontaneously breathe for any reason.

          • b. The patient may not receive adequate tidal volumes.

    4. d. Combined modes. Many newer ventilators now have modes that combine both volume-cycled and pressure-cycled ventilation. It is unclear whether these modes offer a clinical advantage over traditional modes. Theoretically, they help to ensure adequate ventilation (volume-cycled) while attempting to optimize airway pressure and patient comfort (pressure-cycled).

    5. e. Liberation modes. When the underlying reason for intubation and mechanical ventilation are reversed, a spontaneous breathing trial (SBT) should be initiated. An SBT is defined as a period of time (at least ½ hour), in which the patient is placed on a liberation mode (see below) and allowed to breathe spontaneously to assess patient readiness to liberate from mechanical ventilatory support. Modes include:

      1. i. Continuous positive airway pressure (CPAP). The ventilator delivers a continuous pressure, usually 5 cm H2O, throughout inspiration and expiration.

        1. 1. Advantages. The patient does the work of breathing (the low level of support theoretically overcomes the resistance of the ventilator tubing).

        2. 2. Disadvantages. Some positive pressure support is provided, and thus success with normal ventilation is not guaranteed. Some patients with significant heart failure receive cardiovascular support from positive end-expiratory pressure (PEEP), and therefore a weaning trial with CPAP may not predict how the patient will perform when liberated from the ventilator.

      2. ii. T-piece. The endotracheal tube is attached to a T-shaped piece of tubing that delivers only oxygen. The patient is not attached to a ventilator. This method should be used for spontaneous breathing trials (see below) in patients with significant heart failure.

        1. 1. Advantages. The patient does the work of breathing without positive pressure support.

        2. 2. Disadvantages. Because the patient is not attached to a ventilator, no alarms will sound if the patient becomes tachypneic or apneic.

  3. C. Initial Ventilator Settings

    1. a. General principles

      1. i. Hypoxemic respiratory failure. Patients with respiratory failure and a “white chest radiograph” usually have pulmonary interstitial or alveolar disease. Pneumonia, heart failure, and acute respiratory distress syndrome (ARDS) make up most cases. If there is no clinical evidence of heart failure, these patients should be placed on a low tidal volume strategy to prevent the development or worsening of ARDS.

        1. 1. Tidal volume: 6 mL/kg of ideal body weight (IBW).

        2. 2. Respiratory rate: 18–35 breaths/min to ensure adequate ventilation (pH >7.20).

        3. 3. PEEP: 5–24 cm H2O combined with an appropriate fraction of inspired oxygen to maintain PaO2 >55 mm Hg.

        4. 4. In general, tidal volume should be lowered to achieve a plateau pressure <30 cm H2O.

      2. ii. Hypercapnic respiratory failure. Patients with respiratory failure with a “black chest radiograph” usually have airway disease or a central nervous system cause of hypoventilation. Patients with significant airway disease need higher tidal volumes and low respiratory rates to ensure a long expiratory time to empty their lung.

        1. 1. Tidal volume: 8–12 mL/kg IBW

        2. 2. Respiratory rate: 8–12 breaths/min to achieve adequate ventilation while allowing for a long expiratory time to avoid hyperinflation

        3. 3. PEEP can be set at 5 cm H2O and titrated as needed to avoid hyperinflation.

      3. iii. Normal lungs. Intubated for airway protection (e.g., overdose). Combination of the above strategies is acceptable. Tidal volume 6–10 mL/kg IBW, respiratory rate of 12–16 breaths/min, PEEP of 5 cm H2O.

  4. D. Liberation Settings

    1. a. Methods. Liberation is recommended by the following methods:

      1. i. Pressure support. The level of pressure support is turned down gradually. Often used for long-term weaning in patients who can initiate breaths but require more support.

      2. ii. SBT. The patient is placed on continuous positive airway pressure (CPAP) or T-piece for at least 30 minutes once a day as a “stress test” to gauge how the patient will do with very little ventilator support. The patient is assessed throughout and at the end of the trial. The tube is removed if the patient has stable oxygenation, ventilation, and hemodynamics and minimal secretions at the end of the SBT. When coupled with an interruption in sedation, SBT has been shown to improve mortality and decrease time on the ventilator without increasing rate of reintubation. Therefore, a spontaneous breathing trial should be attempted on every day that the criteria in Table 26.1 are met. Remember the mnemonic “WEANS NOW” when reviewing whether a patient is ready for extubation.

        Table 26.1 Criteria to Proceed with Spontaneous Breathing Trial and Extubation

        WEANS NOW: Ventilator Weaning Criteria

        Wake

        Able to protect their airway

        Electrolytes

        No hypomagnesemia or hypophosphatemia

        Acidosis/Alkalosis

        Rule out metabolic causes

        Neuromuscular

        Beware of steroids, paralytics, and aminoglycosides

        Suctioning/Secretions

        Communicate with nursing and respiratory therapy

        Nutritionally intact

        Make a plan with your nutritionist/dietitian

        Obstruction

        If indicated, β‎-agonist ordered?

        Weaning parameters

        RSBI less than 105 breaths/LNIF less than −20 cm H2O

        NIF = negative inspiratory force; RSBI = rapid shallow breathing index.

    2. b. Criteria to proceed with an SBT:

      1. i. Reversal or improvement of the condition causing respiratory failure.

      2. ii. The patient is awake and able to protect the airway.

      3. iii. Fraction of inspired oxygen (Fio2) 0.4 or less with an arterial oxygen tension (PaO2) >60 mm Hg.

      4. iv. PEEP ≤5 cm H2O.

      5. v. Rapid shallow breathing index (RSBI). This is the most predictive measure of successful extubation. The RSBI is the spontaneous respiratory rate/spontaneous tidal volume in liters while off positive inspiratory pressure. A ratio of less than 105 is predictive of successful extubation.

      6. vi. Negative inspiratory force (NIF). This is a test of respiratory muscle strength and is used to assess and trend chest wall weakness. The patient should generate less than −20 cm H2O if ready to extubate. A healthy adult can generate less than −100 cm H2O.

    3. c. Prevention of postextubation respiratory failure. Use of heated, high-flow nasal cannula or noninvasive positive pressure ventilation (both discussed in Chapter 14) has been shown to decrease reintubation and respiratory failure in patients with the following high-risk characteristics:

      1. i. Age >65 years

      2. ii. APACHE II score >12

      3. iii. Body mass index >30

      4. iv. Inadequate secretion management

      5. v. Difficult or prolonged weaning/mechanical ventilation

      6. vi. More than one comorbidity

      7. vii. Heart failure as primary indication for mechanical ventilation

      8. viii. Moderate to severe chronic obstructive pulmonary disease (COPD)

      9. ix. Airway protection problems

  5. E. Ventilator Emergencies. When facing ventilator emergencies, always disconnect the patient from the ventilator and use a bag valve mask and 100% oxygen to ventilate until the problem is resolved.

    1. a. Low airway pressure alarm. Differential diagnoses include the following:

      1. i. Disconnected tubing

      2. ii. Air leak around the cuff (e.g., balloon rupture or tracheal dilation)

      3. iii. Extubation (e.g., tube has slipped into the oropharynx)

      4. iv. Tracheoesophageal fistula (rare)

    2. b. High airway pressure alarm. There are two important pressures to understand on the ventilator.

      1. i. Peak pressure is the highest pressure needed to inflate the lungs during a tidal volume. It is measured during inspiration and represents both airway resistance and elastic (stretch) resistance of the lung/chest wall complex (plateau pressure).

      2. ii. Plateau pressure is the elastic (stretch) pressure applied to the lung/chest wall complex. Importantly, airway resistance is not factored in because this pressure is measured during an end-inspiratory static breath hold (no flow state). Ascites (large volume), obesity, and pleural disease can also increase plateau pressure.

        1. 1. If peak pressure rises without an increase in plateau, there is an increase in airway resistance. This can occur anywhere from the ventilator to the small airways. Examples:

          • a. Kink in ventilator tubing

          • b. Bronchospasm

          • c. Secretions or aspiration of contents into airways

        2. 2. If both peak and plateau pressures are elevated, there is an increase in the stiffness of the lung/chest wall complex. Examples:

          • a. Decrease lung compliance (e.g., pulmonary edema, pneumonia, atelectasis)

          • b. Pneumothorax

          • c. Auto-PEEP (hyperinflation due to stacking breaths)

          • d. Mucus plugging (completely occluding airway)

          • e. Asynchronous breathing (patient resistance to ventilator delivered breath)

    3. c. Apnea alarm. Differential diagnoses include the following:

      1. i. Sedatives

      2. ii. Central nervous system (CNS) depression

      3. iii. Muscle weakness

      4. iv. Neurologic defects

Suggested Further Readings

Burns KE, Meade MO, Premji A, Adhikari NK. Noninvasive positive-pressure ventilation as a weaning strategy for intubated adults with respiratory failure. Cochrane Database Syst Rev 2013:Cd004127.Find this resource:

Hernandez G, Vaquero C, Colinas L, et al. Effect of postextubation high-flow nasal cannula vs noninvasive ventilation on reintubation and postextubation respiratory failure in high-risk patients: a randomized clinical trial. JAMA 2016;316:1565–74.Find this resource:

Tobin MJ. Advances in mechanical ventilation. N Engl J Med 2001;344:1986–96. (Classic Article.)Find this resource: