The mechanical ventilator

The mechanical ventilator device functions as a substitute for the bellows action of the thoracic cage and diaphragm. The mechanical ventilator can maintain ventilation automatically for prolonged periods. It is indicated when the patient is unable to maintain safe levels of oxygen or carbon dioxide by spontaneous breathing even with the assistance of other oxygen delivery devices.
Clinical Indications
 Mechanical Failure of Ventilation
• Neuromuscular disease.
• Central nervous system disease.
• Central nervous system depression (drug intoxication, respiratory depressants, cardiac arrest).
• Musculoskeletal disease.
• Inefficiency of thoracic cage in generating pressure gradients necessary for ventilation (chest injury, thoracic malformation).
 Disorders of Pulmonary Gas Exchange
• Acute respiratory failure.
• Chronic respiratory failure.
• Left ventricular failure.
• Pulmonary diseases resulting in diffusion abnormality.
• Pulmonary diseases resulting in ventilation/perfusion mismatch.
Underlying Principles
 Variables that control ventilation and oxygenation include:
• Ventilator rate—adjusted by rate setting.
• Tidal volume (VT)—adjusted by tidal volume setting; measured as inhaled volume.
• Fraction inspired oxygen concentration (FiO2)—set on ventilator or with an oxygen blender; measured with an oxygen analyzer.
• Ventilator dead space—circuitry (tubing) common to inhalation and exhalation; tubing is calibrated.
• PEEP—set within the ventilator or with the use of external PEEP devices; measured at the proximal airway.
CO2 elimination is controlled by tidal volume, rate, and dead space.
Oxygen tension is controlled by oxygen concentration and PEEP (also by rate and tidal volume).
In most cases, the duration of inspiration should not exceed exhalation.
• Rate, tidal volume, gas flow in liters per minute, and inspiratory pause all control inspiratory time.
• Inverse inspiration:exhalation (I:E) ratio results in “stacking” of breaths or buildup of pressure within the airway. Barotrauma and decreased cardiac output can result when inverse I:E ratio is used.
The inspired gas must be warmed and humidified to prevent thickening of secretions and decrease in body temperature. Sterile water is warmed and humidified by way of a heated humidifier.
Types of Ventilators
 Negative Pressure Ventilators
• Applies negative pressure around the chest wall. This causes intra-airway pressure to become negative, thus drawing air into the lungs through the patient’s nose and mouth.
• No artificial airway is necessary; patient must be able to control and protect own airway.
• Indicated for selected patients with respiratory neuromuscular problems, or as adjunct to weaning from positive pressure ventilation.
• Examples are the iron lung and cuirass ventilator.
 Positive Pressure Ventilators
During mechanical inspiration, air is actively delivered to the patient’s lungs under positive pressure. Exhalation is passive. Requires use of a cuffed artificial airway.
 Pressure limited.
• Terminates the inspiratory phase when a preselected airway pressure is achieved.
• Volume delivered depends on lung compliance.
• Use of volume-based alarms is recommended because any obstruction between the machine and lungs that allows a buildup of pressure in the ventilator circuitry will cause the ventilator to cycle, but the patient will receive no volume.
 Volume limited.
• Terminates the inspiratory phase when a designated volume of gas is delivered into the ventilator circuit (10 to 15 mL/kg body weight—usual starting volume).
• Delivers the predetermined volume regardless of changing lung compliance (although airway pressures will increase as compliance decreases). Airway pressures vary from patient to patient and from breath to breath.
• Pressure-limiting valves, which prevent excessive pressure buildup within the patient-ventilator system, are used. Without this valve, pressure could increase indefinitely and pulmonary barotrauma could result. Usually equipped with a system that alarms when selected pressure limit is exceeded and vents excess inspired air to the atmosphere.
Modes of Operation
 Controlled Ventilation (CV)
• Cycles automatically at rate selected by operator.
• Provides a fixed level of ventilation, but will not cycle or have gas available in circuitry to respond to patient’s own inspiratory efforts. This typically increases work of breathing for patients attempting to breathe spontaneously.
• Possibly indicated for patients whose respiratory drive is absent.
 Assist/Control (A/C)
• Inspiratory cycle of ventilator is activated by the patient’s voluntary inspiratory effort and delivers preset volume.
• Ventilator also cycles at a rate predetermined by the operator. Should the patient stop breathing, or breathe so weakly that the ventilator cannot function as an assistor, this mandatory baseline rate will prevent apnea. A minimum level of minute ventilation (VE) is provided.
• Indicated for patients who are breathing spontaneously, but who have the potential to lose their respiratory drive or muscular control of ventilation. In this mode, the patient’s work of breathing is greatly reduced.
 Intermittent Mandatory Ventilation (IMV)
• Allows patient to breathe spontaneously through ventilator circuitry.
• Periodically, at preselected rate and volume, cycles to give a “mandated” ventilator breath. A minimum level of ventilation is provided.
• Gas provided for spontaneous breaths usually flows continuously through the ventilator.
• Indicated for patients who are breathing spontaneously, but at a tidal volume and/or rate less than adequate for their needs. Allows the patient to do some of the work of breathing.
• Can cause stacked breaths when machine breath and patient-generated breath occur concurrently.
 Synchronized Intermittent Mandatory Ventilation (SIMV)
• Allows patient to breathe spontaneously through the ventilator circuitry.
• Periodically, at a preselected time, a mandatory breath is delivered. The patient may initiate the mandatory breath with own inspiratory effort, and the ventilator breath will be synchronized with the patient’s efforts, or will be “assisted.” If the patient does not provide inspiratory effort, the breath will still be delivered, or “controlled.”
• Gas provided for spontaneous breathing is usually delivered through a demand regulator, which is activated by the patient.
• Indicated for patients who are breathing spontaneously, but at a tidal volume and/or rate less than adequate for their needs. Allows the patient to do some of the work of breathing.
 Pressure Support
• A positive pressure is set.
• During spontaneous inspiration, ventilator circuitry is rapidly pressurized to the predetermined pressure and held at this pressure.
• When the inspiratory flow rate decreases to a preset minimal level (20% to 25% or peak inspiratory flow), the positive pressure returns to baseline. The patient may exhale or complete inspiration without pressure support.
• The patient ventilates spontaneously, establishing own rate, and inspiring the tidal volume that feels appropriate.
• Pressure support may be used independently as a ventilatory mode or used in conjunction with CPAP or SIMV.
Special Positive Pressure Ventilation Techniques
 Positive End-Expiratory Pressure (PEEP)
• Maneuver by which pressure during mechanical ventilation is maintained above atmospheric at end of exhalation, resulting in an increased functional residual capacity. Airway pressure is therefore positive throughout the entire ventilatory cycle.
• Purpose is to increase functional residual capacity (or the amount of air left in the lungs at the end of expiration). This aids in:
 Increasing the surface area of gas exchange.
 Preventing collapse of alveolar units and development of atelectasis.
• Decreasing intrapulmonary shunt.
• Benefits
 Because a greater surface area for diffusion is available and shunting is reduced, it is often possible to use a lower fraction of inspired oxygen concentration (FiO2) than otherwise would be required to obtain adequate arterial oxygen levels. This reduces the risk of oxygen toxicity in conditions such as adult respiratory distress syndrome (ARDS).
 Positive intra-airway pressure may be helpful in reducing the transudation of fluid from the pulmonary capillaries in situations where capillary pressure is increased (ie, left heart failure).
 Increased lung compliance resulting in decreased work of breathing.
• Hazards
 Because the mean airway pressure is increased by PEEP, venous return is impeded. This may result in a decrease in cardiac output (especially noted in hypovolemic patients).
 The increased airway pressure may possibly result in alveolar rupture. The likelihood is greater in patients with noncompliant lungs. This barotrauma may result in pneumothorax, tension pneumothorax, or development of subcutaneous emphysema.
 The decreased venous return may cause antidiuretic hormone formation to be stimulated, resulting in decreased urine output.
• Precautions
 Monitor frequently for signs and symptoms of pneumothorax (increased pulmonary artery pressure, increased size of hemithorax, decreased lung movement, hyperresonant percussion, diminished breath sounds).
 Monitor for signs of decreased venous return (decreased blood pressure, decreased cardiac output, decreased urine output, peripheral edema).
 Abrupt discontinuance of PEEP is not recommended. The patient should not be without PEEP for longer than 15 seconds. The manual resuscitation bag used for ventilation during suction procedure or patient transport should be equipped with a PEEP device. In-line suctioning may also be used so PEEP can be maintained. Some clinicians feel that loss of PEEP for short periods is not detrimental in the lower ranges (less than 10 cm H2O). An exception might be patients with increased intracranial pressure.
 Intrapulmonary blood vessel pressure may increase because of compression of the vessels by increased intra-airway pressure. Therefore, central venous pressure (CVP) and pulmonary artery pressure (PAP) and pulmonary capillary wedge pressure (PCWP) may be increased. The clinician must bear this in mind when determining the clinical significance of these pressures.
Continuous Positive Airway Pressure (CPAP)
• Also provides for positive airway pressure during all parts of a respiratory cycle, but refers to spontaneous ventilation rather than mechanical ventilation.
• May be delivered through ventilator circuitry when ventilator rate is at “0” or may be delivered through a separate CPAP circuitry that does not require the ventilator.
• Indicated for patients who are capable of maintaining an adequate tidal volume, but who have pathology preventing maintenance of adequate levels of tissue oxygenation.
• CPAP has the same benefits, hazards, and precautions noted with PEEP. Mean airway pressures may be lower because of lack of mechanical ventilation breaths. This results in less risk of barotrauma and impedance of venous return.
Newer Modes of Ventilation
 Inverse Ratio Ventilation (IRV)
• I:E ratio is greater than 1 (normally inspiration is shorter than expiration).
• Potentially used in patients who are in acute severe hypoxemic respiratory failure. Oxygenation is thought to be improved.
• Used with heavily sedated patients.
• Still under investigation—only used in rare cases.
 Noninvasive Positive Pressure Ventilation (NIPPV)
• Uses a nasal mask, nasal pillow, oral mask or mouthpiece attached to a standard ventilator. Delivers air through portable ventilator that is either volumecycled or flow-cycled.
• Used primarily in the past for patients with chronic respiratory failure associated with neuromuscular disease. Now, is being used somewhat successfully during acute exacerbations. Some patients are able to avoid invasive intubation. Other indications include weaning and postextubation respiratory decompensation.
• Used easily in home setting—equipment is portable and relatively easy to use.
 High-Frequency Ventilation (HFV)
• Uses very small tidal volumes (less than dead space volume) and high frequency (ratios greater than 100).
• Gas exchange occurs through various mechanisms, not the same as conventional ventilation (convection).
• Types
 High-frequency oscillatory ventilation (HFOV)
 High-frequency jet ventilation (HFJV)
• Theory is that there is decreased barotrauma by having small tidal volumes and that oxygenation is improved by constant flow of gases.
• Has not yet proven to be significantly helpful, but is being tested in neonates (HFOV), and in adults for the treatment of ARDS (HFJV), as well as bronchopleural fistulas (HFJV).
Nursing Assessment and Interventions
 Monitor for complications.
• Airway obstruction (thickened secretions, mechanical problem with artificial airway or ventilator circuitry)
• Tracheal damage
• Pulmonary infection
• Barotrauma (pneumothorax or tension pneumothorax)
• Decreased cardiac output
• Atelectasis
• Alteration in GI function (dilation, bleeding)
• Alteration in renal function
• Alteration in cognitive-perceptual status
 Suction the patient as indicated.
• When secretions can be seen or sounds resulting from secretions are heard with or without the use of a stethoscope.
• After chest physiotherapy.
• After bronchodilator treatments.
• After a sudden rise or the “popping off” of the peak airway pressure in mechanically ventilated patients that is not due to the artificial airway or ventilator tube kinking, the patient biting the tube, the patient coughing or struggling against the ventilator, or a pneumothorax.
• Routine suction is not indicated, but should be based on assessment, patient’s underlying condition, and chest X-ray findings.
 Provide routine care for patient on mechanical ventilator.
 Assist with the weaning process, when indicated (patient gradually assumes responsibility for regulating and performing own ventilations).
• Patient must have acceptable ABGs, no evidence of acute pulmonary pathology, and be hemodynamically stable.
• Obtain serial ABGs and/or oximetry readings, as indicated.
• Monitor very closely for change in pulse and blood pressure, anxiety, and increased rate of respirations.
• The use of anxiolytics to assist with weaning the anxious patient is controversial; they may or may not be beneficial.
 Once weaning is successful, extubate and provide alternate means of oxygen.
• Extubation will be considered when the pulmonary function parameters of tidal volume (VT), vital capacity (VC), and negative inspiratory force (NIF) are adequate, indicating strong respiratory muscle function.