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 CO2 by spontaneous breathing even with the assistance of other oxygen delivery devices.
Mechanical Failure of Ventilation
- Neuromuscular disease
- CNS disease
- CNS depression(drug intoxication, respiratory depressants, cardiac arrest).
- Inefficiency of thoracic cage in generating pressure gradients necessary for ventilation (chest injury, thoracic malforamation).
- When ventilatory support is needed postoperatively.
Disorders of Pulmonary Gas Exchange
- Acute respiratory failure
- Chronic respiratory failure
- Left-sided heart failure
- Pulmonary diseases resulting in diffusion abnormality
- Pulmonary diseases resulting in V/Q mismatch.
- Acute lung injury.
- Variable that control ventilation and oxygenation include:
- Ventilator- adjusted by rate setting.
- VT volume of gas required for onebreath (mL/kg)
- Fraction of inspired oxygen concentration (FiO2)- set on ventilator and measured with an oxygen analyzer.
- Ventilator dead space- Circulatory (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 VT rate, and dead space.
- Oxygen tension is controlled by oxygen concentration and PEEP (also by rate and VT)
- In most cases, the duration of inspiration should not exceed exhalation.
- The inspired gas must be warmed and humidified to prevent thickening of secretions and decrease in body temperature. Sterile or distilled water is warmed and humidified by way of a heated humidifier.
Types of Mechanical Ventilation
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 patients 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 (shell unit) 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 cycled.
- Delivers selected gas pressure during inspiratory phase.
- Volume delivered depends on lungs compliance and resistance.
- Use of volume-based alarms is recommended because any obstruction between the machine and lungs that allows a buildup of pressure in the ventilator circulatory will cause the ventilator to cycle, but the patient will receive no volume.
- Exhaled tidal volume is the variable to monitor closely.
- Volume Limited.
- Designated volume of air is delivered with each breath regardless of resistance and compliance. Usual starting volume is 6 to 8 mL/kg.
- Delivers the predetermined volume regardless of changing lung compliance (although airway pressure will increase as compliance decreases). Airway pressure vary from patient to patient and from breath to breath.
- Pressure-limiting valves, which prevent excessive pressure buildup this value, pressure could increase indefinitely and pulmonary barotrauma could result. Usually equipped with a system that alarms when selected pressure limit is exceeded. Pressure-limited settings terminate inspiration when reached.
Modes of Mechanical Ventilatore
- Patient receives a set number and volume of breaths/minute.
- Provides a fixed level of ventilation, but will not cycle or have gas available in circuitry to respond to patients own inspiratory efforts. This typically increase work of breathing for patients attempting to breathe spontaneously.
- Generally used for patients who are unable to initiate spontaneous breaths.
- Inspiratory cycle of ventilator is activated by the patients voluntary inspiratory effort and delivers a preset full volume.
- Ventilator also cycles at a rate predetermined by the operator. Should the patient not initiate a spontaneous breath, or breath so weakly that the ventilator cannot function as an assistor, this mandatory baseline rate will provide a minimum respiratory rate.
- 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 patients work of breathing is greatly reduced.
Synchronized Intermittent Mandatory Ventilation (SIMV)
- Allows patient to breathe at his or her own rate and volume spontaneously through the ventilator circuitry.
- Periodically, at a preselected time, a mandatory breath is delivered. The mandatory breaths are synchronized with the patient’s inspiratory effort.
- Gas provided for spontaneous breathing flows continuously through the ventilator.
- Ensures that a predetermined number of breaths at a selected VT are delivered each minute.
- Indicated for patients who are breathing spontaneously, but at a VT and/or rate less than adequate for their needs. Allows the patient to do some of the work of breathing.
- Augments inspiration to a spontaneously breathing patient.
- Maintains a set positive pressure during spontaneous inspiration.
- The patient ventilates spontaneously, establishing own rate, VT, and inspiratory time.
- Pressure support may be used independently as a ventilatory mode or used in conjunction with CPAP or synchronized intermittent mandatory ventilation.
Positive Pressure Ventilation Techniques
Positive End-Expiratory Pressure
- 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.
- Improving lung compliance.
- Improving oxygenation.
- Recruiting alveolar units that are totally or partially collapsed.
- Because a greater surface area for diffusion is available and shunting is reduced, it is often possible to use a lower FiO2 than otherwise would be required to obtain adequate arterial oxygen levels. This reduces the risk of oxygen toxicity in conditions such as acute 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 ( i.e left sided heart failure).
- Increased lung compliance resulting in decreased work of breathing.
- Because the intrathoracic pressure is increased by PEEP venous return is impeded. This may result in:
- Decreased cardiac output and decreased oxygen delivery to the tissues (especially noted in hypovolemic patients).
- Decreased renal perfusion.
- Increased intracranial pressure.
- Hepatic congestion.
- The decreased venous return may cause antidiuretic hormone formation to be stimulated, resulting in decreased urine output.
- Because the intrathoracic pressure is increased by PEEP venous return is impeded. This may result in:
- Monitor frequently for signs and symptoms of respiratory distress—shortness of breath, dyspnea, tachycardia, chest pain.
- Monitor frequently for signs and symptoms of pneumothorax (increased PAP, increased size of hemothorax, uneven chest wall movement, hyper resonant percussion, distant or absent breath sounds).
- Monitor for signs of decreased venous return (decreased BP, 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 that PEEP can be maintained.
- Intrapulmonary blood vessel pressure may increase with compression of the vessels by increased intra-airway pressure. Therefore, central venous pressure (CVP), PAP, and pulmonary capillary wedge pressure may be increased. The clinician must bear this in mind when determining the clinical significance of these pressures.
Continuous Positive Airway Pressure
- Assists the spontaneously breathing patient to improve oxygenation by elevating the end-expiratory pressure in the lungs throughout the respiratory cycle.
- 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 VT, but who have pathology preventing maintenance of adequate levels of tissue oxygenation or for sleep apnea.
- 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
- I:E ratio is greater than 1, in which inspiration is longer than expiration.
- Potentially used in patients who are in acute severe hypoxemic respiratory failure. Oxygenation is thought to be improved.
- Very uncomfortable for patients; need to be heavily sedated.
- Pressure-controlled inverse ratio ventilation—used in ARDS and acute lung injury.
- Pressure-regulated volume control ventilator mode is a volume-targeted mode used in acute respiratory failure that combines the advantages of the decelerating inspiratory flow pattern of a pressure-control mode with the ease of use of a volume-control (VC) mode.
Airway Pressure Release Ventilation
- Ventilator cycles between two different levels of CPAP.
- The baseline airway pressure is the upper CPAP level and the pressure is intermittently released.
- Uses a short expiratory time.
- Used in severe ARDS/acute lung injury.
Noninvasive Positive Pressure Ventilation
- Uses a nasal or face mask or nasal pillows. Delivers air through a volume- or pressure-controlled ventilator.
- Used primarily in the past for patients with chronic respiratory failure associated with neuromuscular disease. Now is being used successfully during acute exacerbations. Some patients are able to avoid invasive intubation. Other indications include acute or chronic respiratory distress, acute pulmonary edema, pneumonia, COPD exacerbation, weaning, and postextubation respiratory decompensation.
- Can be used in the home setting. Equipment is portable and relatively easy to use.
- Eliminates the need for intubation, preserves normal swallowing, speech, and the cough mechanism.
- May include BiPAP, which is essentially pressure support with CPAP. The system has a rate setting as well as inspiratory and expiratory pressure setting.
- Uses very small VT (dead space ventilation) and high frequency (rates greater than 100/minute).
- Gas exchange occurs through various mechanisms, not the same as conventional ventilation (convection).
- Types include:
- High-frequency oscillatory ventilation.
- High-frequency jet ventilation.
- Theory is that there is decreased barotrauma by having small VT and that oxygenation is improved by constant flow of gases.
- Successful with infant respiratory distress syndrome, much less successful with adult pulmonary complications.
Nursing Assessment and Interventions
- Monitor for complications:
- Airway aspiration, decreased clearance of secretions, ventilator-acquired pneumonia, tracheal damage, laryngeal edema.
- Impaired gas exchange.
- Ineffective breathing pattern.
- ET tube kinking, cuff failure, mainstem intubation.
- Pulmonary infection.
- Barotrauma (pneumothorax, tension pneumothorax, subcutaneous emphysema, pneumomediastinum).
- Decreased cardiac output.
- Alteration in GI function (stress ulcers, gastric distention, paralytic ileus).
- 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.
- Increased 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.
- Provide routine care for patient on mechanical ventilator Provide regular oral care to prevent ventilator-associated pneumonia. Provide humidity and repositioning to mobilize secretions.
- Assist with the weaning process, when indicated
- Patient must have acceptable ABG values, no evidence of acute pulmonary pathology, and must be hemodynamically stable.
- Obtain serial ABGs and/or oximetry readings, as indicated.
- Monitor very closely for change in pulse and BP, anxiety and increased rate of respirations.
- The patient is awake and cooperative and displays optimal respiratory drive.
- Once weaning is successful, extubate and provide alternate means of oxygen.
- Extubation will be considered when the pulmonary function parameters of VT, VC, and negative inspiratory pressure are adequate, indicating strong respiratory muscle function.