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Dec.29.2020

Mechanical Ventilation: Volume Mode (Respiratory Therapy)

ALERT

Never disable ventilator alarms.

Always plug a ventilator into a power outlet that is supplied by an emergency generator.

OVERVIEW

Positive pressure ventilation (PPV) through an artificial airway is used to maintain or improve oxygenation and ventilation. Respiratory insufficiency or failure, evidenced by apnea, hypoxia, hypercarbia, and increased work of breathing, are indications for mechanical ventilation. Selection of volume or pressure modes is dependent on the available evidence, clinical goals, availability of modes, and the practitioner's preference. There is very little evidence indicating that one mode of ventilation is more effective than another in terms of clinical outcomes (i.e., mortality) and ventilator hours needed.

Positive pressure modes of ventilation have traditionally been categorized into volume mode and pressure mode. However, with the advent of microprocessor technology, sophisticated iterations of traditional volume and pressure modes of ventilation have evolved. Ventilator manufacturers have created different names for the modes, and parameters that require adjustment vary somewhat among the ventilators. Although many of the modes have names that are different from traditional volume and pressure modes, they are similar in function in many cases. There is little evidence that the newer modes improve outcomes.

Summary descriptions of modes, mode parameters, and ventilator alarms are provided within this procedure (Table 1)Table 1 (Table 2)Table 2.

Volume-Limited Ventilation

Volume-limited ventilation has traditionally been the most popular form of PPV, largely because tidal volume (VT) and minute ventilation (MV) are ensured. MV is defined as VT multiplied by respiratory rate. With volume ventilation, a predetermined VT is delivered with each breath, regardless of resistance and compliance. VT is stable from breath to breath but airway pressure may vary.

Several modes of volume-limited ventilation can be delivered. Those modes include controlled mechanical ventilation (CMV), assist-control (A/C) ventilation, and synchronized intermittent mandatory ventilation (SIMV).

Sensitivity

Most ventilators have pressure-sensing mechanisms that trigger flow. This means that the patient must generate a decrease in the system pressure with an inspiratory effort. When the ventilator senses the drop in pressure, flow is delivered. If the ventilator has a flow-triggering option, the flow trigger is selected in L/min. The smaller the number of L/min selected, the more sensitive the ventilator capabilities. Flow triggering is set in conjunction with a base flow (flow in L/min that is provided between ventilator breaths). Flow rate is monitored in the expiratory limb of the ventilator. When flow is disrupted during a spontaneous breath, a decrease in flow downstream is sensed; additional flow is delivered.

Humidity

Humidity is essential to prevent the drying effect of the gases provided by the ventilator. Inspired gases may be humidified with the use of standard cascade or high-volume humidifiers. Many organizations use disposable heat-moisture exchangers (HMEs) in place of conventional humidifiers because HMEs decrease the risk of infection and are inexpensive. HMEs prevent hypothermia, evaporation and thickening of secretions, atelectasis, and destruction of the epithelium in the airway.undefined#ref1">1 The use of HMEs has been associated with decreased incidence of ventilator-associated pneumonia (VAP) in ventilated patients.

Complications

Complications of PPV include pulmonary barotrauma, volume-pressure trauma, hemodynamic changes, and VAP.

  • Pulmonary barotrauma is manifested by pneumothorax, pneumomediastinum, pneumopericardium, pneumoperitoneum, and subcutaneous emphysema.
  • Volume-pressure trauma is evidenced by large volumes being translated into high plateau pressures and subsequent acute lung injury.
  • Hemodynamic changes can be caused by PPV, which can reduce venous return and decrease cardiac output. Auto positive end-expiratory pressure (PEEP), also known as air trapping, is a common complication of mechanical ventilation that can result in hemodynamic compromise and even death.

EDUCATION

  • Provide developmentally and culturally appropriate education based on the desire for knowledge, readiness to learn, and overall neurologic and psychosocial state.
  • Clarify advance directives with the patient and family.
  • During a life-threatening emergency, mechanical ventilation may need to be initiated quickly, with no time for staff to speak with the patient or family members beforehand. As soon as possible, educate the patient and family about mechanical ventilation.
  • Ensure that the patient and family understand the implications of intubation and mechanical ventilation specific to the situation, including why a ventilator is being used. Communicate in a way they understand; "respirator" and "life support" are commonly understood terms.
  • Explain the procedure to the patient and family.
  • Discuss the potential benefits of mechanical ventilation that the patient may experience (e.g., less shortness of breath, less difficulty with the breathing process).
  • Discuss the unpleasant sensations that the patient may experience (e.g., gagging, anxiety). Explain to the patient that medications are given to promote relaxation and tolerance of the treatment. Explain that some patients may require sedation during mechanical ventilation.
  • Explain that the patient will be unable to speak. Establish a method of communication in conjunction with the patient and family before initiating mechanical ventilation, if necessary.
  • Explain to the patient and family what they should expect while the patient is ventilated.
  • Educate the patient and family about ventilator alarms and their meanings. Assure them that staff do hear the alarms and will respond accordingly.
  • Encourage questions and answer them as they arise.

ASSESSMENT AND PREPARATION

Assessment

  1. Perform hand hygiene before patient contact.
  2. Introduce yourself to the patient.
  3. Verify the correct patient using two identifiers.
  4. Assess the need for mechanical ventilation before initiating ventilator support.
    1. Signs and symptoms of respiratory insufficiency or failure (e.g., hypercapnia secondary to hypoventilation, hypoxia)
    2. Decreased peripheral oxygen saturation (SpO2) and arterial oxygen saturation (SaO2)
    3. Altered level of consciousness
    4. Adventitious breath sounds
    5. Acid-base imbalance
    6. Cyanosis
    7. Hypotension or hypertension
    8. Increased work of breathing
    9. Hemodynamic instability

Preparation

  1. Before initiating mechanical ventilation, ensure that the ventilator and associated equipment are functioning properly per the manufacturer's specifications and the organization's practice. Check the system microprocessor or ventilation system, circuit compliance, HME, humidifier, and filters, and perform a circuit leak test.
  2. Ensure that the patient is positioned with the head of the bed elevated 30 to 45 degrees, unless contraindicated.3

PROCEDURE

  1. Perform hand hygiene and don gloves.
  2. Verify the correct patient using two identifiers.
  3. Explain the procedure to the patient and ensure that he or she agrees to treatment.
  4. In collaboration with the authorized practitioner, select the most appropriate mode of volume mechanical ventilation based on the patient's needs (CMV, SIMV, A/C).
    Rationale: Mode selection varies depending on the clinical goal and the practitioner's preference.
    1. CMV is used when a guaranteed volume and rate are desired, which is ensured by setting the sensitivity or flow triggers. When control of ventilation is desired, sedation and paralytic agents may be administered.
    2. SIMV requires setting a frequency and VT that are delivered in synchrony with the patient's respiratory effort. Between mandatory breaths, the patient may initiate breaths at a self-determined volume and rate. In many cases, intermittent mandatory ventilation (IMV) is used in conjunction with pressure support ventilation (PSV) to overcome circuit resistance and to decrease the work of breathing associated with spontaneous effort.
      Rationale: The use of IMV plus PSV has been associated with prolonged weaning times. If respiratory muscle rest is the goal of using IMV plus PSV, the level of PSV should be high enough to provide a V T of 4 to 8 ml/kg and to maintain a total rate (IMV plus PSV breaths) of less than or equal to 20 breaths/min. 3
    3. A/C ventilation requires a control rate and volume to be set. Patient-initiated breaths are delivered at the predetermined volume selected for the control breaths.
  5. Set the VT based on the patient's size.
    1. Generally, 10 ml/kg of ideal body weight (IBW) or greater is not indicated in critically ill patients; typically, 4 to 8 ml/kg of IBW is acceptable.4
    2. Initially, 6 to 8 ml/kg is acceptable without acute respiratory distress syndrome (ARDS). Volumes of less than 6 ml/kg in patients with ARDS may improve mortality.4
    3. The VT may have to be increased if the partial pressure of carbon dioxide (PaCO2) increases causing the patient to become hypercarbic or acidotic.
    4. When lower VTs are used in an attempt to reduce lung injury, the patient may require sedation and paralytics to prevent spontaneous effort. Hypercarbia is an expected outcome of low VT values.
    5. Permissive hypercarbia is generally well tolerated in patients if tissue oxygenation is maintained.4
    6. Occasionally, bicarbonate infusions are used to keep the pH within an acceptable range. However, this maneuver may result in a higher arterial PaCO2 because bicarbonate is metabolized into carbon dioxide and water.
      Do not attempt permissive hypercarbia in a patient with elevated intracranial pressure or a patient with myocardial ischemia, injury, or arrhythmias.
  6. Select a respiratory frequency. In most cases, this frequency is set at a rate of 12 to 16 breaths/min.4
    Rationale: The frequency rate selected depends on whether or not the clinical goal is to rest or work the respiratory muscles.
  7. For inspiratory-to-expiratory (I:E) times, select inspiratory time (TI). (This parameter may be named differently depending on the ventilator manufacturer.) Adjust the flow as necessary to attain patient–ventilator synchrony.4
    1. Examples of flow adjustment names include percentage of TI, flow rate, and peak flow.
    2. A typical TI for an adult is less than 1 second.4
    3. Generally, flow rates of approximately 60 to 80 L/min are used initially and adjusted to provide a TI that synchronizes with the patient's effort.4
      Rationale: Inspiratory flow refers to the speed with which a V T is delivered during inspiration. Increasing the flow rate shortens the T I. Conversely, slowing the flow rate lengthens the T I. Adjusting the inspiratory flow achieves the desired I:E ratio and comfortable breathing patterns.
      A short T I and a longer expiratory time (T E) may be necessary in a patient with an obstructive lung disease (e.g., emphysema, asthma). In contrast, a patient with a restrictive disease such as ARDS has noncompliant lungs; longer T I enhances recruitment and prevents derecruitment in this patient.
  8. Set the trigger sensitivity between –0.5 and –1.5 cm H2O pressure.4
    Rationale: If the sensitivity is set too low, increased patient effort is necessary to initiate a ventilator breath. Dyssynchrony can result.
  9. Place the patient on 100% oxygen unless information is available that identifies a precise fraction of inspired oxygen (FIO2).4 Adjust the FIO2 downward, as tolerated, using SaO2 and arterial blood gas (ABG) values to guide level selection. Titrate the FIO2 to obtain a partial pressure of arterial oxygen (PaO2) of 60 to 80 mm Hg and an SpO2 or SaO2 of 90% or greater.4
    Rationale: Most patients in the acute care setting should be placed on 100% oxygen unless information is available identifying a precise F IO 2. 4 High levels of F IO 2 result in increased risk of oxygen toxicity, absorption atelectasis, and reduction of surfactant synthesis. By initiating PPV with maximum oxygen concentration, hypoxemia can be avoided while optimal ventilator settings are being determined and evaluated. This also permits measurement of the percentage of venous admixture (shunt), which provides an estimate of the severity of the gas-exchange abnormality.
  10. Select the PEEP level. In many cases, the initial setting is 5 cm H2O.4
    1. Adjust PEEP as needed after evaluation of tolerance (e.g., SaO2, PaO2, physical assessment).
    2. Increase PEEP levels to restore functional residual capacity (FRC) and allow reduction of FIO2 to safe levels (i.e., less than or equal to 0.5).4
      Rationale: A PEEP level of 5 cm H 2O is considered physiologic. 4 High levels of PEEP should rarely be interrupted because reestablishing FRC (and Pa O 2) may take hours, especially in a patient with ARDS.
  11. Ensure that all ventilator alarms are set appropriately (Table 2)Table 2.
  12. Provide circuit humidification.
    1. For conventional humidifiers, make sure the humidifier has adequate fluid (sterile distilled water) and that the thermostat setting is adjusted according to the manufacturer's recommendations.
    2. When using a humidifier, maintain the gas temperature between 34°C and 41°C (93.2°F and 105.8°F) at the circuit Y-piece with a relative humidity of 100%.1
      Rationale: Gases are generally humidified before entering the artificial airway.
      In a patient with thick or tenacious secretions, pay attention to the inspired temperature to prevent mucus plugging. In this situation, circuit temperature may need to be closer to body temperature.
    3. When using an HME, place the HME between the patient's airway and the ventilator circuit.
      Rationale: The moisture in warmed, exhaled gases passes through the vast surface area of the HME and condenses. With inspiration, dry gases pass through the HME and become humidified.
      1. Change the HME per the manufacturer's instruction. In many cases, an HME can be used for at least 48 hours; in some patients, it can be used for up to 1 week.1
        Rationale: The longer the HME is inline, the more efficient the humidification; however, inspiratory resistance increases over time. In weaning patients, the additional resistive load added by these humidifiers may preclude their use.
      2. Do not use an HME if secretions are copious or bloody.
        Rationale: Secretions may cause obstruction; an HME is contraindicated when secretions are copious or bloody.
  13. Place the capnography device and appropriate adapter in the ventilator circuit, if ordered, or per the organization's practice.
  14. Discard supplies, remove gloves, and perform hand hygiene.
  15. Document the procedure in the patient's record.

MONITORING AND CARE

  1. Check for secure stabilization and maintenance of the endotracheal (ET) tube. (Commercial ET tube holders are available.)
  2. Confirm ET tube placement, ideally by clinical assessment and continuous waveform capnography. If continuous waveform capnography is not available, use a nonwaveform exhaled carbon dioxide monitor.
  3. Monitor SpO2 continuously.
  4. Monitor the inline thermometer to maintain inspired gas temperature between 34°C and 41°C (93.2°F and 105.8°F).1
    Rationale: There is a risk of thermal injury from overheated inspired gas and risk of poor humidity from underheated inspired gas.
  5. Keep the ventilator tubing clear of condensation. Drain tubing away from the patient toward the expiratory limb.
    Rationale: Condensation in the tube that is drained toward the patient may cause a respiratory infection if the patient inhales contaminated water droplets.
  6. Ensure the availability of a self-inflating manual resuscitation bag (MRB) and an appropriate-size face mask attached to supplemental oxygen at the head of the bed. Attach or adjust the PEEP valve if the patient is on PEEP.
    Rationale: Ventilation and oxygen may be needed immediately to relieve acute respiratory distress caused by hypoxemia or acidosis.
  7. Check the ventilator settings on a routine basis to ensure that they match the prescribing order.
  8. Explore any change in peak inspiratory pressure (PIP) or decreased (sustained) VT on PSV. Immediately explore the cause of high-pressure alarms.
    Rationale: Acute changes in PIP or V T may indicate mechanical malfunction, such as tubing disconnection, cuff or connector leaks, tubing or airway kinks, or changes in resistance and compliance.
    Always consider the possibility of a tension pneumothorax if the patient has a shift in the trachea, decreased breath sounds on one side, and increased peak pressures. If a tension pneumothorax occurs, perform a needle thoracotomy.
  9. Place a bite block between the teeth if the patient is biting on the oral ET tube. If a bite block is unavailable, an oral airway may be used.
    Rationale: An oral airway serves the same purpose as a bite block.
    An oral airway may not be tolerated as well as the bite block because it may induce gagging.
  10. Change the patient's body position as often as possible. Maintain the head of the bed or backrest elevation at 30 to 45 degrees.4
    Rationale: Continuous lateral rotation therapy may be helpful in improving oxygenation. Elevation is one of the most modifiable factors related to VAP.
  11. Evaluate for patient–ventilator dyssynchrony.
    Rationale: Dyssynchrony occurs when the patient's intrinsic breaths oppose or challenge the ventilator and may occur because of patient fatigue or restlessness.
  12. Observe for hemodynamic changes associated with increased VT, PEEP, or decreased cardiac output.
    Rationale: Hemodynamic changes may indicate functional changes in circulating volume caused by positive intrathoracic pressure.
    Always consider the potential for pneumothorax with acute changes, such as a tracheal shift, decreased breath sounds, and increased PIP readings on the ventilator.
  13. Suction the patient, using the closed technique if possible, only when needed (i.e., not routinely).
  14. On an ongoing basis, monitor the patient for complications of mechanical ventilation, such as barotrauma, volutrauma, VAP, pneumothorax, or accidental extubation.
  15. Observe the patient for signs or symptoms of pain. If pain is suspected, report it to the authorized practitioner.

EXPECTED OUTCOMES

  • Maintenance of adequate pH, PaCO2, and PaO2
  • Maintenance of adequate breathing pattern
  • Respiratory muscle rest

UNEXPECTED OUTCOMES

  • Unacceptable pH, PaCO2, or PaO2
  • Hemodynamic instability
  • Pulmonary barotrauma or volutrauma
  • Inadvertent extubation
  • Malpositioned ET tube
  • Nosocomial lung infection
  • Respiratory muscle fatigue
  • Excessive condensation in ventilator circuit
  • Discrepancy between set and measured ventilator settings

DOCUMENTATION

  • Education
  • Completed ventilation system test (pass or fail), date, and initials or signature of respiratory therapist (RT) and credentials
  • Unique identifier of the ventilator and the operational verification
  • Indication for ventilatory assistance
  • Date and time ventilatory assistance was instituted
  • Ventilator settings
    • FIO2
    • Mode of ventilation
    • VT
    • Respiratory frequency (total and mandatory)
    • PEEP level
    • I:E ratio or TI
    • PIP
    • Dynamic lung compliance
    • Static lung compliance
  • ABG values
  • SaO2 readings
  • Patient's responses to PPV
  • Hemodynamic values
  • Vital signs
  • Unexpected outcomes and related interventions
  • Respiratory interventions
  • Tube location verification

OLDER ADULT CONSIDERATIONS

  • A patient who is eligible for invasive long-term care mechanical ventilation in the home requires a tracheotomy tube for ventilatory support but no longer requires intensive medical monitoring services.2

REFERENCES

  1. American Association for Respiratory Care (AARC), Restrepo, R.D., Walsh, B.K. (2012). Humidification during invasive and noninvasive mechanical ventilation: 2012. Respiratory Care, 57(5), 782-788. doi:10.4187/respcare.01766 (classic reference)* (Level VII)
  2. Ishigaki, Y. (2015). Home respiratory management: From COPD to neurological diseases. JMAJ: Japan Medical Association Journal, 58(1-2), 36-39.
  3. Kacmarek R.M. (2021). Chapter 47: Physiology of ventilatory support. In R.M. Kacmarek, J.K. Stoller, A.J. Heuer (Eds.), Egan’s fundamentals of respiratory care (12th ed., pp. 1013-1052). St. Louis: Elsevier.
  4. Kacmarek, R.M. (2021). Chapter 49: Initiating and adjusting invasive ventilatory support. In R.M. Kacmarek, J.K. Stoller, A.J. Heuer (Eds.), Egan’s fundamentals of respiratory care (12th ed., pp. 1072-1104). St. Louis: Elsevier.

ADDITIONAL READING

Gallagher, J.J. (2018). Alternative modes of mechanical ventilation. AACN Advanced Critical Care, 29(4), 396–404. doi:10.4037/aacnacc2018372

Weiss, C.H. and others. (2017). Summary for clinicians: Mechanical ventilation in adult patients with acute respiratory distress syndrome clinical practice guideline. Annals of the American Thoracic Society, 14(8), 1235–1238. doi:10.1513/AnnalsATS.201704-332CME

*In these skills, a "classic" reference is a widely cited, standard work of established excellence that significantly affects current practice and may also represent the foundational research for practice.

Elsevier Skills Levels of Evidence

  • Level I - Systematic review of all relevant randomized controlled trials
  • Level II - At least one well-designed randomized controlled trial
  • Level III - Well-designed controlled trials without randomization
  • Level IV - Well-designed case-controlled or cohort studies
  • Level V - Descriptive or qualitative studies
  • Level VI - Single descriptive or qualitative study
  • Level VII - Authority opinion or expert committee reports
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