Mechanical ventilation is provided to ensure adequate oxygenation and minute ventilation. The means to achieve these goals will vary depending upon the lung pathophysiology. Indications to improve oxygenation include pneumonia, pulmonary edema, and acute respiratory distress syndrome. Clinical situations requiring maintenance of minute ventilation include postoperative patients, asthma, and bronchiolitis. Several modes of mechanical ventilation are available to optimize patient recovery by normalizing pulmonary gas exchange while minimizing lung disease.
There are various ways to deliver a tidal volume to the patient, but understanding the basic terminology helps to understand the benefits or pitfalls of a particular mode.
Breaths are either mandatory and triggered by the machine (control mode) or on demand and triggered by the patient (assist mode).
Variables determining the breath
Trigger: the variable starting the breath. In a spontaneously breathing patient, a change in flow or pressure with an effort can trigger a breath; if the patient becomes apneic, the machine can deliver the breath based on a timed interval.
Cycle: the variable stopping the breath. In a volume mode of ventilation when the volume is delivered, the inspiratory phase will stop—it is volume cycled. In a pressure mode, the time the pressure is maintained will cycle the breath off—it is time cycled. The pressure support mode is flow cycled.
Limit: the goal of the delivered gas flow, either volume or pressure.
The primary input is volume to be delivered from the ventilator. Airway and alveolar pressures generated as a result of the volume delivered will vary depending upon the resistance to flow and the compliance of the respiratory system.
SIMV: synchronized intermittent mandatory ventilation (flow/pressure initiated, volume limited, volume/time cycled)
The volume set can be delivered in synchrony with a patient effort, decreasing the work of breathing.
With apnea, the rate set and the volume delivered will result in the minute ventilation the patient receives.
Assist control (flow/pressure/time initiated, volume limited, volume/time cycled)
Volumes delivered are consistent and synchronized with patient effort.
If there is no patient effort, the rate set and volume delivered determine the minute ventilation.
In contrast to SIMV, every patient effort results in the entire delivered tidal volume with each breath.
IMV: intermittent mandatory ventilation (time initiated, volume limited, volume/time cycled)
Rarely used pediatric mode delivering a volume of breath regardless of patient effort.
Ventilator Modes
| Ventilator Mode | |||
| Variable | Volume Control | Pressure Control | PRVC |
| Tidal Volume | Constant | Variable | Constant |
| Peak Airway Pressure | Variable | Constant | Variable (based on compliance and resistance) |
| Peak Alveolar Pressure | Variable | Constant | Variable (based on compliance) |
| Flow Pattern | Decelerating or Constant | Decelerating | Decelerating |
| Peak Flow | Constant | Variable | Variable |
| Inspiratory Time | Measured value, based on peak flow and RR | Preset | Preset |
| Respiratory Rate | Preset | Preset | Preset |
Primary input is a pressure limit. Variable tidal volumes will result based on the respiratory system compliance and resistance.
Pressure control
SIMV-PC (flow/pressure/time initiated, pressure limited, time cycled) is a synchronized mode of ventilation delivering flows as in IMV-PC, but can be triggered by patient effort.
IMV-PC (time initiated, pressure limited, time cycled) is a control mode of ventilation in which gas flow is rapid at initiation of breath but decreases when the pressure limit is reached, resulting in a constant airway or alveolar pressure during the inspiratory cycle, in turn resulting in higher mean airway pressures than volume modes achieving similar peak inspiratory pressure.
Pressure support (flow/pressure initiated, pressure limited, flow cycled)
Flow from the ventilator will generate the pressure level set. Tidal volume will vary, and each respiratory effort will be supported when triggered. The pressure is maintained until the inspiratory flow decreases to preset levels designed by the ventilator software.
There is no backup respiratory rate; thus, the patient must have an adequate respiratory rate and drive to breathe. This mode is best used as a weaning method from support.
Pressure regulated volume control (flow/time initiated, pressure limited, time or flow cycled)
A mode combining volume ventilation and pressure ventilation
Minute ventilation is guaranteed with set tidal volume and rate
Decelerating inspiratory flows like pressure control apply consistent airway pressures, minimizing peak pressures based on the machine’s algorithm
Can be used in control or assist mode
Adequate minute ventilation is required to meet metabolic demands by having an appropriate respiratory rate and tidal volume (MV = RR × TV)
Rate
Respiratory rate is set initially for physiologic normative rate for the patient’s age; rates generally are higher for younger children. Inspiratory time (iT) defines the time allowed for gas to flow and is lower in infants and children than for adults; times vary from 0.4 to 1 second.
With higher rates, short iT is necessary to allow for exhalation time and avoiding breath stacking. When exhaled time is prolonged such as in acute asthma, the iT may again be shortened to allow complete exhalation.
Tidal volume
Volume mode
Volumes generally delivered are 6 to 10 mL/kg of body weight. Ideal body weight calculations may be necessary in very obese patients, as the tidal volumes used based on actual weight will result in overdistention of the lungs and high peak inspiratory pressure.
Peak inspiratory pressure and plateau pressure are monitored closely with this mode of ventilation, as they may increase greatly with decreased lung compliance.
High peak pressure (greater than 30 cm H2O) is associated with alveolar overdistention and lung damage. Illnesses such as ARDS and poor lung compliance frequently result in high peak pressure. Strategies for volume ventilation in this instance include using small tidal volumes, 4 to 6 mL/kg, and increased frequency to achieve the desired level of gas exchange.
Pressure mode
Pressure input limits peak pressures and generally will be limited to less than 30 cm H2O.
Tidal volumes in this mode are variable based on resistance to flow and compliance of the respiratory system. Pressure limits for neonates are lower than for children.
Limited pressures above the end-expiratory pressure are set from 12 to 30 centimeters of water.
Oxygenation goals are primarily met by adjusting inspired oxygen concentration and positive end-expiratory pressure (PEEP). Subtle adjustments in inspiratory flows will alter mean airway pressure—adjustments that may alter oxygenation.
Supplemental oxygen can be supplied from 21% to 100% oxygen. Many newer-generation ventilators can substitute gases other than nitrogen, such as helium, for therapeutic use while maintaining appropriate tidal volumes.
PEEP is maintained by flow through the ventilator circuit to keep alveolar pressures at set levels. With minimal lung disease, PEEP is set at 3 to 5 cm H2O. Increasing PEEP increases alveolar surface area, thereby increasing functional residual capacity and improving oxygenation. With ARDS, PEEP levels are higher and are not uncommonly 15 to 20 cm H2O. Higher levels of PEEP may lead to interstitial emphysema, pneumothorax, and pneumomediastinum, as well as alveolar overdistention and worsened perfusion and ventilation mismatch.
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