Ch 5: Selecting the Ventilator and the Mode Flashcards

Framework for Selecting a Ventilator and Mode

• Indication for Support: Clinicians must determine why the patient requires ventilatory support. This involves assessing the specific clinical indicators and triggers for mechanical ventilation.

• Pathology and Specialized Modes: The nature of the ventilatory problem or underlying disease process (pathology) determines if a specific mechanical ventilation mode is required to address the condition.

• Therapeutic and Treatment Goals: Selection is guided by the specific therapeutic outcomes the clinical team aims to achieve for the patient.

• Patient Interface: A decision must be made regarding whether the patient requires invasive ventilation (intubation) or can be managed with noninvasive interfaces, such as a mask.

• Location of Care: The setting where ventilatory support is provided influences equipment selection. Typical locations include:

  • Intensive Care Unit (ICU).

  • The patient's home.

  • Extended care facilities.

• Duration of Support: The anticipated length of time the patient will require support (brief versus long-term) is a critical factor in ventilator selection.

• Staff Training and Familiarity: The level of familiarity the medical staff has with specific ventilators or modes is a key practical consideration for patient safety and efficacy.

Noninvasive Positive Pressure Ventilation (NIV)

• Selecting Patient Interfaces: NIV is commonly administered using nasal masks or full-face masks.

• Methods of Delivery:

  • Continuous Positive Airway Pressure (CPAP): Provides a single level of continuous pressure.

  • Noninvasive Positive Pressure Ventilation (NIV): Often involves two levels of pressure support.

• Ventilator Types for NIV:

  • Pressure-triggered, pressure-limited, flow-cycled devices.

  • Critical-care ventilators adapted for noninvasive use.

Full and Partial Ventilatory Support

• Full Ventilatory Support (FVS):

  • The ventilator provides all the energy necessary to maintain effective alveolar ventilation.

  • Typically characterized by respiratory rates > 8\text{ breaths/min}.

  • The ventilator delivers an adequate Tidal Volume ($V_T$) for the patient using preset volume or pressure targets.

• Partial Ventilatory Support (PVS):

  • The patient participates in the work of breathing (WOB).

  • Typically characterized by respiratory rates < 6\text{ breaths/min}.

Components of Breath Delivery and Modes of Ventilation

• Variables Determining Breath Delivery:

  • Type of Breath: Categorized as Mandatory, Spontaneous, or Assisted.

  • Targeted Controlled Variable: Categorized as Volume or Pressure.

  • Timing of Breath Delivery: Includes Continuous Mandatory Ventilation (CMV), Intermittent Mandatory Ventilation (IMV), and Continuous Spontaneous Ventilation (CSV).

• Definitions of Breath Types:

  • Mandatory Breaths: The ventilator controls the timing, tidal volume, or inspiratory pressure.

  • Spontaneous Breaths: The patient controls the timing and the tidal volume. The delivery is based on patient demand and the patient’s lung characteristics (compliance and resistance).

  • Assisted Breaths: These exhibit characteristics of both mandatory and spontaneous breaths. All or part of the breath is generated by the ventilator.

Targeting Volume as the Controlled Variable

• Characteristics: The volume remains constant while the pressure varies based on the patient’s lung characteristics.

• Advantages:

  • Guarantees the delivery of a specific volume and the volume of expired gas.

  • Assists in maintaining a specific level of $\text{PaCO}_2$.

• Disadvantages and Considerations:

  • Issues become evident when the patient’s lung condition worsens.

  • Risk of generating high pressures to achieve volume targets.

  • Requires careful management of flow and sensitivity settings; inadequate settings can lead to patient-ventilator asynchrony.

Targeting Pressure as the Controlled Variable

• Characteristics: Pressure is established as the independent variable. Volume delivery fluctuates as the lung characteristics (compliance and resistance) change.

• Clinical Implications:

  • Considered a lung-protective strategy as it limits peak pressures.

  • May offer increased comfort for the spontaneously breathing patient.

• Advantages:

  • Allows the clinician to set a maximum pressure limit.

  • Reduces the risk of alveolar overdistention.

  • Utilizes a decelerating flow pattern which may aid gas distribution.

• Disadvantages:

  • The delivered $V_T$ is variable and depends on lung characteristics.

  • Clinicians may lack technical familiarity compared to volume control.

  • Tidal volume ($V_T$) and expired minute volume ($V_E$) decrease if the patient's lung condition deteriorates.

Timing of Breath Delivery and Clinical Modes

• Continuous Mandatory Ventilation (CMV):

  • Breaths are either time-triggered (by the machine) or patient-triggered.

  • Controlled Ventilation: A variation where the patient is "locked out" by making the ventilator insensitive to effort; this is rarely advisable.

  • Volume-Controlled CMV (VC-CMV): Also known as volume-targeted CMV. All breaths are mandatory. If the patient triggers a breath, it is defined as an assisted breath. Although designed to reduce WOB, studies indicate patients may still perform $33\%$ to $50\%$ (or more) of the work of inspiration.

  • Pressure-Controlled CMV (PC-CMV): Also called pressure-targeted CMV or Pressure-Controlled Ventilation (PCV). All breaths are time-triggered or patient-triggered, pressure-targeted, and time-cycled. The ventilator provides constant pressure during inspiration. The operator sets the inspiratory time, pressure level, and backup rate. The resulting $V_T$ depends on compliance, resistance, patient effort, and the set pressure. Decelerating ramp flow curves in this mode may improve gas distribution and allow variability in flow during spontaneous efforts.

• Intermittent Mandatory Ventilation (IMV):

  • Involves periodic mandatory (volume- or pressure-targeted) breaths delivered at set time intervals.

  • The patient can breathe spontaneously between these mandatory machine breaths at the baseline pressure.

  • Spontaneous breaths do not trigger a full mandatory breath.

  • Most modern ventilators can provide Pressure Support for these spontaneous breaths.

• Continuous Spontaneous Ventilation (CSV):

  • All breaths are spontaneous and patient-triggered.

  • Spontaneous Breathing: Patients breathe through the ventilator circuit without mandatory breaths (similar to a T-piece).

  • Continuous Positive Airway Pressure (CPAP): Used to improve oxygenation in patients suffering from refractory hypoxemia and a low Functional Residual Capacity (FRC).

  • Pressure Support Ventilation (PSV): The ventilator provides a constant pressure during inspiration once an inspiratory effort is sensed. It requires the delivery of appropriate flow at the start of inspiration.

Advanced Settings and Specialized Modes

• Pressure Support Settings: Modern ICU ventilators allow adjustments to the slope of pressure and flow curves via variables such as:

  • Flow acceleration percent.

  • Inspiratory rise time / Inspiratory rise time percent.

  • Slope adjustment.

  • Inspiratory cycle percent / Inspiratory flow termination / Expiratory flow sensitivity.

• Bilevel Positive Airway Pressure (Bilevel PAP): Also referred to as biphasic positive airway pressure or bilevel pressure assist. This is a form of pressure ventilation commonly used in noninvasive settings.

• Pressure Augmentation (PAug): Features pressure-limited ventilation with a guaranteed volume delivery target for every breath. Also known as Volume-Assured Pressure Support (VAPS).

• Pressure-Regulated Volume Control (PRVC): A volume-targeted, pressure-controlled breath type where the ventilator adapts the pressure to achieve a set volume.

• Volume-Support Ventilation (VSV): Functionally similar to pressure support but includes a volume target.

• Mandatory Minute Ventilation (MMV):

  • Also known as minimum minute ventilation or augmented minute ventilation.

  • Primary use is for weaning patients from the ventilator.

  • The ventilator ensures a minimum minute ventilation by increasing either the breathing rate or the preset pressure if the patient's own effort is insufficient.

• Adaptive Support Ventilation (ASV):

  • A variation of MMV where the clinician sets a target based on the patient’s ideal body weight and estimated dead space volume.

  • The ventilator automatically selects the optimum $V_T$ and respiratory rate based on changes in the patient's lung mechanics.

• Airway Pressure-Release Ventilation (APRV):

  • Provides two levels of CPAP and allows spontaneous breathing at both levels.

  • The high CPAP level is briefly interrupted to allow pressures to drop (release).

  • Expiratory flow is generally not allowed to return to baseline zero, intentionally creating auto-PEEP (Positive End-Expiratory Pressure).

• Proportional Assist Ventilation (PAV):

  • Pressure, flow, and volume delivery are directly proportional to the patient’s spontaneous effort.

  • Ventilator output depends on patient-demanded inspiratory flow/volume and the degree of clinician