Mechanical Ventilation Alarms and Complications

Apnea Alarm

• Triggered by cessation of breathing (apnea) or circuit disconnection.
• Set with a 15- to 20-second delay to avoid false alarms.
• Potential causes include circuit disconnection, airway obstruction, and increased airway resistance.

High Frequency Alarm

• Triggered when respiratory frequency exceeds set limit, indicating potential respiratory distress.
• Set 10 breaths per minute above observed frequency.
• Consult "Management of Mechanical Ventilation" chapter for detailed troubleshooting.

High and Low F₂O₂ Alarms

• High F₂O₂ alarm set 5% to 10% above analyzed F₂O₂ value.
• Low F₂O₂ alarm set 5% to 10% below analyzed F₂O₂ value.

Hazards and Complications of Mechanical Ventilation

• Increased risk of complications with prolonged ventilation.
• Common complications include ventilator disconnections, infections, and other complications.

Apnea Alarm

• Triggered by a cessation of breathing (apnea) or circuit disconnection
• Set with a 15- to 20-second delay to account for variations in ventilator triggers
• Troubleshooting: Check for circuit disconnection, water in the circuit, kinking of the endotracheal tube, airway secretions, bronchospasm, mucus plugs, tension pneumothorax, and increased airway resistance

High Frequency Alarm

• Triggered when respiratory frequency exceeds 10 breaths per minute above the observed frequency
• Set 10 breaths per minute above the observed frequency
• Represents potential respiratory distress

High and Low F₂O₂ Alarms

• High F₂O₂ Alarm: Set 5% to 10% above the analyzed F₂O₂
• Low F₂O₂ Alarm: Set 5% to 10% below the analyzed F₂O₂

Hazards and Complications of Mechanical Ventilation

• Potential hazards include disconnections, infections, and complications
• The frequency of these hazards is directly related to the duration of mechanical ventilation
• Longer ventilation periods increase the risk of complications

General Information

• Ventilator alarms are essential to alert medical professionals to issues with the ventilator or patient.
• Although ventilator alarm systems vary, common alarms include:
  • Low exhaled volume alarm
  • Low inspiratory pressure alarm
  • High inspiratory pressure alarm
  • Apnea alarm
  • High frequency alarm
  • F₁O₂ alarm
• These alarms should have a backup battery source in case of electrical failure.

Low Exhaled Volume Alarm

• The low exhaled volume alarm is triggered when the patient fails to exhale the required tidal volume.
• It is set at 100 mL below the normal tidal volume.
• This alarm typically detects system leaks or circuit disconnections.

Low Inspiratory Pressure Alarm

• The low inspiratory pressure alarm is triggered when the peak inspiratory pressure falls below the set limit.
• It is set 10 to 15 cm H₂O below the normal peak inspiratory pressure.
• This alarm complements the low exhaled volume alarm and also detects system leaks or circuit disconnections.

High Inspiratory Pressure Alarm

• The high inspiratory pressure alarm is triggered when the peak inspiratory pressure reaches or exceeds the set limit.
• It is set 10 to 15 cm H₂O above the normal peak inspiratory pressure.

Tidal Volume Changes

• Increased tidal volume increases inspiratory time (I Time), decreases expiratory time (E Time), and decreases I:E Ratio.
• Decreased tidal volume decreases I Time, increases E Time, and increases I:E Ratio.

Frequency Changes

• Increased frequency (fff) results in a minimal change to inspiratory time (I Time), decreases expiratory time (E Time), and decreases I:E Ratio.
• Decreased frequency (fff) results in a minimal change to I Time, increases E Time, and increases I:E Ratio.

Initial FiO2

• Patients with severe hypoxemia or abnormal cardiopulmonary function may require an initial FiO2 of 100%.
• This may be necessary in cases like post-resuscitation, smoke inhalation, and ARDS.
• The FiO2 should be evaluated using arterial blood gas analyses.

Adjusting FiO2

• After the patient stabilizes, adjust the FiO2 to maintain an appropriate PaO2.

Maintaining FiO2

• Once stable, maintaining an FiO2 of 50% is recommended.
• This helps to avoid oxygen-induced lung injuries.

Tidal Volume

• Initial Tidal Volume: Typically set between 10 and 12 mL/kg of predicted body weight.
• Predicted Body Weight: Can be used for selecting tidal volume, unless the patient is significantly underweight or overweight.
• Lower Tidal Volume: Might be appropriate for certain patients, especially those with ARDS.
• ARDS: Tidal volumes as low as 6 mL per kg of predicted body weight have been recommended.
• Benefits of Lower Tidal Volume: Minimizes airway pressures and the risk of barotrauma.
• Risks of Lower Tidal Volume: Hypercapnia, deadspace ventilation, increased work of breathing, dyspnea, severe acidosis, and atelectasis.
• COPD Patients: Benefit from reduced tidal volume settings due to reduced expiratory flow rates.
• COPD Tidal Volume Recommendation: Decreasing the tidal volume by 100–200 mL helps prevent air trapping and improve exhalation.
• Lung Resection Patients: May require lower tidal volumes due to reduced lung volumes.

Gas Leakage and Circuit Compressible Volume

• Actual Tidal Volume: Usually lower than the set tidal volume due to gas leakage and circuit compressible volume.
• Gas Leakage: Can occur in the ventilator circuitry, cuff, and circuit compressible volume loss.

Initial Frequency

• The initial ventilator frequency is typically set between 10 and 12 breaths per minute (bpm) to achieve a normal carbon dioxide level (PaCO2) in the blood.
• This frequency, along with a typical tidal volume (amount of air per breath) of 10 to 12 mL/kg, usually provides sufficient air flow to normalize PaCO2.
• Frequencies above 20 bpm can lead to unintended positive end-expiratory pressure (PEEP) and should be avoided.

Alternative Method to Determine Initial Frequency

• You can calculate the frequency by dividing estimated minute volume by tidal volume.
• Formula: Frequency = Estimated minute volume / Tidal volume
  • Estimated Minute Volume:
    • Males: 4.0 * BSA (body surface area)
    • Females: 3.5 * BSA (body surface area)
• Body Surface Area (BSA) can be found using a nomogram (e.g., Dubois body surface area chart).

Adjusting Frequency

• The initial frequency of 10-12 bpm assumes normal carbon dioxide production and physiological dead space.
• If carbon dioxide production is high or dead space is increased, minute volume needs to be adjusted to maintain a normal PaCO2.
• Increase frequency is usually more appropriate than increasing tidal volume to avoid high airway pressure.

Blood Gas Monitoring

• Blood gases should be checked 15-30 minutes after stabilizing on the ventilator.
• Higher than normal PaCO2 (e.g., >45 mm Hg or >50 mm Hg for patients with chronic CO2 retention) indicates the need for an increase in minute volume, typically by increasing frequency.
• Lower than normal PaCO2 (e.g., <35 mm Hg) indicates a potential overventilation, and a decrease in minute volume, typically by decreasing frequency, may be necessary.

Mechanical Breath Variables

• Control variable: Determines how a breath is delivered. Examples include pressure-controlled or volume-controlled ventilation.
• Trigger variable: Initiates inspiration. Can be triggered by patient pressure or flow, or by the ventilator using a time trigger.
• Cycle variable: Ends inspiration. Can be volume-cycled, pressure-cycled, flow-cycled, or time-cycled.

Breath Sequence

• Continuous mandatory: All breaths are controlled by the ventilator, with no spontaneous breaths permitted. An example is CMV.
• Intermittent mandatory: The ventilator provides a set number of mandatory breaths. Spontaneous breaths are allowed between mandatory breaths, as in SIMV.
• Continuous spontaneous: All breaths are spontaneous, either with assistance (pressure support ventilation (PSV)) or without assistance (continuous positive airway pressure (CPAP)).

Control or Target Scheme

• Set point: The ventilator's target for achieving a desired goal. For example, the set point for pressure-controlled ventilation is pressure.
• Servo: The ventilator adjusts its output based on patient-specific variables. For example, proportional assist ventilation adjusts its pressure to create appropriate flow to meet a patient's flow needs.
• Adaptive: The ventilator modifies the set point to reach a different target. For example, pressure-regulated volume control adjusts pressure by altering flow and inspiratory time to reach a targeted volume.
• Optimal: The ventilator employs a mathematical model to adjust set points to achieve a specific goal. Adaptive support ventilation alters frequency, tidal volume, and pressure to attain a desired minute ventilation.

Ventilation Modes

• Intermittent mandatory ventilation (IMV): A set number of breaths are provided by the ventilator, allowing for spontaneous breaths between.
• Synchronized intermittent mandatory ventilation (SIMV): Similar to IMV, but synchronized with the patient's breathing pattern.
• Mandatory minute ventilation (MMV): The ventilator provides a set number of breaths per minute, regardless of the patient's breathing pattern.
• Pressure support ventilation (PSV): The ventilator assists spontaneous breaths by providing pressure support.

Operating Modes of Mechanical Ventilation

• Ventilator modes define the operating characteristics of a ventilator
• Modes describe how inspiration and expiration are triggered and cycled, limits placed on variables during inspiration, and breath spontaneity
• Modern ventilators also control FiO2, inspiratory flow rate, and have various alarms
• Regardless of the operating mode, four main goals should always be considered:
  • Adequate ventilation and oxygenation
  • Avoiding ventilator-induced lung injury
  • Patient-ventilator synchrony
  • Successful weaning from mechanical ventilation
• There are over 23 different ventilation modes available on various ventilators
• Modes can be combined to achieve specific effects
• Spontaneous: Patient breathes independently
• Positive end-expiratory pressure (PEEP): Adds pressure at the end of exhalation
• Continuous positive airway pressure (CPAP): Maintains positive pressure throughout respiration
• Bilevel positive airway pressure (BiPAP): Two different pressures are applied for inhalation and exhalation
• Controlled mandatory ventilation (CMV): Delivers preset tidal volume and frequency
• Assist/control (AC): Supports patient breathing with preset parameters, assisting when insufficient

Airway Pressure Release Ventilation (APRV)

• APRV is a form of continuous positive airway pressure (CPAP) with two distinct pressure levels.
• APRV maintains spontaneous breathing throughout the ventilatory cycle.
• APRV is time-triggered, pressure-limited, and time-cycled.

APRV Key Features

• Time-triggered: The high and low pressures, and the inspiratory times for each pressure level are set by the clinician.
• Pressure-limited: The higher pressure helps keep the alveoli inflated and enhances recruitment.
• Time-cycled: The time interval at the higher pressure (Thigh) is longer than the time spent at the lower pressure (Tlow).
• Pressure release: The release of pressure from the higher to lower pressure setting helps remove CO2.
• Patient triggering: Most ventilators allow patient triggering of a breath (either pressure or flow).

APRV: Clinical Significance

• Inflated alveoli: The initial setting of higher pressure maintains inflated alveoli and enhances recruitment.
• CO2 removal: The release of pressure between a higher to lower pressure setting helps in the removal of CO2, as the lower pressure interval is established using the time triggering method.
• Mean airway pressure: The higher pressure is typically set above the lower inflection point of the lung's pressure-volume curve, close to the mean airway pressure during pressure-controlled ventilation.
• Spontaneous breaths: The patient may or may not receive pressure support while in the spontaneous breathing portion, depending on ventilator settings.

APRV: Additional Features

• Some manufacturers offer pressure support during the spontaneous portion at the higher CPAP level.

Automode

• Combines pressure and volume support into a single mode.
• Delivers mandatory breaths in a time-triggered, pressure-limited, and time-cycled mode when there is no spontaneous effort.
• Switches to volume-support ventilation (VSV) with patient-triggered breaths when spontaneous effort is present.

Proportional Assist Ventilation (PAV)

• The ventilator proportionally assists the patient's spontaneous ventilation by amplifying the delivered pressure in proportion to the measured inspiratory flow and volume.

Automatic Tube Compensation

• Automatically compensates for the resistance of the endotracheal tube, adjusting pressure based on tube size and type.
• Eliminates airway resistance.
• Active during inspiration and expiration.
• Reduces air trapping and intrinsic PEEP.

Additional details

• Clinician sets volume targets, PEEP, and pressure limits.
• The next breath's inspiratory pressure increases if the tidal volume falls below the target.

Dual Control within-a-Breath

• Ventilator switches from pressure-controlled to volume-controlled during inspiration
• Starts as a pressure controller, delivering a breath with a pre-set tidal volume target
• Ventilator measures the delivered tidal volume and adjusts the pressure
• Includes methods for augmentation and volume-assured pressure support

Dual Control Breath-to-Breath

• Clinician sets a volume target for each breath
• Ventilator delivers pressure-controlled breaths to achieve the desired tidal volume
• Operates in pressure support or pressure-controlled mode, with adjustments to the pressure limit

Pressure-Limited Time-Cycled Breaths

• Begin with pressure-limited inspiration (pressure increases to a target value)
• Inspiration ends after a specified time
• Clinician sets a target tidal volume and a maximum pressure
• Ventilator delivers a sample breath, calculates airway resistance, and lung compliance
• Pressure is automatically adjusted in increments of 1 to 3 cm H₂O to reach the desired volume or until the maximum pressure is reached
• If the target volume isn't met, an alarm alerts the clinician
• Examples include volume control plus (VC+) and pressure-regulated volume control (PRVC)

Pressure-Limited Flow-Cycled Breaths

• Start as pressure-support breaths with a target tidal volume
• Inspiration ends when the inspiratory flow falls to a set level
• Inspiratory flow rate is tracked

Intermittent Mandatory Ventilation (IMV)

• IMV allows patients to breathe spontaneously between time-triggered ventilator breaths.
• Patients can breathe at the same FO2 and baseline pressure, without needing to trigger a mandatory breath.
• Spontaneous breaths can be augmented with pressure support to increase tidal volume and reduce inspiratory work.

Pressure Support

• Pressure support augments spontaneous breathing effort with positive pressure.
• Patients trigger each breath, and a preset pressure is delivered until flow reaches 10% to 40% of peak inspiratory flow.
• Expiration begins with variable flow increasing to maintain desired pressure support.
• Figure 3-14: Pressure-time scalar graph for pressure-controlled mode with labels for inspiration, end-expiration/inspiration, pressure plateau, end-inspiration/expiration, end-expiration, inspiratory time, expiratory time, and total cycle time.
• Figure 3-15: Scalar presentation of IMV with mechanical and spontaneous breath displays.

Patient Case Notes

• Patient is diagnosed with pneumonia in the lower lobe.
• Patient is experiencing hypoxia.
• Patient's positioning is not specified.

Mechanical Ventilation Modes

• A mechanical ventilation mode consists of: a specific breathing pattern, control type and operational algorithms.
• Modern ventilators utilize microprocessors for control, increasing available modes and complexity.
• Understanding ventilation modes is crucial for matching patient need and ventilation.

Volume-Controlled Ventilation

• Clinicians set the breath volume in volume-controlled ventilation.
• Pressure varies depending on patient lung compliance and airway resistance.
• Delivered volume remains constant despite changes in patient condition.
• Volume control allows control over both tidal volume and minute ventilation.

Pressure-Controlled Ventilation

• Clinicians set the peak inspiratory pressure in pressure-controlled ventilation.
• Volume and minute ventilation vary depending on patient lung compliance and airway resistance.
• Pressure remains constant despite changes in patient condition.
• Volume and minute ventilation decrease if lung compliance worsens or airway resistance increases.
• Pressure control protects the lungs from excessive pressure, reducing risk of ventilator-induced lung injury (VILI).