Mechanical Ventilation Basics

Basic Mechanical Ventilation

• Presented by TCMC Pulmonary Services.
• By Kevin McQueen, RCP, RRT.

Objectives

• Review of basic respiratory cycles.
• Discussion of negative pressure breathing vs. positive pressure breathing.
• Definition of basic terminology of mechanical ventilation.
• Inspiratory to Expiratory ratio (I:E).
• Comparison of the different types of positive pressure ventilation.
• Review of the different modes of mechanical ventilation.
• Review of traditional vs. lung protective approaches to mechanical ventilation.
• Review of troubleshooting of ventilator alarms.

Definitions

• Tidal Volumes (VT) is the volume of gas moved into and out of the lungs in a single breath; normal is approximately 500ml.
• Expired minute ventilation (VE) is the volume of gas moved out of the lungs per minute; normal (ideal) adult male is 5-6 LPM.
• Frequency or Rate is the number of ventilator cycles or breaths per minute.
• The I:E Ratio expresses the relationship between inspiratory and expiratory time.
  • For example, if inspiratory time is one second and expiratory time is three seconds, the I:E ratio is 1:3.
• I:E needs monitoring because of its adverse effects on the cardiovascular system.
• Compliance represents the relationship of change in volume and change in pressure.
• Normal compliant lungs have volume that will increase or change with minimal change in pressure.
• Non-compliant lungs require a significant change in pressure for a normal volume change.

Objectives of Mechanical Ventilation

• Physiological objectives include:
  • Support or manipulation of pulmonary gas exchange, including alveolar ventilation and oxygenation.
  • Increase lung volume to prevent or treat atelectasis.
  • Reduce the work of breathing.
• Clinical objectives include:
  • Reverse acute respiratory failure and Reverse respiratory distress.
  • Reverse hypoxemia, prevent or treat atelectasis, and reverse ventilatory muscle fatigue.
  • Permit sedation and/or paralysis.
  • Reduce oxygen consumption and intracranial pressure.
  • Stabilize the chest wall.

Negative vs. Positive Pressure Breathing

• Humans naturally breathe via negative pressure, drawing air into the lungs.
• Positive pressure works oppositely, forcing air into the lungs.

Types of Positive Pressure Ventilation

• Ventilators are universally classified by the physical parameter that ends the inspiratory cycle.
• Types:
  • Volume cycled ventilation
  • Pressure cycled ventilation
  • Time cycled ventilation

Controversy: Volume vs. Pressure

• Volume-limited ventilation advantages: known tidal volume delivery, but it may cause high peak pressure.
• Pressure-limited ventilation advantages: less risk of excessive peak pressures, but it may fluctuate Tidal Volume and Minute Ventilation due to impedance changes.

Volume Ventilation

• A volume cycled ventilator ends the inspiratory breath cycle upon delivering a preset volume.
• Volume cycled ventilators achieve a preset tidal volume with a pressure limit alarm setting to inform of possible changes in lung compliance or airway resistance.

Pressure Ventilation

• A pressure cycled ventilator ends the inspiratory breath cycle when a preset pressure is reached.

Modes of Ventilation

• CMV.
• Assist/Control.
• SIMV.

CMV (Control Mode Ventilation/Continuous Mandatory Ventilation)

• Assumes a ventilator and circuitry insensitive to patient effort or response.
• It's characterized by predetermined frequency and depth of VT, with patients unable to alter or influence any portion of the ventilatory cycle.
• Clients are usually non-responsive or sedated to reduce spontaneous breaths.
• This mode is not found on many modern ICU Ventilators.

A/C (Assist Control)

• Time or patient triggered.
• Characterized by a minimal rate or frequency, with volume or pressure set.
• Triggering occurs because the ventilator is sensitive to sub-baseline pressure or flow changes.
• The time intervals between assist breaths will vary as the patient varies the intervals between breaths.
• Volume controlled assist/control ventilation may be associated with significant work of breathing, especially if the trigger threshold is high.
• It is important to check for correct sensitivity levels if the patient appears to be breathing hard.

SIMV (Synchronized Intermittent Mandatory Ventilation)

• Periodic volume or pressure breaths occur at preset time intervals.
• Between mechanical breaths, patients spontaneously breathe without getting a preset mandatory breath.
• Patients breathe from continuous flow or a demand valve.
• Each mandatory breath is synchronized.

Adjuncts to Ventilation

• SPONT/CPAP/PEEP.
• Pressure Support Ventilation.

CPAP/SPONT MODE (Continuous Positive Airway Pressure/Spontaneous)

• All breaths are controlled by the patient; no mechanical breaths are provided unless the patient becomes apneic.
• CPAP and PEEP techniques apply airway pressure to the spontaneously breathing patient.
• The entire breathing cycle may have positive airway pressure, or just at the end of the breath.

PSV (Pressure Support Ventilation or Inspiratory Support)

• Airway resistance may be a significant factor causing difficulty during the weaning phase.
• PSV provides a preset level of positive pressure above PEEP during the inspiratory cycle of the spontaneous inspiratory effort.
• It is similar to an IPPB breath.

Improving Oxygenation

• FiO2 (higher levels of 02 may cause side effects).
• PEEP (positive end expiratory pressure) establishes and maintains preset airway pressures greater than ambient at end of exhalation.
• PEEP increases functional residual capacity (FRC).
  • This increase in FRC either occurs by increasing alveolar volumes or by recruiting previously collapsed alveoli.

Troubleshooting

• Assessment should be done by determining how severe the problem is, starting from the patient and moving outward toward the ventilator.
• Determine if the patient requires immediate resuscitation or other intervention
• If in doubt, remove the patient from the ventilator and manually ventilate.
• Check:
  • That the chest is moving and moving symmetrically.
  • If the patient is cyanotic, and monitoring their oxygen saturation.
  • If the patient is hemodynamically stable.
• The next step is to diagnose the problem.
• Differentiate vent/circuit issues from endotracheal tube/patient issues by taking the patient off the ventilator and manually bagging with 100% oxygen.
• Monitor mean airway pressure (Paw) and end-inspiratory pressure.
  • If both are high, the problem is likely due to a decrease in compliance (e.g., pneumothorax).
  • If only airway pressure is high, consider increased resistance (e.g., partially blocked ETT, bronchospasm).

Troubleshooting/Alarms

• High pressure causes include coughing, airway plugs, excessive secretions, changes in patient position, pneumothorax, incorrect ETT position, ETT kinked, patient biting, kinked ventilator circuit, or excessive water in tubing.
  • Interventions include clearing secretions, repositioning the patient, assessing breath sounds and chest movement, and checking ETT placement.
• A low-pressure alarm is triggered by a sudden decrease in peak airway pressure, potentially due to disconnection from the ventilator or a leak in the circuit.
  • The intervention is to check for disconnect and evaluate/tighten circuit connections.
• Gradual low-pressure decreases:
  • If the alarm needs to be adjusted, it may indicate an improvement/decrease in airway resistance or an increase in lung compliance.
• Desaturation causes include endobronchial intubation, accidental extubation, pneumothorax, pulmonary embolus, increased intrapulmonary shunt, respiratory failure, or ventilator malfunction.
  • Management includes increasing FiO2 (100%), checking chest movement, manually ventilating if needed, treating the underlying cause, altering ventilator settings, and calling for a CXR.