Mechanical Ventilator Components and Common Ventilation Modes
Modern mechanical ventilators consist of microprocessor-controlled systems with electromechanical valves for gas flow regulation, extensive monitoring capabilities, and the ability to deliver multiple ventilation modes including pressure support 1.
Key Ventilator Components
Modern ventilators share several essential features that enable sophisticated respiratory support:
- Microprocessor control systems that regulate operational and monitoring functions
- Electromechanical valves (typically solenoid valves) that control and adjust gas flow patterns
- Feedback electronics that adjust inspiratory flow based on expiratory flow to meet preset volumes
- Extensive monitoring systems with alarms for pressure, flow, and volume parameters 1
Essential Monitoring and Alarm Systems
Ventilators must include disconnection alarms, pressure alarms (both high and low), and flow monitoring capabilities 2. The low pressure alarm detects disconnection or excessive leakage, while high pressure alarms warn of conditions that make the patient difficult to inflate. Flow alarms are particularly informative as they can detect changing leakage, worsening airflow obstruction, or partially occluded tubing 2.
Common Ventilation Modes
The terminology for ventilation modes can be confusing as manufacturers use different names for similar modes 2. Understanding modes requires focusing on the control variable (pressure vs. volume), breath sequence, and targeting schemes rather than proprietary names 3.
1. Controlled Mechanical Ventilation (CMV)
In CMV mode, full ventilatory support is provided with no patient effort required 2. The clinician sets:
- Inflation pressure OR tidal volume
- Respiratory frequency
- Timing of each breath
Pressure control: The resulting tidal volume depends on ventilator tubing resistance, airflow limitation, and lung/chest wall compliance.
Volume control: Tidal volume is preset and the resulting pressure is determined by circuit compliance and thoracic mechanics 2.
2. Assist/Control Ventilation (ACV)
ACV delivers a preset number of mandatory breaths per minute while permitting patient triggering 2. Key characteristics:
- A minimum respiratory rate is guaranteed in the absence of patient effort
- Patient-triggered breaths receive identical support to mandatory breaths
- A "lock out" period prevents breath stacking and excessive inflation
- Also called Synchronized Intermittent Mandatory Ventilation (SIMV) or Spontaneous/Timed (S/T) mode on NIV machines 2
Common pitfall: Setting a long expiratory time creates a long lock-out period that may lead to poor patient tolerance 2.
3. Assisted Spontaneous Breathing (Pressure Support)
In pressure support (PS) mode, the patient's respiratory effort triggers the ventilator both on and off 2. Critical features:
- Respiratory frequency and breath timing are entirely patient-determined
- Pressure is the controlled variable
- If the patient fails to make respiratory effort, no assistance occurs (though many manufacturers now incorporate a backup rate of 6-8 breaths/min) 2
This mode requires adequate patient respiratory drive and is inappropriate for patients with minimal respiratory effort.
4. Continuous Positive Airway Pressure (CPAP)
CPAP is employed primarily to correct hypoxemia, not to provide ventilatory support 2. Mechanisms of action:
- Permits higher inspired oxygen content than other supplementation methods
- Increases mean airway pressure
- Improves ventilation to collapsed lung areas (similar to PEEP in intubated patients)
- Unloads inspiratory muscles and reduces inspiratory work
- In COPD exacerbations, offsets intrinsic PEEP, potentially reducing ventilatory work and PaCO2 2
Important caveat: In hyperinflated patients with airflow obstruction, CPAP may adversely affect inspiratory muscle function by further increasing lung volume 2.
Flow requirements: Distressed patients with high minute ventilation and peak inspiratory flows exceeding 60 L/min require high-flow CPAP generators to prevent pressure drops 2.
Evidence-Based Ventilation Strategy for ARDS
For adult patients with ARDS, use low tidal volume (LTV) ventilation with tidal volumes of 4-8 mL/kg predicted body weight and plateau pressures <30 cm H2O 4. This recommendation is based on:
- Meta-analysis of 9 RCTs including 1,629 patients
- Trials with larger tidal volume gradients (greater difference between LTV and control) showed significantly lower mortality risk (P = 0.002)
- When combined with higher PEEP, LTV showed greater mortality benefit (RR 0.58; 95% CI 0.41-0.82) 4
Predicted body weight calculation:
- Males: 50 + 0.91 × [height (cm) - 152.4] kg
- Females: 45.5 + 0.91 × [height (cm) - 152.4] kg 4
Minimum Specifications for Hospital NIV Ventilators
Essential features 2:
- Pressure controlled capability
- Pressure capability ≥30 cm H2O
- Support inspiratory flows ≥60 L/min
- Assist-control AND bi-level pressure support modes
- Rate capability ≥40 breaths/min
- Sensitive flow triggers
- Disconnection alarm
Desirable features: Short/adjustable pressure rise time, adjustable inspiratory/expiratory triggers, adjustable I:E ratio in assist-control mode, internal battery (≥1 hour), accessible control panel with lock-out facility 2.
Practical recommendation: Use a single ventilator model (or minimal number of types) within each hospital for ease of staff training 2.