What are the ventilation modes on Hamilton mobile ventilators and how does each mode function?

Ventilation Modes on Hamilton Mobile Ventilators

Hamilton mobile ventilators offer several core ventilation modes that can be understood through a framework of control type and patient interaction, with the most commonly used being pressure-targeted modes that allow for patient-triggered breaths and leak compensation.

Core Ventilation Modes

Controlled Mechanical Ventilation (CMV)

This mode provides full ventilatory support with no patient effort required 1. The clinician sets either:

• Pressure control: Sets inspiratory pressure; resulting tidal volume depends on airway resistance, lung compliance, and chest wall mechanics
• Volume control: Sets tidal volume; the ventilator generates whatever pressure is needed to deliver that volume

The respiratory rate and timing of each breath are completely machine-determined. This mode is rarely used in non-invasive settings but may be appropriate for heavily sedated or paralyzed patients.

Assist/Control Ventilation (ACV) / Spontaneous-Timed (S/T) Mode

This mode delivers a preset minimum number of mandatory breaths per minute while allowing patient triggering 1. Key features:

• If the patient triggers a breath, the ventilator delivers an identical breath to the mandatory breaths
• A "lock-out" period prevents breath stacking and excessive inflation
• Critical pitfall: Setting a long expiratory time creates a long lock-out period, which can lead to poor patient tolerance 1
• Triggered breaths delay the next machine-determined breath, creating synchronization (SIMV)
• On Hamilton NIV machines, this is often labeled as S/T or IE mode 1

Pressure Support (PS) / Assisted Spontaneous Breathing (ASB)

The patient's respiratory effort triggers the ventilator both on and off, with the patient controlling respiratory frequency and timing 1. This is the most commonly used mode for non-invasive ventilation:

• The clinician sets the inspiratory pressure level
• Critical limitation: If the patient fails to make respiratory effort, no assistance occurs
• Most manufacturers now incorporate a backup rate of 6-8 breaths per minute to prevent apnea 1
• This mode maximizes patient comfort and synchrony

Continuous Positive Airway Pressure (CPAP)

CPAP maintains constant positive pressure throughout the respiratory cycle and is primarily used to correct hypoxemia, not to provide ventilatory support 1:

• Increases mean airway pressure and recruits collapsed lung units (similar to PEEP)
• Permits higher inspired oxygen content than standard oxygen delivery
• Unloads inspiratory muscles and reduces work of breathing
• In COPD exacerbations, offsets intrinsic PEEP, potentially reducing respiratory rate and PaCO2
• Important consideration: In hyperinflated patients with airflow obstruction, CPAP may worsen inspiratory muscle function by further increasing lung volume 1

Bi-Level Pressure Support

This combines pressure support with CPAP, using two pressure levels 1:

• IPAP (Inspiratory Positive Airway Pressure): The higher pressure during inspiration
• EPAP (Expiratory Positive Airway Pressure): The lower pressure during expiration
• The difference between IPAP and EPAP provides the ventilatory assistance
• EPAP recruits underventilated lung, offsets intrinsic PEEP (improving triggering), and vents exhaled gas through the exhaust port
• This is the most commonly used configuration for acute non-invasive ventilation

Advanced Modes (Hamilton-Specific)

Adaptive Support Ventilation (ASV) and INTELLiVENT-ASV are proprietary Hamilton modes:

• These are closed-loop ventilation modes that automatically adjust settings to target the lowest work and force of breathing 2
• Recent evidence shows INTELLiVENT-ASV reduces transpulmonary driving pressure in moderate-to-severe ARDS compared to conventional pressure control 2
• The mode continuously adapts tidal volume, respiratory rate, and pressure based on patient mechanics

Practical Considerations for Mode Selection

Pressure-targeted ventilation is preferred for non-invasive ventilation because 3:

• Constant pressure delivery avoids uncomfortable pressure spikes seen with volume control
• Compensates for air leak (inevitable with non-invasive interfaces)
• Positive expiratory pressure flushes CO2 from the mask and aids triggering

Common pitfall: Terminology varies significantly between manufacturers—what Hamilton calls one mode may have a different name on other ventilators 1, 4. Always verify the actual function (control variable, breath sequence, targeting scheme) rather than relying solely on the mode name.

For acute hypercapnic respiratory failure, start with bi-level pressure support (S/T mode with backup rate) to ensure adequate minute ventilation even if patient effort diminishes 3. For pure hypoxemic failure without ventilatory compromise, CPAP may suffice initially 1.