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Modes of ventilation

Up Dysynchrony e-lectures Figure 5a & 5b Modes of ventilation Non-invasive ventilation Physiological effects Specific ventilators


Assist control
BIPAP

Nomenclature

The nomenclature used on different ventilators is often confusing. It is, therefore, important to consult the appropriate instruction manual to determine what a particular mode actually does. Where possible we have used generic names for modes.

Control mode ventilation

- time preset pattern of inspiratory flow that delivers a set tidal volume
- when time preset pattern of inspiratory pressure is delivered = PCV
- patient initiated breaths not possible
- this mode is not found on modern ICU ventilators

Synchronized intermittent mandatory ventilation

- patient can breathe spontaneously but also receives a set number of mechanical breaths
- ventilator assisted breaths are synchronized with the patients' breathing to prevent the possibility of a mechanical breath on top of a spontaneous breath (see fig 5a & 5b)
- advantages over CMV thought to be: improved weaning, ventilator synchrony and intrapulmonary gas distribution; minimization of sedation and prevention of respiratory muscle atrophy
- no proven advantage over T-piece trials in weaning
- SIMV breaths often fails to unload respiratory muscles and impose same work as intercurrent spontaneous breaths. ie at the same overall level of ventilator support, effort is more or less independent of whether the breath is assisted or not. This is probably due to the inability of the respiratory system to adjust inspiratory effort on a breath to breath basis.
– by varying the difference between the preset rate and the SIMV rate it is possible to manipulate the I:E ratio in apnoeic patients. This is because the cycle time and thus the absolute inspiratory time is determined by the preset rate not the SIMV rate. This feature of SIMV may be useful in patients with severe asthma/COPD.

Inverse ratio ventilation

- increases mean airway pressure and therefore oxygenation by prolonging inspiratory time
- usually pressure-controlled but can be volume controlled
- in pressure-controlled mode allows increased mean airway pressures without increased peak pressures, theoretically improving oxygenation without increasing the incidence of barotrauma. However reduced expiratory time predisposes to inadequate emptying of lung regions with long expiratory time constants resulting in air trapping and intrinsic PEEP. This may lead to overdistension and rupture of alveoli. 26% incidence of barotrauma has been reported with PC-IRV which is similar to incidence of barotrauma reported with CMV
- like extrinsic PEEP, intrinsic PEEP increases alveolar recruitment and thus improves oxygenation. However, can decrease delivered volumes in pressure-limited ventilation and can increase peak airway pressure in volume-limited ventilation. Decrease in delivered volumes may be compensated for by an improvement in ventilation-perfusion matching and thus decreased dead space
- exact mechanism by which IRV improves oxygenation not clear
- although widely used there is little data to support its superiority over other modes of ventilation. Primarily used in acute lung injury when toxic levels of inspired oxygen required using conventional modes of ventilation
- "unnatural" respiratory timing necessitates sedation
- greater haemodynamic effect due to increased mean intrathoracic pressure

Airway pressure release ventilation

- like a variable level of CPAP
- upper level of airway pressure chosen to provide a mean airway pressure that prevents hypoxia
– CO2 elimination occurs by:

  • spontaneous patient ventilation
  • intermittent reduction of airway pressure (to a lower or zero level of CPAP) allowing for gas release from the lungs

- total ventilation is a function of effectiveness of spontaneous ventilation; frequency, duration and pressure difference of each pressure release; and intrinsic impedance of patients lung and chest
- peak airway pressure never exceeds preset upper level. Theoretically reduces morbidity due to peak airway pressure elevations
- resembles IRV in that peak airway pressures are lowered while mean pressures are increased by prolonged inspiratory times and short expiratory time decreases potential for airway closure
- unlike IRV allows patient to breath spontaneously with little sedation
- exact role not clear at present

Inspiratory flow waveforms

- little evidence to suggest that any one waveform superior for any given condition
- decelerating waveform associated with improved distribution of ventilation in patients with chronic airflow limitation but also results in higher mean airway pressure and therefore predisposes to potentially adverse haemodynamic effects

PEEP & CPAP

- PEEP applied by placing an expiratory resistance in circuit. May be spring-loaded valve, weighted valve, under-water column, venturi valve, electronically controlled scissor valve or pressure actuated solenoid valve
- threshold resistor mechanism preferred (minimal resistance to flow above opening pressure). Minimizes expiratory work and barotrauma during coughing or straining
- CPAP circuit may use demand valve or continuous fresh gas flow. Demand valve may be flow triggered or pressure triggered. Advantage of demand valve system is that ventilator monitoring is still available but work of breathing is increased by need to trigger valve. Respiratory work imposed by circuit and demand valve. Older ventilators imposed respiratory work similar to normal work of breathing. Modern ventilators: relatively low work. Newer ventilators have flow-by system
- in patients with dynamic hyperinflation and auto-PEEP, extrinsic PEEP may reduce respiratory work during spontaneous or patient initiated breaths. Due to reduction in threshold work
– recruits partially collapsed alveoli reducing shunt
– redistributes intra-alveolar pulmonary oedema from the alveolar to the interstitial compartment but doesn’t decrease total lung water


©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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