PSV/ASB Ventilation

Ambulance staff transport a ventilated patient on a stretcher to the ambulance

PSV/ASB ventilation is a form of ventilation that facilitates spontaneous breathing by means of targeted pressure support. It can be flexibly combined with other modes such as SIMV and BIPAP – though PSV ventilation is used particularly with CPAP. This allows ventilation therapies to be adapted individually to suit each patient.

Read this article to find out more about the function of PSV/ASB ventilation, its applications, its benefits, and how WEINMANN ventilators support the PSV ventilation mode. 

Definition: PSV/ASB ventilation explained

PSV (Pressure Support Ventilation), also called ASB (Assisted Spontaneous Breathing), is a pressure-supported form of ventilation which supports patients with spontaneous breathing. Both terms, PSV and ASB, are used synonymously, and refer to the same form of ventilation.

PSV/ASB ventilation is preferred for use in patients with weakened respiratory muscles, and is employed primarily as a non-invasive form of ventilation – frequently in combination with CPAP ventilation.1

The intensity of breathing support/the pressure administered is preset, and is based on a patient’s individual breathing power. Common pressure settings are between 10 and 20 mbar, with higher pressure leading to a greater inspiratory volume.2

A prerequisite for PSV/ASB ventilation is adequate respiratory drive, as the ventilator works entirely on the basis of patient initiation. Ventilation is triggered when a pressure or flow threshold is exceeded:

  • Pressure trigger: A vacuum below the PEEP (positive end-expiratory pressure) level is generated by the patient’s efforts to breathe in. The ventilator detects this vacuum and delivers synchronized pressure support.
  • Flow trigger: When the patient breathes in, the resulting flow of air is detected by the ventilator. A pressure-controlled mechanical breath is then administered, synchronized with the patient’s breath.

Expiration can be initiated by a variety of mechanisms:

  • Flow control: Inspiration ends when the inspiratory flow drops to a predefined percentage of peak flow or falls below a certain minimum value.
  • Pressure control: Expiration is initiated when the patient causes a rise in pressure by making efforts to breathe out.
  • Timed: Inspiration is ended once a preset time has elapsed, regardless of the patient’s efforts to breathe out.

Safety precautions

In PSV/ASB ventilation, it is the patient who determines the respiratory rate. Although pressure support reduces the work of breathing, and thus makes spontaneous breathing easier, if there is no spontaneous breathing, a mechanical breath cannot be triggered. As a consequence, safety precautions are always required3:

  • Apnea ventilation/backup: If no spontaneous breathing is detected within a specified period, a backup mode is activated automatically. This so-called apnea ventilation ensures that the patient receives controlled ventilation even if there is no spontaneous breathing. It maintains adequate ventilation, and prevents both hypoxemia and other associated risks.
  • Alarm function: The ventilator generally has an alarm function that informs medical staff about potentially critical changes. An apnea alarm is triggered if no ventilation is detected or if the preset respiratory rate/minute volume is undershot. The alarm function is key in enabling timely intervention.4

Application

PSV/ASB ventilation is among the most frequently-used forms of ventilation in intensive care medicine. It is used for various types of respiratory insufficiency in both long-term ventilation and in the weaning phase. The ability to combine it with numerous other ventilation modes makes it extremely versatile.5 For example, patients with severe oxygenation disorders can be treated successfully with PSV/ASB ventilation with a sufficiently high PEEP – as long as they have an intact respiratory drive. It has also proved particularly effective in chronic obstructive pulmonary disease (COPD).

Ventilation modes which can be combined with ASB ventilation

PSV/ASB ventilation can be combined with various other ventilation modes. The following are among the most common combinations:

CPAP + ASB (Continuous Positive Airway Pressure + Assisted Spontaneous Breathing)

In CPAP + ASB ventilation, a continuous positive airway pressure (CPAP) is maintained whilst the patient is breathing spontaneously. Every breath is furthermore pressure-supported, significantly reducing the work of breathing and allowing more stable, more efficient breathing.

BIPAP/BiLevel + ASB (Biphasic Positive Airway Pressure + ASB)

In this mode, the patient breathes spontaneously at two different pressure levels: a higher inspiratory pressure (pInsp) and a lower expiratory pressure (PEEP). In addition, every breath during BIPAP-ASB ventilation is pressure-supported.

SIMV + ASB (Synchronized Intermittent Mandatory Ventilation + ASB)

This form of ventilation combines a mechanical ventilation frequency with the option of breathing spontaneously within a specified trigger window. In this process, spontaneous breathing is supplemented by pressure support in order to achieve smooth synchronization with controlled mechanical breaths, and to provide relief to the respiratory muscles.6

PRVC + ASB (Pressure Regulated Volume Control + ASB)

This process specifies a target tidal volume to be achieved by means of pressure-controlled ventilation. At every breath, the ventilator automatically adapts the pressure to achieve the desired tidal volume. The patient also receives pressure support for every spontaneous breath.7

Ventilation parameters for PSV/ASB ventilation 

A variety of parameters can be set for PSV/ASB ventilation so as to achieve the best possible patient support, and to take account of individual requirements such as respiratory drive power.

Ventilation parameters for PSV/ASB ventilation

A variety of parameters can be set for PSV/ASB ventilation so as to achieve the best possible patient support, and to take account of individual requirements such as respiratory drive power.

PSV/ASB ventilation: Settings

The following parameters can be set for PSV/ASB ventilation:

  • PEEP: The pressure maintained in the lung at the end of expiration.
  • PS/ΔpASB: The additional pressure applied to support every spontaneous breath.
  • Inspiratory time: The duration of the inspiratory phase.
  • Expiratory time: The duration of the expiratory phase.
  • Inspiratory trigger: The sensitivity with which the ventilator reacts to the patient’s efforts to breathe in.
  • Expiratory trigger: The sensitivity with which the ventilator detects the start of breathing out.
  • Ramp: The speed at which pressure rises at the start of inspiration.

Control parameters

The following control parameters are relevant for checking the efficacy of PSV mode in ventilation:

  • Respiratory rate 
  • Minute volume 
  • Tidal volume 
  • End-tidal CO₂ (etCO₂) 
  • Oxygen saturation 

Benefits of PSV/ASB ventilation

As most intensive care patients are able to breathe spontaneously, and do not require complete mechanical ventilation, PSV/ASB ventilation provides suitable support.

Synchronization with the ventilator is improved by patient-triggered breathing, where the patient determines the respiratory rhythm, ventilation cycle, and inspiratory time. This not only leads to greater patient comfort, but also frequently allows ventilation to take place with reduced sedation or none at all. At the same time, it reduces strain on the circulatory system due to the generally low mean airway pressure.

Another key benefit is the improvement in oxygenation, especially if patients are switched from another ventilation mode to pressure support. Pressure support reduces the work of breathing, and thus reduces the oxygen consumption of the respiratory muscles. This prevents fatigue, and may prevent or reduce the development of rapid, shallow breathing (tachypnea).

PSV/ASB ventilation is also capable of compensating for the additional work of breathing resulting from the tracheal tube, the ventilation hoses, and the trigger valves. Pressure support of 5 mbar to 10 mbar over and above the set PEEP level is generally required for this. There is furthermore the option of infinitely reducing pressure support, making it easier, as well as safer, to wean patients off ventilation.8

Risks

Correct calibration is critical for PSV/ASB ventilation. Setting too high a support pressure may lead to overinflation of the lung (volutrauma), whilst at the same time, setting too low a support pressure may cause inadequate ventilation (hypoventilation).9

The setting for trigger sensitivity is also critical. Too low a setting may lead to the ventilator initiating undesired mechanical breaths which may cause hyperventilation. Conversely, too high a trigger setting may increase the effort the patient has to make to breathe, with the result that patients may not have sufficient strength to trigger pressure support at all. 

When using a pressure trigger, the patient has to breathe against a closed valve at the start of inspiration in order to initiate the ventilation cycle. This may lead to exhaustion and respiratory pump failure.10

ASB ventilation at WEINMANN

MEDUMAT Standard² and MEDUVENT Standard ventilators from WEINMANN provide a number of ventilation modes, including various options with ASB:

  • CPAP + ASB (spontaneous) 
  • BiLevel + ASB (pressure-controlled) 
  • PRVC + ASB (pressure-controlled, volume-guaranteed) 
  • SIMV + ASB (volume-controlled) 

On both devices, ASB ventilation is available as an additional option, and can be added as required.

MEDUVENT Standard

MEDUVENT Standard is one of the smallest, lightest-weight emergency and transport ventilators in the world. Weighing just 2.1 kg, it is especially handy and, at the usual ventilation settings, can supply adults with oxygen for 7.5 hours without the need for an external compressed gas supply. Inspiratory oxygen concentrations of 21% to 100% can be administered in this process.

MEDUMAT Standard²

Weighing 2.5 kg, MEDUMAT Standard² is likewise lightweight and handy, and its battery runtime of 10 hours makes it particularly suitable for prolonged applications. It can ventilate infants weighing 3 kg or more, and is thus suitable for all patient groups.

Both devices have clear monitoring features, with pressure and flow curves to visualize patients’ vital signs. The displays also have a night mode so that they remain perfectly legible, even in the dark.

1 https://zbi-gruppe.com/neues-aus-der-zbi-gruppe/asb-psv-beatmung-ablauf-vor-nachteile

2 Lang, Hartmut (2020), Beatmung für Einsteiger, Theorie und Praxis für die Gesundheits- und Krankenpflege [Ventilation for beginners, theory and practice for healthcare]. Berlin: Springer.

3 Larsen R, Ziegenfuß T (2013). Beatmung [Ventilation]. 5th edition, Berlin: Springer.

4 www.ncbi.nlm.nih.gov/pmc/articles/PMC7531439/

5 Larsen R, Ziegenfuß T (2018). Beatmung [Ventilation]. Berlin Heidelberg: Springer-Verlag.

6 R. Larsen, T. Ziegenfuß (2017). Pocket Guide Beatmung [Pocket Guide to Ventilation]. Berlin Heidelberg: Springer

7 https://www.weinmann-emergency.com/topics/ventilation/types-of-ventilation

8 Larsen R, Ziegenfuß T (2013). Beatmung [Ventilation]. 5th edition, Berlin: Springer.

9 Lang, Hartmut (2020), Beatmung für Einsteiger, Theorie und Praxis für die Gesundheits- und Krankenpflege [Ventilation for beginners, theory and practice for healthcare]. Berlin: Springer

10 Larsen R, Ziegenfuß T (2013). Beatmung [Ventilation]. 5th edition, Berlin: Springer.