Synchronized Intermittent Mandatory Ventilation: SIMV Ventilation
In emergency medicine, SIMV ventilation is an essential form of ventilation to support patients in critical situations. Reliable breathing support is absolutely essential in patient transport, where ambient conditions and a patient’s condition can change quickly.
This article explains how SIMV ventilation differs from SIMV+ASB, the applications for the modes, and how WEINMANN ventilators support ventilation with SIMV and SIMV+ASB.
Definition: SIMV ventilation explained
SIMV (Synchronized Intermittent Mandatory Ventilation) is a further development of IMV (Intermittent Mandatory Ventilation). In contrast to IMV ventilation, in which mechanical breaths are administered without taking into account the breaths taken independently by the ventilated individual, SIMV ventilation is a combination of controlled and assisted ventilation. Here, the ventilation mode can be implemented on both a volume-controlled (VC-SIMV) and a pressure-controlled (PC-SIMV) basis.
In SIMV ventilation, a minimum frequency of mechanical breaths per minute is specified to ensure that the patient is sufficiently ventilated. Patients additionally have the option of breathing spontaneously during the expiratory time. A spontaneous breath is expected within a certain time window, the so-called trigger window. If this does not occur, a non-synchronized mechanical breath is administered.1 The trigger mechanisms for detecting spontaneous breathing can be set to suit individual patients.
If patients take additional spontaneous breaths, the mandatory minute volume and actual ventilation frequency will be slightly above the set rate.2 In the case of synchronized mechanical breaths, the respiratory rate and mandatory minute volume are unchanged. The process allows the independent breaths to be facilitated by means of pressure support administered by the respirator (ASB).
Mechanical breaths in SIMV ventilation can accordingly be administered under both ventilator and patient control, guaranteeing a minimum level of ventilation and increasing patient comfort by synchronizing with the patient’s breathing.
VC-SIMV ventilation vs. PC-SIMV ventilation
SIMV ventilation can be implemented in the form of either volume-controlled SIMV ventilation (VC-SIMV) or pressure-controlled SIMV ventilation (PC-SIMV). The variants differ in terms of ventilation parameter settings.
A fixed tidal volume (Vt) is specified in VC-SIMV ventilation. The resulting airway pressure depends on the patient’s individual compliance and resistance. If pressure during inspiration reaches the set upper limit – that is, the maximum ventilation pressure (pMax) – this pressure is maintained until the end of the inspiratory time; the ventilator then switches to expiration. It may be that not all of the set tidal volume is administered in this process.3
In PC-SIMV ventilation, on the other hand, it is the inspiratory pressure (pInsp) which is set, whilst the tidal volume varies as a function of compliance and resistance.4
In general terms, SIMV – whether volume-controlled or pressure-controlled - facilitates spontaneous breathing activity between two mechanical breaths, and triggering of inspiration.5
SIMV-PSV ventilation
SIMV-PSV (Pressure Support Ventilation), also called SIMV-ASB (Assisted Spontaneous Breathing), is a volume-controlled form of ventilation with a specified mandatory minute volume. A preset maximum pressure limit (pMax) guarantees patient safety here.
As in regular SIMV ventilation, patients can initiate pressure-controlled mechanical breaths within a defined trigger window in the expiratory phase. In addition, however, SIMV-PSV ventilation also facilitates spontaneous breaths with pressure support during the remaining time.6
Parameter
- Setting mode
VC-SIMV ventilation: Volume-controlled
PC-SIMV ventilation: Pressure-controlled
SIMV-PSV ventilation: Volume-controlled with pressure support
- Target
VC-SIMV ventilation: Guaranteed tidal volume
PC-SIMV ventilation: Guaranteed airway pressure
SIMV-PSV ventilation: Guaranteed airway pressure + support
- Control variable
VC-SIMV ventilation: Tidal volume
PC-SIMV ventilation: pInsp
SIMV-PSV ventilation: Tidal volume at pMax
- Trigger window
VC-SIMV ventilation: Yes
PC-SIMV ventilation: Yes
SIMV-PSV ventilation: Yes
- Spontaneous breathing
VC-SIMV ventilation: Facilitates spontaneous breathing between controlled breaths
PC-SIMV ventilation: Facilitates spontaneous breathing between controlled breaths
SIMV-PSV ventilation: Supports all spontaneous breaths with pressure support
Benefits and risks
Benefits and applications of SIMV
SIMV ventilation delivers a range of benefits that make it a valuable option in a variety of clinical scenarios.
Adjusting the ventilation frequency allows the work of breathing to be adapted perfectly to suit patients’ capabilities. It is thus possible to transition from fully-controlled ventilation at a high frequency to largely spontaneous breathing at a low respiratory rate.
This is of particular benefit when weaning a patient off ventilation, as in this case, the frequency is reduced in stages, encouraging patients to breathe independently.7 As the reflex control of breathing is maintained by pulmonary and thoracic baroreceptors, this also means fewer coordination problems with respiratory muscles during weaning.8
SIMV ventilation allows patients largely to determine their own respiratory rhythm, whilst at the same time guaranteeing a preset minimum ventilation.9 This combination of patient safety and comfort makes this form of ventilation especially suitable for patient transport, as the fact that the ventilation frequency can be adapted during transport makes it possible to react quickly to changes in the patient’s condition.
SIMV ventilation is also used for diseases which involve weakness of the respiratory muscles such as Guillain-Barré Syndrome (GBS) and myasthenic crisis.10
It is also used in neonatology to ventilate neonates. Low tidal volumes and high ventilation frequencies are generally selected in such cases. Accurate control and monitoring of tidal volume can improve patient outcomes.11
Other benefits of SIMV ventilation are lower airway and pleural pressures that reduce the risk of barotrauma and lung injury, as well as patients having the option to influence their own ventilation and thus carbon dioxide partial pressure paCO₂ in the blood, reducing the risk of hyperventilation.12
Risks
Although an established form of ventilation, SIMV ventilation does have potential risks that require careful monitoring.
SIMV may extend the time taken to wean a patient off a respirator, as it may not relieve the respiratory muscles enough, and independent breathing phases may be disrupted by mechanical breaths.13 Alternatives to SIMV ventilation such as BIPAP may be a better option in such cases.14
If too high a ventilation frequency is set, this furthermore increases the risk of hyperventilation with respiratory alkalosis, whilst if too low a frequency is set, this may cause hypoventilation with respiratory acidosis as spontaneous breathing slows down.15
In the case of shallow breathing, SIMV ventilation may also lead to inadequate lung inflation, intensifying the risk of atelectases. However, it is possible to counteract this risk by combining the mode with suitable pressure support.16
SIMV ventilation with demand flow, in particular, may require additional inspiratory work, which is why it has come in for criticism in the treatment of patients with chronic obstructive pulmonary disease (COPD).17
Poorly-responding trigger valves, or those with too insensitive a setting, can furthermore make the work of breathing more difficult, as patients have to breathe against the resistance of the device. This is because, if the ventilator does not correctly detect attempts to breathe spontaneously, patients often have to breathe without the synchronized mechanical breath delivered by the device. These reasons make it important to use high-quality ventilators which guarantee accurate, sensitive detection of patient breathing – this is the only way to ensure effective, patient-centered support.
SIMV ventilation with WEINMANN
SIMV ventilation guarantees a minimum level of ventilation whilst simultaneously supporting spontaneous breathing. This gives patients in an alarming situation a high degree of comfort, as well as providing optimum care during transport.
WEINMANN offers SIMV (+ ASB, Assisted Spontaneous Breathing) as an optional ventilation mode in the MEDUVENT Standard and MEDUMAT Standard² ventilators.
SIMV ventilation with WEINMANN devices allows maximum patient safety and flexible adaptation to patients’ individual requirements if the following ventilation parameters are set:
- Vt: Tidal volume in ml
- Freq.: Ventilation frequency in 1/min
- PEEP: Positive end-expiratory pressure in mbar
- pMax: Maximum inspiratory pressure in mbar
- I:E: Inspiratory-expiratory time ratio
- Inspiratory trigger
- Expiratory trigger (with ASB)
In this case, the ventilation frequency defines the number of mechanical breaths. For rapid ventilation in an emergency, however, WEINMANN ventilators can also be started simply by entering the patient’s height. This dispenses with the need to enter numerous parameters, allowing guideline-compliant ventilation to start at once. This function is of particular significance in emergency medicine, as it may save vital time in critical situations.
MEDUVENT Standard
MEDUVENT Standard is one of the smallest, lightest-weight turbine-driven emergency and transport ventilators in the world. Despite its low weight of 2.1 kg, it has a battery runtime of 7.5 hours, assuming typical ventilation settings for adults, and does not need an external gas supply. It allows inspiratory oxygen concentrations from 21% to 100% to be administered to ensure optimum support for differing degrees of respiratory insufficiency and for different clinical pictures. MEDUVENT Standard itself consumes no oxygen during the process.
MEDUMAT Standard²
MEDUMAT Standard² is a versatile ventilator with a battery runtime of 10 hours, making it suitable for a wide variety of situations, especially for prolonged applications. With a weight of 2.5 kg, it is handy and easy to transport. MEDUMAT Standard² can ventilate any patient with a body weight of 3 kg or more, allowing treatment across the board, from infants to adults.
Both ventilators feature intuitive operation and clear, extensive monitoring options. Pressure and flow curves allow all vital signs to be monitored continuously, whilst a night mode means that patient data can be read off easily, even in the dark.
The clear arrangement of symbols and controls, together with acoustic and visual warning signals, guarantee a high level of patient safety. Automatic function checks and hygiene filters contribute further to patient safety and to the smooth functioning of the devices.
1 https://www.thieme-connect.com/products/ejournals/pdf/10.1055/s-0041-105578.pdf
2 R. Larsen, T. Ziegenfuß (2017). Pocket Guide Beatmung [Pocket Guide to Ventilation]. Berlin Heidelberg: Springer
3 https://www.weinmann-emergency.com/de/themen/notfallbeatmung/volumenkontrollierte-beatmung
4 https://www.amboss.com/de/wissen/maschinelle-beatmung/
5 Humberg, Alexander et al.: 2017, Beatmung in Pädiatrie und Neonatologie [Ventilation in pediatrics and neonatology], DOI: 10.1055/b-0036-140160
6 https://www.weinmann-emergency.com/de/themen/notfallbeatmung/volumenkontrollierte-beatmung
7 https://flexikon.doccheck.com/de/SIMV
8 Larsen R, Ziegenfuß T (2017) Pocket Guide Beatmung [Pocket Guide to Ventilation]. 2nd edition Berlin: Springer
9 Larsen, Reinhard (2012). Anästhesie und Intensivpflege für die Fachmedizin [Anesthesia and intensive care for specialists]. 8th edition Berlin Heidelberg: Springer
10 Schwab et. al. (2011). Neurointensiv [Neurointensive care]. 2nd edition Berlin Heidelberg: Springer
11 https://www.thieme-connect.com/products/ejournals/pdf/10.1055/s-0041-105578.pdf
12 Larsen R, Ziegenfuß T (2017) Pocket Guide Beatmung [Pocket Guide to Ventilation]. 2nd edition Berlin: Springer
13https://link.springer.com/chapter/10.1007/978-3-662-45989-8_14
14 https://link.springer.com/content/pdf/10.1007/BF03364455.pdf
15 https://viamedici.thieme.de/lernmodul/6772238/4915521/beatmung
16 Larsen R, Ziegenfuß T (2017) Pocket Guide Beatmung [Pocket Guide to Ventilation]. 2nd edition Berlin: Springer