© SICS 2005-2023. Scottish Intensive Care Society SCIO registered in Scotland charity number SC050112
By the end of this module you should be able to
1. Safely set up a ventilator for a new admission to ICU
2. Understand the basic differences between modes of ventilation
3. Identify potential side effects of ventilation
4. Know how to respond to hypoxaemia or hypercarbia in ICU
Mechanical ventilation is one of the most common interventions made in intensive care. It is often life saving, but there are life threatening side effects associated with its use.
The main purpose of ventilation is to provide life support without causing harm to the lungs and the patient. A major form of harm is ventilator induced lung injury (VILI).
Ventilator Induced Lung Injury (VILI)
This is lung damage caused by mechanical ventilation. The main elements of this are: Barotrauma: damage to the lungs as a result of high airway pressures, especially plateau Paw over 30 CmH2O. Volutrauma: damage to the lungs as a result of high tidal volumes (more than 6ml/kg in patients with ARDS). Atelactrauma: damage to the lungs as a result of cyclical collapse (atelactasis) and opening of the alveoli. This is prevented by the use of an appropriate level of PEEP. Oxygen toxicity: damage to the lungs as a result of prolonged exposure to high FiO2. The mechanism is uncertain, but it is thought that high FiO2 results in the generation of high levels of free radicals which are toxic to lung tissues. |
A sound knowledge of the underlying principles of mechanical ventilation is required for its effective and safe implementation. The information required can be obscured by the confusing terminology attached to the various modes of ventilation.
FiO2
Fraction of inspired oxygen is essentially the percentage oxygen delivered to the patient. The FiO2 range is from 0.21 (room air) to 1 (100% O2).
Positive End Expiratory Pressure (PEEP)
Positive end expiratory pressure is the amount of pressure in the breathing circuit at the end of exhalation. The initial PEEP for patients admitted to ICU is usually between 5 and 10 cm H2O.
Volume control
Refers to modes of ventilation where the volume of the tidal breath is set. E.g. we set the ventilator to deliver a breath of 500 ml. Setting the rate is normally mandatory for this method of controlling ventilation. E.g. we set the tidal volume (Vt) at 500 ml to be delivered at a rate of 14 bpm (breath per minute). The ventilator in this example is set to deliver a minute volume of 500 x 14 = 7 litres.
The airway pressure in this mode of ventilation will depend on the lung and chest wall compliance. Indeed the airway pressure might change from breath to breath depending on many factors.
Pressure control
Refers to modes of ventilation where the pressure of the tidal breath is set. E.g. we set the ventilator to deliver a breath pressure of 30 cm H20. Setting the rate is normally mandatory for this method of controlling ventilation. E.g. we set the breath pressure at 30 cm H20 to be delivered at a rate of 14 bpm (breath per minute).
The tidal volume in this mode of ventilation will depend on the lung and chest wall compliance. Indeed the tidal volume might change from breath to breath depending on many factors.
I:E ratio
Inspiratory:Expiratory ratio refers to the ratio of inspiratory time:expiratory time. In normal spontaneous breathing, the expiratory time is about twice as long as the inspiratory time. This gives an I:E ratio of 1:2 and is read “one to two”.
This ratio is typically changed in asthmatics due to the prolonged time of expiration. They might have an I:E ratio of 1:3 or 1:4. Even longer expiratory times are required sometimes, but this is best done by an experienced clinician.
An inverse ratio refers to when the I:E ratio is 2:1 or higher and is typically used to ventilate non-compliant lungs. This requires some level of expertise. Consult a senior clinician if you are unsure. Pressure control modes of ventilation should be used when employing inverse ratios as the use of volume control modes might lead to “breath stacking” and an increase in airway pressures.
Synchronisation
Is a feature of some modes of ventilation that allows the tidal volume to be delivered synchonised with the patient’s breathing. Different ventilators do this differently; consult your ventilator manual for a detailed description of the synchronisation method used by your ventilator.
In general, a synchronised mode of ventilation is better tolerated by patients.
The purpose of ventilation is to provide ‘life support’ without causing harm to the lungs and the patient. A major form of harm is ventilator induced lung injury (VILI).
As the patient arrives in ICU, we need to decide on the setting the following:
We will now discuss these in turn.
FiO2 |
PEEP |
Spontaneous or controlled mode |
Pressure or volume controlled |
Peak and plateau pressure |
Tidal volume |
I:E ratio |
Start at 1.0. It should be reduced quickly using SpO2 as a guide. If the patient is already established on lower levels of FiO2 (e.g. patient from theatre) and oxygenation is adequate, continue at that level.
High FiO2, or at least high tissue PO2, may contribute to VILI. FiO2 should normally be chosen to achieve a normal SpO2. If FiO2 is > 0.5, the aim should be SpO2 of 92%, once other ventilator settings have been optimised. This is termed permissive hypoxia. Relative hypoxaemia is also tolerated in some patients who are chronically hypoxic.
Starting levels on arrival in ICU of 5 – 10 cmH2O.
In many patients, PEEP improves oxygenation by:
In addition it will reduce the opening and closing of alveoli that occurs with each breath (‘atelectrauma’), and therefore reduce VILI.
The precise level of PEEP is controversial and will depend on the underlying condition. It will be set between 5 – 15cm H2O in the majority of patients.
FiO2 |
PEEP |
Spontaneous or controlled mode |
Pressure or volume controlled |
Peak and plateau pressure |
Tidal volume |
I:E ratio |
A spontaneous mode of ventilation is one that only delivers support to the patient’s breathing, if the patient has some respiratory effort. The ventilator then provides additional positive pressure during inspiration in order reduce the work of breathing and improve gas exchange.
A controlled mode of ventilation is one that delivers ventilation regardless of the patient’s respiratory effort. This can be volume or pressure controlled (see below).
In general, as a patient arrives in ICU, a controlled mode of ventilation will be the initial mode. Examples of this is SIMV.
The choice between these two modes of ventilation is often a matter of clinician preference, as there is no evidence that either mode is superior to the other in relation to any clinically important outcome.
If pressure controlled mode is chosen, the inspiratory pressure is set. You have to then note what Vt is being delivered to the patient to make sure that it is the Vt that you want to deliver.
If Volume controlled mode is used, the Vt is set. Here the effect on airway pressures has to be noted to make sure that the airway pressures are within safe limits.
This refers to the highest airway pressures (peak) and the airway pressure during the plateau phase of volume controlled ventilation. Peak Paw is normally taken to represent pressure in the major airways, while the plateau pressure represents pressure at the alveolar level.
FiO2 |
PEEP |
Spontaneous or controlled mode |
Pressure or volume controlled |
Peak and plateau pressure |
Tidal volume |
I:E ratio |
During mechanical ventilation, Vt is the volume delivered by the ventilator per breath.
Inspiratory:Expiratory ratio refers to the ratio of inspiratory time:expiratory time. In normal spontaneous breathing, the expiratory time is about twice as long as the inspiratory time. This gives an I:E ratio of 1:2 and is read “one to two”.
FiO2 |
PEEP |
Spontaneous or controlled mode |
Pressure or volume controlled |
Peak and plateau pressure |
Tidal volume |
I:E ratio |
Initial settings that would be reasonable for most patients are:
These settings should then be adjusted to the following important end points:
Airway pressure: the plateau pressure should be less than 30 cmH2O, except in exceptional circumstances. This avoids ‘barotrauma’.
Tidal volume: when lung compliance is low (e.g. ARDS), the tidal volume should be limited to 6-8 ml/kg. This avoids ‘volutrauma’. The tidal volume is slightly less important when breathing spontaneously, or when compliance is good, as long as airway pressures are controlled.
Respiratory rate should be adjusted to control the CO2. Initial upper limit of 20/min.
Often the rate will have to be high (e.g. 30/min) but this should be discussed with a senior clinician, as it can be dangerous.
We will now describe some common modes of ventilation that you will come across in ICUs in Scotland. You should be aware that every ventilator will have an algorithm for delivering a certain mode of ventilation. You should consult the ventilator manual for a detailed description of ventilation modes e.g.how and when synchronisation occurs for a particular mode of ventilation.
Ventilator mode | VC/PC | Spont/Cont | Sync |
IPPV | VC or PC | Controlled | No |
SIMV | VC or PC | Controlled | Yes |
PCV and BiPAP | PC | Controlled | Yes |
PS (or ASB) | PC | Spontaneous | Yes |
VC = volume control Cont = controlled
PC = pressure control Sync = synchronised
Spont = spontaneous
IPPV = Intermittent positive pressure ventilation. Can be volume or pressure controlled, is a controlled mode of ventilation (does not require any spontaneous respiratory effort from the patient), and is not synchronised with patient’s respiratory effort.
SIMV = Synchronised Intermittent mandatory ventilation. Can be volume or pressure controlled. Is a controlled mode of ventilation (does not require any spontaneous respiratory effort from the patient), and is synchronised.
PCV and BiPAP = Pressure controlled ventilation and Bilevel positive airway pressure. Are pressure controlled modes of ventilation. They are controlled modes of ventilation, but allow synchronisation if the patient takes spontaneous breaths. The two modes are slightly different in the way synchronisation occurs.
PS (ASB) = Pressure support (assisted spontaneous breathing). Is a spontaneous mode of ventilation that is pressure controlled. It is fully synchronised with the patient’s breath.
SIMV and PCV are almost always combined with PS to allow the support of any spontaneous breaths that the patient takes on top of the mandatory breaths delivered.
Common complications of mechanical ventilation |
VILI |
Haemodynamic instability |
Ventilator associated pneumonia |
Pneumothorax |
We will now discuss two of these common complications.
Increasing intrathoracic pressure with positive pressure ventilation has haemodynamic consequences.
In general, it causes a reduction in cardiac preload with a corresponding decrease in cardiac output and organ perfusion. This may be masked by vasoconstriction, which will protect blood pressure.
The effect of an increase in intrathoracic pressure in an individual patient will depend upon:
The precise effect in an individual patient is difficult to predict.
Infection of the lungs that develops after 48 hours of ventilation.
The presence of an endotracheal tube bypassing natural defences against infection, drugs causing gastric stasis and incompetence of gastrointestinal sphincters, patient positioning, and the general hypoimmunity seen in many critically ill patients all predispose to the development of ventilator associated pneumonia.
Responding to blood gas abnormalities:
Check machine, circuit and airway device. Increase FiO2 to 1.
The response following this depends on the severity of the hypoxaemia and the speed of the deterioration. Always examine the patient. While doing so, arrange CXR and ECG which provide diagnostic information and exclude some treatable causes.
Exclude:
History, targeted examination and investigation should take place rapidly. Treatment depends on the cause, but remember that a pneumothorax developing during positive pressure ventilation may quickly become life threatening.
SpO2 of 92% is acceptable. In severe respiratory failure, many experts would accept saturations of over 85%.
Clearly the treatment depends on the cause, so reaching a diagnosis is vital. History, examination and investigation should proceed in the usual fashion. Short-term measures which may help are:
The correct response will depend on the circumstances, but keep in mind the principles of ventilator management referred to above (tidal volume, airway pressure, respiratory rate).
Permissive hypercarbia is an accepted part of respiratory management in ICU. The question that should be answered when asked to deal with hypercarbia is: will the treatment required to lower the CO2 cause more damage to the patient than the hypercarbia itself?
If the treatment is to increase airway pressure or tidal volume above the safe values referred to above, the answer is likely to be yes: therefore the hypercarbia should be tolerated. If the treatment is to increase respiratory rate, the answer is likely to be no. If the treatment is to lighten sedation, the answer is likely to be no.
Some experts would prescribe sodium bicarbonate to counteract the respiratory acidosis, but this is not universally accepted.
If a patient is breathing spontaneously, there is a danger that ‘taking over’ a patient’s ventilation only causes a further rise in CO2. Be sure that you can increase the overall minute ventilation, in the face of reduced compliance caused by controlling ventilation, before attempting to do so.
General principles of trouble shooting poor gas exchange for patients on mechanical ventilation |
Increase FiO2 |
History, quick targeted examination of patient and equipment, and CXR and ECG |
Definitive treatment if diagnosis made. E.g. bronchodilators for spasm, diuretic for fluid overload, decompression for tension pneumothorax |
Improve patient’s synchronisation with ventilator. E.g. change to different mode of ventilation or give muscle relaxant |
Make sure lungs are well perfused |
Try increasing PEEP (max 15 cm H20). This can take a few minutes to work |
Increasing RR is worth trying before increasing Vt (if this is > 7 ml/kg) |
Call for help (do this first if hypoxia develops quickly and you do not know why) |
© SICS 2005-2023. Scottish Intensive Care Society SCIO registered in Scotland charity number SC050112