Tidal Volume
Tidal volume describes how much air is supplied to a patient during ventilation. It has major impacts on the success of ventilation therapy. Particularly with regard to lung-protective ventilation, the tidal volume must be set carefully to avoid serious lung damage.
In this article, you will learn how different tidal volumes affect the lungs and which settings are ideal for you to use when ventilating patients – both adults and children.
Tidal volume: Definition
The tidal volume (Vt), sometimes referred to as breathing volume, describes the volume administered per mechanical breath. It is indicated in millilitersand is subject to strong individual fluctuations.1 In adults, it varies between 350 and 800 ml at rest.2 A distinction is occasionally made between the tidal volume during spontaneous breathing, called the tidal volume, and the mechanically administered tidal volume, called mechanical breath volume.3
For ventilator users, the tidal volume is a pivotal setting parameter. It plays an important role, particularly during volume-controlled ventilation where we distinguish between 2 values:
- The inspiratory tidal volume (Vti), which is set directly on the ventilator,
- and the expiratory tidal volume (Vte), which can be measured at the end of exhalation.
A secure tidal volume ensures regular and reliable ventilation. However, the increase in ventilation pressure can be difficult to predict. Therefore, the maximum airway pressure pMax defines a pressure limit to prevent overpressure and the resultant lung damage.4
In pressure-controlled ventilation, by contrast, the tidal volume is not set directly, but rather determined indirectly by the level of inspiratory pressure (pInsp) and the lung mechanics.5
Setting the tidal volume during ventilation
The tidal volume setting plays a decisive role in ventilation when it comes to preventing ventilator-associated lung damage. For that reason, the lowest possible tidal volume is recommended for lung-protective ventilation in order to prevent volume and barotrauma.
The guide value on the basis of which you should calculate the total tidal volume for ventilation is a maximum of 6 ml/kgBW – measured against the patient’s ideal body weight (IBW). In this context, the IBW and not the actual body weight is used, as the size of the lungs is proportional to a person’s height and does not change if they are over- or underweight.6
In patients with ARDS, the S3 guidelines on invasive ventilation recommend a tidal volume of 6 ml/kg standard body weight (BW) to minimize systemic inflammation as well as mortality. For patients without ARDS, a setting of 6 to 8 ml/kgBW is recommended.
Tidal volume in the ventilation of children
For the ventilation of children, there is no clear evidence that a low tidal volume has a lung-protective effect. The value of 6 ml/kgBW, established for adults, is conventionally extrapolated to children.
Nevertheless, studies with different tidal volumes – ranging between < 7 ml and > 12 ml/kgBW – have not shown any significant differences in mortality. Conversely, that lack of differences may also be attributable to the use of pressure-controlled ventilation, where smaller tidal volumes are generally administered in more severely ill children.7
Under general anesthesia, children typically receive tidal volumes of 6 to 10 ml/kgBW, with pressure-controlled ventilation modes being preferred.8
Deviations from the normal range
Deviations from the tidal volume normal range during ventilation can be associated with various risks for the patient.
Low tidal volume
A too low tidal volume often leads to hypoventilation, where the gas exchange is disturbed and CO₂ is not eliminated sufficiently by exhalation. This can increase the carbon dioxide partial pressure (paCO₂), which in turn can cause hypercapnia and respiratory acidosis. Clinically, this manifests itself in symptoms such as fatigue, disturbance of consciousness and, in extreme cases, CO₂ narcosis. The risk of hypoventilation can be monitored by measuring end-tidal carbon dioxide (etCO₂) using capnography.9
Elevated tidal volume
On the other hand, a too high tidal volume during ventilation can initiate hyperventilation. In such cases, the paCO₂ value drops, which can lead to respiratory alkalosis.10 Furthermore, a study by Sjoding et al. suggests that patients with a tidal volume above 8 ml/kgBW had an approximately 8 % higher mortality risk than those who were ventilated with a lower tidal volume. Although the study only investigated a small sample, the correlation suggests that tidal volume has an impact on patient outcomes.
Measuring/calculating tidal volume
The expiratory tidal volume (Vte) indicates in milliliters how much air escapes from the lungs during expiration. Vte is measured via a flow sensor at the patient valve which detects the volume of air during exhalation. The inspiratory tidal volume (Vti) describes the set or measured tidal volume during inspiration.
In the case of single-patient circuits, the inspiratory and expiratory tidal volume can be measured directly at the tracheal tube using a proximal flow sensor.
In the case of double-patient circuits, the expiratory tidal volume is usually measured using a flow sensor on the device.
As a rule, the difference between Vti and Vte tends to be minor during mechanical ventilation. However, if larger differences arise, they indicate possible disturbances and risks such as leaks in the respiratory system. In particular, if the exhalation volume is repeatedly lower than the inhalation volume, overinflation of the lungs may occur. Therefore, the tidal volume should always be monitored closely.11
Factors that influence tidal volume
During ventilation, the tidal volume depends on various parameters. Particularly during pressure-controlled ventilation, multiple factors influence the tidal volume:
- Compliance: Compliance describes the elasticity of the lungs. The lower the compliance, the lower the resulting tidal volume.¹²
- Airway resistance: Airway resistance describes the frictional forces that the airflow has to overcome in the airways. When airway resistance increases, tidal volume decreases.¹³
- PEEP: A greater pressure difference between positive end-expiratory pressure (PEEP) and inspiratory pressure (pInsp) leads to a higher tidal volume. In order to pump a larger tidal volume at a constant pressure difference, the PEEP value should be kept rather low.
- pInsp: The inspiratory pressure (pInsp) determines how much tidal volume is applied during pressure-controlled ventilation. A higher pInsp leads to a larger tidal volume.¹⁴
Moreover, there are several situations that can likewise affect the tidal volume during ventilation:
- Leakage: Leaks can occur at the tracheal tube, the mask or in the breathing circuit. During volume-controlled ventilation, leaks lead to a reduced tidal volume, which correspondingly reduces ventilation. Therefore, a reduced volume during volume-controlled ventilation often indicates leaks. During pressure-controlled ventilation, by contrast, ventilation is maintained within a certain scope.¹⁵
- Incorrect positioning of the tracheal tube: If the tracheal tube slips forward into a main bronchus – an event that occurs in around 10 % of cases – then only one lobe of the lung is supplied with the tidal volume and inadequate ventilation results.¹⁶
- Obstructions, kinking of the tracheal tube or inadequate sealing of the cuff can also reduce the air flow.
- An obstruction increases resistance and can impair ventilation and pulmonary gas exchange. If an obstruction prevents the tidal volume from not being exhaled sufficiently, there is a risk that overinflation of the lungs and barotrauma occur.¹⁷
- Chest compressions: If chest compressions are performed during cardiopulmonary resuscitation (CPR), the tidal volume is low and will range between 7.5 ml and 41.5 ml without additional ventilation, which results in ventilation being inadequate.¹⁸
- Even with additional manual or mechanical ventilation, the tidal volume is influenced by chest compressions and is reduced by approx. 30 % compared to the set value in volume-controlled ventilation.¹⁹
Application scenarios
The tidal volume is set directly during volume-controlled ventilation and is used in the following ventilation modes:
IPPV ventilation: IPPV stands for Intermittent Positive Pressure Ventilation and describes a form of ventilation in which both a Vt and a PEEP are set. The pAw is determined by the tidal volume setting and can be limited by the maximum airway pressure (pMax).
S-IPPV ventilation: During this further development of the IPPV mode, the ventilator can detect spontaneous breathing during expiration within a trigger window of 100 % and support this in a synchronized manner.
SIMV ventilation: SIMV stands for Synchronized Intermittent Mandatory Ventilation. This form combines mandatory mechanical breaths with spontaneous breaths. During spontaneous breathing, the device can synchronize the patient’s own breathing during expiration within a trigger window of 20 %. This leads to an increased minute volume during spontaneous breathing.
SIMV + ASB: This extension of the SIMV mode enables additional pressure support for spontaneous breaths.
Monitoring tidal volume during ventilation
Monitoring the tidal volume is crucial for ensuring reliable ventilation of patients. On WEINMANN ventilators, the respiratory volume can be monitored via the graphical display of the flow curve and the numerical display of the measured values for Vte and MVe.
Additionally, alarm functions can be set that are activated whenever there are upper or lower excursions of the specified minute volume. These alarms are particularly relevant for pressure-controlled ventilation modes and spontaneous breathing modes, as the tidal volume depends on the ventilation pressure and the patient’s own breathing activity. Thereby, the alarm range should be close to the specified minute volume limits, i.e., around ±20 %.20
2 Larsen, R. & Mathes, A. (2023): Beatmung. 7th Edition, Berlin Heidelberg: Springer Verlag, p. 32.
3 Larsen, R. & Mathes, A. (2023): Beatmung. 7th Edition, Berlin Heidelberg: Springer Verlag, p. 244.
4 Lang, H. (2017): Außerklinische Beatmung. Berlin Heidelberg: Springer Verlag, p. 134, 279.
5 Larsen, R. & Mathes, A. (2023): Beatmung. 7th Edition, Berlin Heidelberg: Springer Verlag, p. 278.
6 Lang, H. (2020): Beatmung für Einsteiger. Berlin Heidelberg: Springer Verlag, p. 100.
8 Larsen, R. & Mathes, A. (2023): Beatmung. 7th Edition, Berlin Heidelberg: Springer Verlag, p. 472, 476.
9 Lang, H. (2017). Außerklinische Beatmung. Berlin Heidelberg: Springer Verlag, p. 34, 279, 331.
10 Lang, H. (2017). Außerklinische Beatmung. Berlin Heidelberg: Springer Verlag, p. 333f.
11 Lang, H. (2017). Außerklinische Beatmung. Berlin Heidelberg: Springer Verlag, p. 105f, 307.
12 Lang, H. (2020): Beatmung für Einsteiger. Berlin Heidelberg: Springer Verlag, p. 215f.
13 Lang, H. (2020): Beatmung für Einsteiger. Berlin Heidelberg: Springer Verlag, p. 212f.
14 Lang, H. (2020): Beatmung für Einsteiger. Berlin Heidelberg: Springer Verlag, p. 100ff.
15 Larsen, R. & Mathes, A. (2023): Beatmung. 7th Edition, Berlin Heidelberg: Springer Verlag, p. 317, 318, 385.
16 Larsen, R. & Mathes, A. (2023): Beatmung. 7th Edition, Berlin Heidelberg: Springer Verlag, p. 407
17 Larsen, R. & Mathes, A. (2023): Beatmung. 7th Edition, Berlin Heidelberg: Springer Verlag, p. 149, 545, 551.
19 Orlob S et al Reliability of mechanical ventilation during continuous chest compressions: a crossover study of transport ventilators in a human cadaver model of CPR. Scand J Trauma Resusc Emerg Med. 2021 Jul 28;29(1):102. doi: 10.1186/s13049-021-00921-2. PMID: 34321068; PMCID: PMC8316711.
20 Larsen, R. & Mathes, A. (2023): Beatmung. 7th Edition, Berlin Heidelberg: Springer Verlag, p. 298, 385.