Inspiratory Tidal Volume (VTi) during Ventilation

The inspiratory tidal volume (VTi) and the expiratory tidal volume (VTe) are two tidal volume parameters. During ventilation, VTi provides important indications of potential discrepancies – especially when compared to VTe. Large deviations between VTi and VTe can indicate leaks and draw attention to the risk of intrinsic PEEP. 

This article provides an overview of inspiratory tidal volume and how it can be reliably monitored with WEINMANN ventilators.

Definition: What is VTi in ventilation?

VTi describes the volume of air that is delivered into the patient’s lungs via the ventilation hose in the ventilator during inspiration. It is usually stated in milliliters (ml) and is crucial to ensure adequate ventilation.¹

The VTi setting on the ventilator defines how much respiratory gas the patient can receive per breath. The value does not take into account any possible leaks in the ventilation system.

During ventilation, VTi can be calculated using the positive area under the flow curve. This makes it possible to visualize how the flow is developing and how much volume is actually being applied. The graph below shows the flow curve during pressure-controlled ventilation, with the blue area under the curve corresponding to VTi.²

Factors influencing VTi

The choice between pressure-controlled or volume-controlled ventilation determines which factors influence the tidal volume: 

Pressure-controlled ventilation and VTi 

With pressure-controlled ventilation, VTi is not set directly, but results from various ventilation variables. Primarily, the inspiratory tidal volume depends on the difference between the positive end-expiratory pressure (PEEP) and the inspiratory ventilation pressure (pInsp), with a greater pressure difference resulting in a higher tidal volume. 

However, increasing the pressures can reduce respiratory system compliance, which reduces the volume administered. This means that with PEEP of 5 mbar and pInsp of 15 mbar, the tidal volume is higher than with PEEP of 20 mbar and pInsp of 30 mbar, although the difference is identical in both cases. The following parameters can also have an impact on VTi:

Volume-controlled ventilation and VTi 

With volume-controlled ventilation, the inspiratory tidal volume (VTi) is fixed. This means that it is administered independently of factors such as compliance or resistance of the lungs. As a result, the set mechanical breath volume can be precisely controlled, which enables precise control of paCO₂ and the pH value.

However, VTi is not always fully applied. Maximum airway pressure (pMax) is set in advance to protect the lungs from barotrauma. If the airway pressure reaches pMax, the mechanical breath can be terminated prematurely. Alternatively, the air pressure can be maintained at the limited level until the inspiratory time has ended.⁴

In addition, leaks during inspiration result in a reduction in ventilation, as part of the volume escapes.⁵

Difference between VTi and VTe during ventilation

The term ‘tidal volume’ covers both VTi and VTe, but there are differences between the two parameters. VTi indicates how much air enters the patient’s lungs during inspiration. In contrast, VTe measures the amount of air that is actually exhaled during expiration. It therefore represents the volume that participated in the gas exchange in the lungs.

Deviations between VTi and VTe are caused by leaks in the ventilation system, which can result in part of the tidal volume escaping. Significant differences between the parameters occur in particular during non-invasive ventilation with a CPAP mask, as this can never be completely sealed. VTi is higher than VTe in such cases. For this reason, the expiratory volume (VTe) is more important during NIV, as it indicates the effective ventilation.⁶

During controlled ventilation, differences between the two values are normally minor. If deviations occur, the ventilation system should be checked for leaks. These can occur both in the device and in the patient. Examples of leaks in the patient include a pneumothorax and a tracheal rupture.⁷

If VTi is permanently higher than VTe, this can be problematic: There is a danger of intrinsic PEEP forming, where increased pressure remains in the lungs.⁸ This can result in right ventricular strain and obstruction of spontaneous breathing, leading to desynchronization with the ventilator.⁹

VTe can be measured with single or double breathing circuits.¹⁰

Standard values of VTi

VTi is based on standard values of 6-8 ml/kgBW IBW (ideal body weight) for ventilation. Ideal body weight is used as lung capacity is proportional to height and does not depend on the patient’s actual weight.¹¹

Deviations from standard values

Possible causes of deviations from the inspiratory tidal volume and what to do about them are explained here to help you recognize and treat them at an early stage.

VTi too high during ventilation: What to do?

If VTi is too high during ventilation, this can be triggered by various factors. Typical causes include:

The following measures should be taken in such cases:

1. Check the patient’s condition
2. Check the breathing circuit and the adapter for the expiratory module for leaks
3. Check therapy and alarm settings and adjust if necessary

VTi too low during ventilation: What to do?

If VTi is too low during ventilation, the reasons may include the following:

• Obstruction/constriction of the airways, e.g. due to
  • Secretions
  • Obstruction
  • Bronchoconstriction¹²
• Leakage (with volume-controlled or non-invasive ventilation)

To correct VTi that is too low, you can proceed as follows:

1. Check the patient’s condition and airways
2. Check the breathing circuit for possible obstructions
3. Check therapy and alarm settings and adjust if necessary¹³

Monitoring VTi during ventilation – with WEINMANN ventilators

Continuous monitoring of VTi during ventilation is crucial in order to detect deviations at an early stage. WEINMANN ventilators offer several options for this. 

VTi can be monitored directly via the graphical representation of the respiratory curve. On the flow curve, high or low VTi can be recognized by the height and width of the upper spike.

In contrast to VTe, there are no direct alarms for VTi. However, there are signals that can indicate an abnormal VTi. Examples of this are messages such as “Airway pressure high” or “Airway pressure low” during pressure-controlled ventilation.

In addition, some devices display the inspiratory minute volume (MVi). This value is calculated by multiplying VTi by the ventilation frequency per minute and can therefore also provide information about VTi.

¹ Lang, H. (2020): Beatmung für Einsteiger. Theorie und Praxis für die Gesundheits- und Krankenpflege, 3rd edition, Berlin Heidelberg: Springer Verlag, p. 87ff.

² Lang, H. (2020): Beatmung für Einsteiger. Theorie und Praxis für die Gesundheits- und Krankenpflege, 3rd edition, Berlin Heidelberg: Springer Verlag, p. 225ff.

³ Lang, H. (2020): Beatmung für Einsteiger. Theorie und Praxis für die Gesundheits- und Krankenpflege, 3rd edition, Berlin Heidelberg: Springer Verlag, p. 100ff.

⁴ Lang, H. (2017): Außerklinische Beatmung, Berlin Heidelberg: Springer Verlag, p. 129.

⁵ Larsen, R. & Mathes, A. (2023): Beatmung. 7th edition, Berlin Heidelberg: Springer Verlag, p. 317.

⁶ Lang, H. (2020): Beatmung für Einsteiger. Theorie und Praxis für die Gesundheits- und Krankenpflege, 3rd edition, Berlin Heidelberg: Springer Verlag, p. 238ff.

⁷ https://intensiv.anthroposophische-pflege.de/beatmung/

⁸ Lang, H. (2020): Beatmung für Einsteiger. Theorie und Praxis für die Gesundheits- und Krankenpflege, 3rd edition, Berlin Heidelberg: Springer Verlag, p. 249.

⁹ https://www.ai-online.info/abstracts/pdf/dacAbstracts/2010/15_Huschak.pdf

¹⁰ Lang, H. (2020): Beatmung für Einsteiger. Theorie und Praxis für die Gesundheits- und Krankenpflege, 3rd edition, Berlin Heidelberg: Springer Verlag, p. 95f.

¹¹ Lang, H. (2020): Beatmung für Einsteiger. Theorie und Praxis für die Gesundheits- und Krankenpflege, 3rd edition, Berlin Heidelberg: Springer Verlag, pp. 100 & 127.

¹² Lang, H. (2020): Beatmung für Einsteiger. Theorie und Praxis für die Gesundheits- und Krankenpflege, 3rd edition, Berlin Heidelberg: Springer Verlag, p. 213.

¹³ https://document.resmed.com/documents/rc-clinical-guides/astral-series/clinical-guide/astral-100-150_clinical-guide_row_ger.pdf (pp. 174 & 176)