Original post written on 5th April 2023 by Giuseppe Natalini
During mechanical ventilation, inspiratory support should be modulated to maintain optimal activity of the inspiratory muscles.
Respiratory muscle activity can be measured as the pressure they develop during inspiration (Pressure-time product, PTP, see also this post from 30/10/2016, English translation not yet available) (Figure 1), which is the sum of: a) the pressure generated by the respiratory muscles to expand the lungs, expressed by the reduction in esophageal pressure during inspiration (PTPL, hatched area); b) the pressure required to expand the rib cage (PTPCW, dotted area), which is calculated at each moment of inspiration as the product of rib cage elastance (calculated from esophageal pressure) and inspired volume.
The sole function of inspiratory support is to support the activity of the respiratory muscles, which is measured by esophageal pressure. With that being said, it would seem obvious that the most important pressure to measure in ventilated patients should be esophageal pressure... In clinical practice, esophageal pressure is rarely measured, and the appropriateness of inspiratory support is mainly determined by evaluating respiratory rate and tidal volume, as well as indirect indicators of respiratory muscle activity such as P0.1 (post dated 27/06/21), PMI (post dated 08/05/2016, English translation not yet available), dPocc (1), and Flow Index, the topic of this post.
The Flow Index.
The Flow Index is a dimensionless number that describes the shape of the inspiratory flow curve from its peak to the point where cycling occurs (2). If this portion of the flow curve has an upward concavity, the Flow Index assumes a value <1 (the smaller the value, the more pronounced the upward concavity), if it decreases linearly, the Flow Index is 1, and if the flow curve has a downward concavity, the Flow Index is >1 (the greater the value, the more pronounced the downward concavity) (Figure 2).
Figure 2
The Flow Index stems from the understanding that during pressure-controlled ventilation, inspiratory flow decreases if the patient is passive, while the reduction in alveolar pressure due to respiratory muscle activity produces a downward concavity.
A picture is worth a thousand words: in Figure 3, we can see how the flow profile changes with increasing levels of pressure support (PS).
Figure 3
With a PS of 20, the patient, after the activation of the inspiratory trigger, becomes passive throughout inspiration, and the flow decreases. As inspiratory support is reduced (moving from right to left in Figure 3), inspiratory activity becomes progressively more intense, and concurrently, the flow curve assumes a morphology with an increasingly pronounced downward concavity. Consequently, as respiratory muscle activity increases, the Flow Index becomes higher.
This concept is not at all new for those who have participated in our ventilation courses or follow ventilab: evaluating respiratory muscle activity from the flow curve has already been proposed in our first mechanical ventilation course in 2007. It was the subject of one of the first posts I wrote on ventilab (dated 25/03/2010, English translation not yet available), and it is an important part of the RESPIRE method (particularly its "I" letter), presented in its first version in the post dated 20/08/2017 (English translation not yet available).
The Flow Index has simply transformed the subjective and qualitative evaluation of the flow curve into an objective and quantitative measurement. Studies on the Flow Index (2-4) have demonstrated two important things:
- The morphology of the inspiratory flow curve during pressure-controlled ventilation is effective for a non-invasive and continuous assessment of respiratory muscle activity. Before the Flow Index, we had only the theoretical rationale for this approach (which was already significant), but now experimental evidence has also been added to it.
- Assessing respiratory rate and tidal volume to gain an understanding of inspiratory effort cannot replace the information provided by the Flow Index; at most, it should complement the Flow Index. In fact, the strong association between the Flow Index and inspiratory effort remains even when adjusting for tidal volume and respiratory rate.
How to calculate the Flow Index
A brief technical note. The Flow Index examines the portion of flow following the peak and preceding cycling (Figure 4) and applies the same equation used on airway pressure to calculate the stress index:
flow=a+b⋅timec
The parameter c, the exponent of time, is the Flow Index. Here are two examples:
Figure 4
Currently, the Flow Index can only be calculated through a mathematical analysis procedure of flow data. However, in the future, it could easily be automatically calculated by mechanical ventilators if further validations are added to the existing studies confirming its ability to identify over-assisted or under-assisted patients (3,4). In the meantime, the qualitative analysis of the flow curve remains highly applicable in clinical practice, allowing for the easy visual identification of patients with high or reduced inspiratory activity, i.e., with or without a downward concavity in the flow curve.
The uniqueness of the Flow Index compared to other estimates of respiratory muscle activity.
As mentioned earlier, in addition to the Flow Index, there are other indicators of inspiratory muscle activity. However, there is an important difference between the Flow Index and other indices of respiratory muscle activity. P0.1, PMI and dPocc are influenced by both pre-trigger and post-trigger inspiratory activity, whereas the Flow Index is influenced solely by post-trigger inspiratory activity. The clinical implication of this difference is crucial: let's try to understand why. Figure 5 presents the PTP, which we previously saw in Figure 1, in a slightly more complex form.
Figure 5
Inspiratory activity begins at line "A" (as can be seen from the onset of esophageal pressure reduction), but inspiratory flow starts only at line "C". The pre-trigger inspiratory activity, occurring before line "C" in the PTP, is due to both autoPEEP (PTPpeepi) and trigger activation (PTPtr) thresholds. The portion of PTP beyond line "C" (PTPpost) represents post-trigger inspiratory activity, occurring exclusively after the onset of inspiratory support (i.e., the increase in airway pressure, Paw), and coinciding with the portion of flow analyzed by the Flow Index.
Therefore, the Flow Index is sensitive only to the part of inspiratory effort that originates during inspiratory support. In other words, the Flow Index is a specific indicator of post-trigger inspiratory activity and among the ventilator parameters, it is influenced by the only parameter acting post-trigger, namely inspiratory support.
On the contrary, P0.1, PMI and dPocc are influenced by both pre- and post-trigger effort, and thus, both pre-trigger ventilator settings (inspiratory trigger and PEEP in relation to autoPEEP) and post-trigger settings (inspiratory support) have an impact on them.
The clinical implication is that in a patient with signs and symptoms of excessive respiratory muscle activity, a Flow Index > 1 (flow with downward concavity) suggests, as a first step, increasing inspiratory support. Conversely, a Flow Index ≤ 1 (flow that decreases linearly or with upward concavity) should indicate that post-trigger effort is already reduced and that it would be more effective to reduce pre-trigger effort by adjusting PEEP or the trigger, or by reducing autoPEEP with bronchodilator therapy and/or sitting position and/or diuretic therapy. Figure 6 provides an example of a patient with these characteristics.
Figure 6
Respiratory muscle activity begins at the white dashed vertical line, and flow initiation occurs at the red dashed vertical line. The entire decrease in esophageal pressure (Pes) occurs between these two lines. After the onset of flow, esophageal pressure does not further decrease, indicating minimal or no inspiratory muscle activity during this phase.
A significant weakness of the respiratory muscles (as evidenced by a very low maximum inspiratory pressure, see post dated 28/06/2013) may be the only exception to this approach. In this case, post-trigger inspiratory activity is low at any level of inspiratory support because the respiratory muscles are unable to generate higher pressure. In this condition, tidal volume solely depends on inspiratory support, and it is the only clinical condition where, in an active patient, pressure support needs to be primarily adjusted to achieve the desired tidal volume.
Conclusions
Here is a brief summary of the main points from today's post:
-Inspiratory variation in esophageal pressure is the true objective of inspiratory support. Therefore, esophageal pressure should be measured in patients under assisted ventilation, at least in those with prolonged weaning.
-The Flow Index is a numerical measurement of the concavity of the inspiratory flow and indirectly estimates post-trigger respiratory muscle activity.
-A Flow Index ≤ 1 (upward concavity or linear decay of inspiratory flow) indicates respiratory muscle passivity during inspiratory support (post-trigger).
-A Flow Index > 1 (downward concavity) indicates inspiratory activity during inspiratory support (post-trigger): the higher the Flow Index (i.e., the greater the downward concavity of the flow), the greater the inspiratory activity.
-In patients with signs of excessive inspiratory activity, it is advisable to increase pressure support if the Flow Index is > 1 (downward concavity), while optimizing PEEP and trigger or reducing autoPEEP is preferred if the Flow Index is ≤ 1 (upward concavity or linear decay). This approach may not be appropriate for patients with low maximum inspiratory pressure.
A smile and happy Easter to all of ventilab’s friends.
References
1. Bertoni M, Telias I, Urner M, et al.: A novel non-invasive method to detect excessively high respiratory effort and dynamic transpulmonary driving pressure during mechanical ventilation. Crit Care 2019; 23:346
2. Albani F, Pisani L, Ciabatti G, et al.: Flow Index: a novel, non-invasive, continuous, quantitative method to evaluate patient inspiratory effort during pressure support ventilation. Crit Care 2021; 25:196
3. Albani F, Fusina F, Ciabatti G, et al.: Flow Index accurately identifies breaths with low or high inspiratory effort during pressure support ventilation. Crit Care 2021; 25:427
4. Miao M-Y, Chen W, Zhou Y-M, et al.: Validation of the flow index to detect low inspiratory effort during pressure support ventilation. Ann Intensive Care 2022; 12:89
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