Assessing inspiratory efforts in intubated patients

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Aman Thind
Critical care medicine fellow at the Cleveland Clinic. Interests: Cardiopulmonary physiology, shock, POCUS, mechanical ventilation, and ARDS. Music genres: Blues, Rock, and Heavy metal

The Pre-brief

A 48-year old patient with ARDS is being mechanically ventilated. His respiratory mechanics and imaging are improving but he is not ready for extubation. He is on PRVC mode with set tidal volume of 5 cc/kg. He appears to be having increased work of breathing. How can you objectively assess for inspiratory muscle effort in this patient?

Introduction

In a recent post, we discussed the utility of CVP and PAWP tracings to estimate the intensity of inspiratory efforts. We also alluded to the possibility of performing this assessment in intubated patients using certain maneuvers. The objective of this post is to learn more about these.

What are we trying to estimate? – The gold standard:

From the mechanics standpoint, we are trying to evaluate the intensity of inspiratory Pmus (muscular effort of the patient). The pressure-time product (PTP) of Pmus is the gold standard for quantifying respiratory effort (Figure 1).

The direct estimation of Pmus requires insertion of an esophageal balloon that allows measurement of esophageal pressure (Pes): a surrogate for pleural pressure (Ppl). During inspiration, Pmus is estimated as follows – 

Pmus =  {Ew x V(t)} – ΔPes(t)

Where,

Ew = Chest wall elastance. This can either be measured in passive conditions by calculating ΔPpl  with a tidal breath. Alternatively, empirical values of Ew can be used.

V(t) = volume of gas inhaled at time ‘t’ after initiation of the breath

ΔPes(t) = change in Pes from baseline at time ‘t’ after initiation of the breath. Note that the negative sign here means that its value is additive. E.g. if Pes goes from zero to –5 cmH2O, ΔPes = –5 cmH2O.

 

A detailed discussion on the theoretical basis of this measurement requires a separate discussion. However, the key point to note is that the value of Pmus always exceeds the value of ΔPes. This excess is a function of (i) chest wall elastance, and (ii) change in volume.

Alternative method: Pocc – 

Since esophageal pressure monitoring (more on that here and here) is not widely available, alternative methods of estimating inspiratory effort are required.

What is ΔPocc?

This newer technique for the estimation of inspiratory Pmus was recently described in 2019 by Bertoni et al. To measure ΔPocc, an end-expiratory airway occlusion is performed. There is a distinct option to perform this in most common ventilators. On some ventilators (e.g. PB840), a ‘NIF maneuver’ would have to be used to create end-expiratory airway occlusion.

Patient’s respiratory effort against the occluded airway creates a negative deflection in the airway pressure (Paw). The difference between the baseline Paw and the nadir of Paw during the negative deflection is defined as ‘ΔPocc’. Mechanistically, ΔPocc precisely mirrors ΔPes as the airway is occluded and there is no airflow during the inspiratory effort. A representative example (case from last post’s pre-brief) is shown in Figure 2.

How does ΔPocc compare with actual inspiratory Pmus ?

There are two major considerations while estimating Pmus from ΔPocc:

  1. Remember that ΔPocc mirrors ΔPes during the ‘occluded breath’. As discussed before, Pmus always exceeds ΔPes. Hence, the peak inspiratory Pmus during the occluded breath would actually exceed the measured ΔPocc value.
  2. For a given respiratory drive and neural stimulus, inspiratory Pmus generated during an occluded airway would always be higher than that generated during a tidal breath. The mechanism of this effect is rooted in the force-velocity relationship of skeletal muscles (Figure 3). Briefly, force generated during muscle contraction is inversely proportional to the velocity of muscle shortening. Since there is no respiratory muscle shortening during an occluded breath, the force generated is significantly higher.

How do I use this information?

The concept of lung protective ventilation is widely appreciated in patients with ARDS. More recently, the concept of ‘diaphragm protective ventilation’ has been described. The core idea behind this is the observation is that both under-assistance and over-assistance from the ventilator can lead to diaphragm dysfunction so we should aim for the happy medium. Of note, these two concepts are sometimes at odds and the clinician has to make the decision of when to enforce one over the other. E.g. in early severe ARDS, lung protective ventilation should take precedence even if that comes at the cost of requiring neuromuscular blockade (the ultimate form of ‘over-assistance’). ‘Diaphragm protection’ can perhaps be made a priority as the disease process starts to improve.

Inspiratory efforts can certainly be examined clinically. Patients in respiratory distress with high work of breathing recruit accessory muscles e.g. sternomastoid. Activation of sternomastoid can be assessed via inspection and palpation. However, the ability to objectively measure inspiratory Pmus can be very helpful while titrating ventilatory support to avoid both over- and under-assistance. Goligher et al propose a predicted Pmus 5 to 10 cmH2O (ΔPocc 8 to 20 cmH2O) as the ideal range for inspiratory effort. Let’s review the case in the pre-brief. Setting very low tidal volume on PRVC mode is a common cause of ventilatory under-assistance. Switching to pressure support is a viable option. This is a good scenario where objective assessment of Pmus (with ΔPocc) would be helpful while titrating pressure support.*

The Debrief

  • In assisted mechanical ventilation, the ventilatory load is shared by Pvent (positive airway pressure applied by the ventilator) and Pmus (inspiratory muscular pressure of the patient).
  • The concept of ‘diaphragm-protective ventilation’ strives to provide adequate Pvent, so that both over-assistance and under-assistance are prevented.
  • Although the gold standard of Pmus measurement requires utilization of an esophageal balloon, ΔPocc obtained by applying end-expiratory occlusion can be used to reliably estimate peak inspiratory Pmus.
  • This information can then be used to fine-titrate ventilatory support (Pvent).*

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 23, 346 (2019). 
  2. Mauri T, Guérin C, Hubmayr R. The ten pressures of the respiratory system during assisted breathing. Intensive Care Med. 2017 Oct;43(10):1504-1506.
  3. Goligher EC, Dres M, Patel BK et al. Lung- and Diaphragm-Protective Ventilation. Am J Respir Crit Care Med. 2020 Oct 1;202(7):950-961.
  4. Goligher EC, Jonkman AH, Dianti J et al. Clinical strategies for implementing lung and diaphragm-protective ventilation: avoiding insufficient and excessive effort. Intensive Care Med. 2020 Dec;46(12):2314-2326.

*Appendix

In an under-assisted patient with very high Pmus, the hope is that if we increase the ventilatory support (Pvent), Pmus will go down so that the tidal volume remains about the same. This is indeed what happens in many patients. However, this may not always be the case. E.g., certain patients may have very high respiratory drive due to pain, anxiety, acidosis etc. These patients may maintain the high Pmus despite an increase in Pvent, thereby resulting in an increase in tidal volume and minute ventilation. Attention to these factors that increase respiratory drive is thus warranted.



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