When Simple Solutions Spark Unnecessary Complexity

We are honored that a figure of Mr. Chatburn’s stature has dedicated attention to our work. We interpret this letter as another of his many efforts to improve conceptual accuracy and clarity in respiratory care, a tradition that values precise language as a cornerstone of both teaching and safe clinical practice. We agree: consistency and clarity are essential in science. That said, we offer a few clarifications and reflections.

The critique centers on our inclusion of the equation “P_mus = P_occ × k₁,” from which we omitted the “Δ” and the negative sign, potentially leading to clinical confusion. To clarify: we did not use that equation in our methods or analyses. It appears in the introduction only, and only as background information. It was intended to illustrate the broader interest in airway occlusion pressure (P_occ) as a surrogate for inspiratory effort.

We did not estimate the muscle pressure (P_mus). We did not apply or compute k₁. There were no regressions or transformations involving P_mus. Our aim was simpler: we compared P_occ values under different ventilatory conditions.

Our goal was to validate the P_occ in the context of noninvasive ventilation (NIV) using a simulated model. The reference was P_occ obtained with an intubated manikin connected to a simulator (ASL 5000, Ingmar Medical, Pittsburgh, Pennsylvania). In short, we compared P_occ-NIV to P_occ-ventilator. Estimating P_mus was not among our objectives, and the constant k₁, validated by Bertoni et al,¹ played no role in our methods.²

We did mention the P_mus levels programmed in the ASL5000, but only to report the settings used. We could have called them “low,” “medium,” and “high,” and our findings would remain unchanged.

Chatburn notes that we reported k₁ as 0.75, whereas Bertoni et al defined it as −0.75.¹ However, we didn’t apply either value. We never estimated P_mus. Our comparator was P_occ, not simulator-generated P_mus.

We understand the concern, when converting a negative pressure (ΔP_occ) to a positive muscular effort (P_mus), a negative coefficient is algebraically required. That makes perfect sense if one is performing that transformation.

Still, the observation prompts a valid academic question: Why didn’t we use the simulator-generated P_mus as the accepted standard? Because in mechanically ventilated patients, ΔP_occ is known to overestimate P_mus, as documented in Bertoni’s validation study¹ and confirmed by others.³ That’s why the original authors applied a correction factor: ΔP_occ × (−0.75). In simulation, the programmed P_mus will always exceed the pressure measured during an occlusion. As Chatburn rightly notes, “ΔP_occ should be smaller due to undetected losses of force in the system.” Using programmed P_mus as a comparator would predictably yield higher values than measured P_occ, recreating the discrepancy described: “Measured values of ΔP_occ were 4.2, 7.9, and 11.7 cm H₂O. Thus, k₁ was: 5/4.2 = 1.19, 10/7.9 = 1.27, and 15/11.7 = 1.28 (mean k₁ = 1.25).”

That would have added a new k₁, this time 1.25, but offered no clinical insight or physiological validity. Hence, we chose as GS the Pocc measured in an intubated configuration, allowing us to compare two ways of measuring the same variable: NIV-P_occ versus ventilator-P_occ.

Chatburn argues that, “in a perfect world,” P_occ and the airway occlusion pressure at 100 milliseconds after the onset of inspiration (P_0.1) should be negative, as they represent pressure drops, and that abandoning this convention may hinder effective teaching. The rationale is well-founded: both P_0.1 and P_occ reflect negative variations in airway pressure. However, during an occlusion, the equation P_aw + P_mus = 0 applies, meaning that P_mus equals “−P_aw.” In this context, the pressure drop (P_aw) is negative, whereas the muscular pressure (P_mus) is positive; two sides of the same equation.⁴ They describe the same physical phenomenon, viewed from different perspectives. The negative deflection in P_aw reflects the inspiratory effort generated by the patient. This duality is both physically accurate and entirely consistent with Newton’s third law.

As a community, we must be open to revisiting conventions, provided we retain conceptual clarity. Effective communication sometimes requires simplifying, not distorting, technical language. For instance, tidal volume (V_T) is routinely reported without the “Δ” symbol, even though it represents change in volume over time. Expired volume (V_Te) is reported as a positive value, though strictly [Vol(t₂) − Vol(t₁)] would be negative. Likewise, inspiratory flow is defined as positive, and expired volume is reported as a positive value despite being derived from a negative flow.

As you note, this makes algebraic sense if expiratory flow is defined as negative. But this is a ventilator-specific convention. In spirometry, the flow out of the patient is positive.

We believe that our notational choice is neither novel nor careless. Several highly cited studies^5–7 present negative pressures using positive values, often explicitly. Chatburn suggests that our formatting choice may have been driven by a desire to avoid rejection. On the contrary, it was a reasoned editorial decision. We aimed to align our work with the conventions most familiar to the clinical community, especially those engaged in noninvasive ventilation.

So we ask: is it always best to report a physiologically negative value as negative? Or does clarity sometimes justify reporting it as positive? Whitelaw, in 1975, has offered perspective.⁸ You can report pressure from the point of view of P_aw (negative) or P_mus (positive). Both are valid.

In today’s literature, P_occ and P_0.1 often function as quasi-nominal variables, indicating magnitude or threshold more than precise algebraic value. We used the dominant clinical convention, not to endorse confusion, but to reflect prevailing practice and facilitate interpretation. We appreciate Mr. Chatburn’s insightful commentary. Scientific disagreement, particularly when expressed respectfully, helps refine both thinking and practice.

Author Disclosure Statement

Mr. Gogniat provided services for Tecme Argentina. Mr. Steinberg received remuneration for teaching programs for Tecme, Argentina. Mr. Plotnikow received remuneration for teaching programs for Medtronic Argentina, ICUMedical Latin America, and Vapotherm Inc., USA.

Funding Information

No funding was received for this article.