In this post hoc analysis of the FACCT trial, we observed that 331 of 359 or 92% of patients had an mPAP exceeding 20 mmHg (3.3 kPa). This incidence is similar to a prospective study by Beiderlinden et al. [15] of 103 patients who reported a pulmonary hypertension incidence of 92.2% in ARDS patients. As ARDS-related pulmonary hypertension progresses, the frequency of ARDS-related Acute Cor Pulmonale (ACP) increases, occurring in up 28% of all ARDS patients [16]. Importantly, it is associated with increased mortality [16].
As the echocardiographic features of ACP are reversible and may be treated by a range of therapeutic modalities, including adjustment of ventilator settings, prone positioning, fluid management and vasoactive drugs early diagnosis is important. Dessap et al. performed a prospective study of 752 patients with moderate-to-severe ARDS receiving lung protective ventilation and found that 22% had evidence of ACP [3]. Splitting patients into a derivation and validation cohort, they identified four variables: pneumonia as a cause of ARDS, driving pressure ≥ 18 cm H2O, arterial oxygen partial pressure to fractional inspired oxygen (PaO2/FiO2) ratio < 150 mmHg and arterial carbon dioxide partial pressure ≥ 48 mmHg, which comprised the ACP risk score. The ACP risk score was subsequently validated by Li et al. [17] in a retrospective analysis of 2434 patients. Additionally, the ACP score correlated with an increased incidence of pulmonary hypertension in patients with ARDS, with each increase in ACP score associated with an increase in the prevalence of pulmonary hypertension.
We have used mPAP as a marker of pulmonary hypertension [13] and as mPAP reflects right ventricular afterload, which is a major determinant of ACP, a surrogate marker of the risk of ACP. We investigated the association of mPAP with several ventilation parameters in ARDS from our previously described derivation cohort. All the ventilation parameters demonstrated a significant correlation with mPAP in our initial univariate analysis, except tidal volume (once adjusted for respiratory rate).
The subsequent multivariate analysis demonstrated that the blood pH level, P/F ratio, PaCO2 level, mean airway pressure and the mechanical power indexed to compliance were independently associated with mPAP. All of these variables represent potential independent surrogate markers of the risk of ACP. Amongst these variables, the mechanical power indexed to compliance, which is a representation of the energy applied per minute to the amount of well-aerated lung tissue in ARDS patients, has never previously been shown to have an association with mPAP. This may help to partially explain the results of a previous study in 2020 by Copolla et al. [18] demonstrating that mechanical power normalised to compliance is independently associated with intensive care mortality in ARDS patients.
To date, the only published description of right ventricular protective mechanical ventilation was by Paternot et al. in 2016 [6]. In this opinion article, they describe an approach targeting four variables; plateau pressure < 28 cmH20, driving pressure < 18 cmH20, P/F ratio > 150 mmHg and PaCO2 < 48 mmHg. The optimal PEEP for this approach was stated to be controversial but was described as the PEEP associated with the best PaO2/FIO2 ratio without altering compliance. In our post hoc analysis, all the components suggested by Paternot et al. have demonstrated a strong correlation in our univariate analysis with mPAP. Additionally, the P/F ratio and PaCO2 were demonstrated to be independently associated with the prevalence of raised mPAP. Both of these results add further validation to this approach.
Given the data from this analysis, it is possible to further elucidate what represents right ventricular protective ventilation. Specifically, the addition of respiratory rate appears an important variable for consideration in any future approach. Respiratory rates above 17 cycles per minute demonstrated an incremental increase in mPAP in the final extended nonlinear analysis. The respiratory rate is also an integral part of the mechanical power equation, which when indexed to compliance is independently associated with mPAP. Therefore, increases in tidal volume (within the limitation of driving pressure < 18 cmH20) may represent a more right ventricular protective way to control CO2 and pH. This is consistent with a prospective study by Vieillard-Baron et al. [19] which concluded that high respiratory rates in ARDS patients did not improve CO2 clearance, produced dynamic hyperinflation and impaired right ventricular ejection.
There are a number of weaknesses to this study. The retrospective nature of this post hoc analysis means these data are hypothesis generating only. We used mean pulmonary artery pressure as a marker of pulmonary hypertension in ARDS, to align with international definitions [13]. However, we appreciate there are other markers of PVR that could have been used in its place [20]. As the other arm of the trial used for this post hoc analysis was fluid administration in ARDS, there is a risk of unequal fluid resuscitation in our derivation cohort which could also affect our findings [11]. Another weakness of this analysis is that the cohort identified had a very high mortality rate and we were unable to perform a mortality analysis with enough statistical power due to this imbalance.
It should also be noted that the pulmonary haemodynamic data used for this analysis have come from this singular trial, and that data from PACs are subject to errors both in measurement and interpretation of their waveform and generated haemodynamic data as demonstrated by Parviainen et al. [21]. This trial did not undertake routine echocardiography on all participants to be able to explore whether there would be echocardiographic surrogate findings along the same lines as the PAC data. This is particularly pertinent due to the prominence and ubiquity of echocardiography in current critical care practice, as well as having dedicated echocardiographic protocols and definitions of right heart dysfunction and ACP. Whether the data used to inform such a decision-making process in the future should be echocardiogram finding-driven or PAC-driven is beyond the scope of a post hoc analysis. However, a recent publication by Cloverdale et al. [22] has questioned whether there could be a role for the return of the PAC to intensive care practice.
In conclusion, the associations identified from this analysis would suggest that classical ARDS lung protective strategies, including low tidal volume ventilation and permissive hypercapnia, may negatively impact the management of the subset of ARDS patients with associated right ventricular dysfunction or ACP. The index of suspicion of such dysfunction, coupled with the ACP score, may help generate a dedicated study to evaluate for this pathology, and subsequently tailor ventilatory management not just to a lung protective strategy, but to a lung and right heart protective one. The previously described right ventricular protective ventilation strategy by Paternot et al. has been validated. The addition of respiratory rate to future right ventricular protective strategies, given its strong association with mPAP, its integral part in the mechanical power equation and its inflexion point at 17 cycles per minute, is strongly recommended.