PEEP levels in COVID-19 pneumonia

To the Editor:

Recently, the Surviving Sepsis Campaign COVID-19 guidelines and ATS suggest that a ventilatory strategy complying with the ARDSnet protocol should be applied to manage COVID-19 pneumonia [1,2,3]. However, “COVID-19 lung” pathophysiology seems to be divergent from the “ARDS lung”; hence, heart-lung interactions may be more pronounced than initially considered [2, 3].

We studied 17 patients (March 20–April 14, 2020) treated in two Greek University Intensive Care Units. Patients had COVID-19 pneumonia fulfilling the Berlin criteria of acute respiratory distress syndrome (ARDS) and were on the 2nd or 3rd day of invasive mechanical ventilation. Positive end-expiratory pressure (PEEP) was set according to predefined criteria [1,2,3]. Mean tidal volume (± standard deviation) was 6.8 ± 0.9 ml/kg ideal body weight (469 ± 64 ml), respiratory rate was 29.5 ± 3.7 breaths/min, and the fraction of inspired oxygen was 82 ± 12%. After measuring respiratory mechanics, arterial blood gases, and hemodynamics, we decreased PEEP by 25–30% (other mechanical ventilation variables remained stable). We re-evaluated measurements (1 h later) focusing on the effects of PEEP reduction on respiratory mechanics, hemodynamics, and fluid balance in a 12-h window before and after the PEEP change.

Mean PEEP reduction by 29% significantly increased respiratory system compliance and reduced hypercapnia, while oxygenation (PaO₂/FiO₂) did not worsen (Table 1). PEEP reduction was not accompanied by lung de-recruitment, as oxygenation was not deteriorated. Rather PEEP reduction decreased lung overdistension as interpreted by the increase in respiratory system compliance and decrease in dead space ventilation (reduced PaCO₂). Concerning hemodynamics, PEEP reduction was followed by a substantial decrease in noradrenaline dose, most likely indicating improvement in cardiac output that allowed decrease in vasopressor dose, while blood pressure remained constant.

Table 1 Respiratory and hemodynamic data in 17 mechanically ventilated patients with COVID-19, before and after PEEP reduction

Application of unnecessary high PEEP (i.e., when PEEP does not result in lung recruitment) increases transpulmonary pressures, forcing West Zone 3 lung regions to Zones 2 and 1. This phenomenon increases dead space ventilation, resulting in hypercapnia when minute ventilation remains constant. Pulmonary vascular resistance increases, leading to a variable degree of right ventricular dysfunction due to ventriculo-arterial uncoupling. PEEP effects may be exacerbated when the pulmonary vascular bed is injured, as in ARDS, where intravascular obstruction is observed due to thromboemboli [4]. A hypercoagulable state and extensive lung capillary thrombosis has also been reported in COVID-19 [5].

When lung compliance is relatively normal, substantial amount of the alveolar pressure is transmitted to the pleura. Therefore, relatively high PEEP in a non-recruitable lung with almost normal compliance may significantly increase pleural pressure and have a detrimental impact on hemodynamics by deteriorating venous return. A conservative or de-resuscitative fluid strategy is recommended in the management of patients with ARDS [6]. Yet, in our patients, a strategy of increased PEEP approximating the ARDSnet protocol was accompanied by substantial vasopressor dosage and 12-h fluid balance. PEEP de-escalation led to significant reduction of cumulative fluid balance during the following 12 h and a three-fold decrease of vasopressor dosage. Decreased need for vasopressors and better fluid management translates in increased cardiac output and organ perfusion accompanied with less fluid accumulation in the lungs.

COVID-19 lung involvement is unique having a “pneumonia pattern” than being a typical “ARDS pattern” at least in the initial phase during the first days after intubation, and this was rather the case in our patient group [5]. Therefore, mechanical ventilation should be applied differently from the routinely followed ARDS protocol. Intensivists have to be cautious in discriminating between pneumonia and typical ARDS. Failure to discriminate between COVID-19 pneumonia and ARDS and adoption of erroneous ventilatory strategy may have detrimental effects on hemodynamics, resulting in organ hypoperfusion and ultimately multiorgan failure.