The oesophageal balloon for respiratory monitoring in ventilated patients: updated clinical review and practical aspects

Abstract

There is a well-recognised importance for personalising mechanical ventilation settings to protect the lungs and the diaphragm for each individual patient. Measurement of oesophageal pressure (P _oes) as an estimate of pleural pressure allows assessment of partitioned respiratory mechanics and quantification of lung stress, which helps our understanding of the patient's respiratory physiology and could guide individualisation of ventilator settings. Oesophageal manometry also allows breathing effort quantification, which could contribute to improving settings during assisted ventilation and mechanical ventilation weaning. In parallel with technological improvements, P _oes monitoring is now available for daily clinical practice. This review provides a fundamental understanding of the relevant physiological concepts that can be assessed using P _oes measurements, both during spontaneous breathing and mechanical ventilation. We also present a practical approach for implementing oesophageal manometry at the bedside. While more clinical data are awaited to confirm the benefits of P _oes-guided mechanical ventilation and to determine optimal targets under different conditions, we discuss potential practical approaches, including positive end-expiratory pressure setting in controlled ventilation and assessment of inspiratory effort during assisted modes.

FIGURE 1

Equation of motion of the respiratory system, including the components of the lung and chest wall transmural pressures. Resistive pressure (P_res) is the pressure needed to overcome airway resistance. Elastic pressure (P_el) is the pressure needed to expand the lungs and the chest wall. P₀ is the pressure inside the respiratory system at the end of expiration, which is zero in non-ventilated patients, since all pressures are measured relative to atmospheric pressure, or is referred to as total positive end-expiratory pressure in ventilated patients. P_alv: alveolar pressure; P_rs: transmural pressure of the respiratory system (transrespiratory system pressure); P_L: transmural pressure of the lungs (transpulmonary pressure); P_cw: transmural pressure of the chest wall (pressure across the chest wall); P_pl: pleural pressure; E_L: lung elastance; E_cw: chest wall elastance.

FIGURE 2

Conceptual illustrations of key respiratory physiology during a) spontaneous breathing in a non-ventilated subject, b) fully controlled mechanical ventilation with a passive patient and c) assisted mechanical ventilation. For full description, see main text. All pressures are described in cmH₂O. a) During spontaneous breathing in healthy conditions, pleural pressure (P_pl) is slightly negative at the end of expiration (situation 2). The pressure generated by the respiratory muscle pump (P_mus) creates a further drop in P_pl. P_pl is transmitted to the alveoli resulting in a negative alveolar pressure (P_alv) and a pressure gradient between the airway opening pressure (P_ao) and P_alv allowing tidal volume to enter (situation 2). P_mus is the pressure needed to generate chest wall expansion as well as a drop in P_pl; therefore, P_mus is the difference between the chest wall pressure (P_cw) and the P_pl, and maximum P_mus occurs at the end of inspiration (situation 3). P_cw is calculated by multiplying the instantaneous lung volume by the chest wall elastance (E_cw). E_cw can be obtained during passive lung inflation or calculated as 4% of predicted vital capacity [20]. b) During passive ventilator insufflation P_pl increases and represents the P_cw. Circuit occlusions are required to assess static transmural pressure of the lungs (transpulmonary pressure (P_L)). When there is no flow and with the airways fully open, P_alv represents the airway pressure (P_aw) during the occlusion: total positive end-expiratory pressure (PEEP_tot) and plateau pressure (P_plat) for end-expiratory and end-inspiratory occlusions, respectively. P_plat is thus the sum of the P_L and P_cw during the end-inspiratory occlusion. Likewise, the respiratory system driving pressure (ΔP=P_plat−PEEP_tot) includes both the lung driving pressure (ΔP_L) and the driving pressure expanding the chest wall (ΔP_cw). The direct method and elastance-derived method to calculate P_L are presented. c) During assisted mechanical ventilation, both the ventilator pressure (P_vent) and P_mus contribute to lung inflation. The swing in dynamic P_L (ΔP_L,dyn) is computed as peak P_L−end-expiratory P_L and therefore is different from the static ΔP_L (circuit occlusions), which can be difficult to obtain/read in actively breathing patients. P_alv is thus not necessarily equal to P_aw at end-expiration and end-inspiration. P_oes: oesophageal pressure; E_rs: respiratory system elastance; E_L: lung elastance.

FIGURE 3

Oesophageal manometry: a practical step-by-step approach to oesophageal pressure (P_oes) measurements in clinical practice. For further clinical and scientific details, see main text. P_aw: airway pressure; P_L: transmural pressure of the lungs (transpulmonary pressure).

FIGURE 4

Summary of suggested steps for oesophageal pressure (P_oes)-guided titration of mechanical ventilation in acute respiratory distress syndrome during controlled mechanical ventilation. The procedure should be performed sequentially with step a), then step b) and finally step c). The level of evidence is mentioned. All pressures are described in cmH₂O. P_L: transmural pressure of the lungs (transpulmonary pressure); PEEP: positive end-expiratory pressure; P_aw: airway pressure; P_oes,end-exp: end-expiratory P_oes; PEEP_tot: total PEEP; V_T: tidal volume; P_L,end-exp: end-expiratory P_L; P_L,end-insp: end-inspiratory P_L; P_plat: plateau pressure; E_rs: respiratory system elastance; E_L: lung elastance.

FIGURE 5

Example of oesophageal manometry during assisted mechanical ventilation. A double-balloon naso-gastric catheter was inserted for simultaneous measurement of oesophageal pressure (P_oes) and gastric pressure (P_ga) and the resulting transdiaphragmatic pressure (P_di). Dynamic transpulmonary pressure (P_L,dyn) was obtained in real-time as airway pressure (P_aw)−P_oes. P_oes measurements revealed patient–ventilator asynchrony delayed cycling-off (grey area and arrows in P_aw signal): at the time of ventilator cycling-off, P_oes and P_di were already back to their baseline value, indicating that the patient's neural inspiratory time was shorter than the ventilator inspiratory time. In addition, the patient demonstrated high breathing effort with P_oes swings of 15 cmH₂O and P_di swings of 20 cmH₂O, resulting in ΔP_L,dyn >25 cmH₂O. Note that the end of neural inspiratory time (start of grey area) is presented just after the nadir in P_oes, but the exact timing is debatable.