The Pressure Paradox: Abdominal Compression to Detect Lung Hyper-Inflation in COVID-19 ARDS

Impact of Supine Versus Semirecumbent Body Posture on the Distribution of Ventilation in Acute Respiratory Distress Syndrome

In some patients with acute respiratory distress syndrome (ARDS), a paradoxical improvement in respiratory system compliance (CRS) has been observed when assuming a supine (head of bed [HOB] 0°) compared with semirecumbent (HOB 35-40°) posture. We sought to test the hypothesis that mechanically ventilated patients with ARDS would have improved CRS, due to changes in ventilation distribution, when moving from the semirecumbent to supine position. We conducted a prospective, observational ICU study including 14 mechanically ventilated patients with ARDS. For each patient, ventilation distribution (assessed by electrical impedance tomography) and pulmonary mechanics were compared in supine versus semirecumbent postures. Compared with semirecumbent, in the supine posture CRS increased (33 ± 21 vs. 26 ± 14 mL/cm H2O, p = 0.005), driving pressure was reduced (14 ± 6 vs. 17 ± 7 cm H2O, p < 0.001), and dorsal fraction of ventilation was decreased (48.5 ± 14.1% vs. 54.5 ± 12.0%, p = 0.003). Posture change from semirecumbent to supine resulted in a favorable physiologic response in terms of improved CRS and reduced driving pressure-with a corresponding increase in ventral ventilation, possibly related to reduced ventral overdistension.

Paradoxical response to chest wall loading predicts a favorable mechanical response to reduction in tidal volume or PEEP

Background

Chest wall loading has been shown to paradoxically improve respiratory system compliance (CRS) in patients with moderate to severe acute respiratory distress syndrome (ARDS). The most likely, albeit unconfirmed, mechanism is relief of end-tidal overdistension in ‘baby lungs’ of low-capacity. The purpose of this study was to define how small changes of tidal volume (VT) and positive end-expiratory pressure (PEEP) affect CRS (and its associated airway pressures) in patients with ARDS who demonstrate a paradoxical response to chest wall loading. We hypothesized that small reductions of VT or PEEP would alleviate overdistension and favorably affect CRS and conversely, that small increases of VT or PEEP would worsen CRS.

Methods

Prospective, multi-center physiologic study of seventeen patients with moderate to severe ARDS who demonstrated paradoxical responses to chest wall loading. All patients received mechanical ventilation in volume control mode and were passively ventilated. Airway pressures were measured before and after decreasing/increasing VT by 1 ml/kg predicted body weight and decreasing/increasing PEEP by 2.5 cmH2O.

Results

Decreasing either VT or PEEP improved CRS in all patients. Driving pressure (DP) decreased by a mean of 4.9 cmH2O (supine) and by 4.3 cmH2O (prone) after decreasing VT, and by a mean of 2.9 cmH2O (supine) and 2.2 cmH2O (prone) after decreasing PEEP. CRS increased by a mean of 3.1 ml/cmH2O (supine) and by 2.5 ml/cmH2O (prone) after decreasing VT. CRS increased by a mean of 5.2 ml/cmH2O (supine) and 3.6 ml/cmH2O (prone) after decreasing PEEP (P < 0.01 for all). Small increments of either VT or PEEP worsened CRS in the majority of patients.

Conclusion

Patients with a paradoxical response to chest wall loading demonstrate uniform improvement in both DP and CRS following a reduction in either VT or PEEP, findings in keeping with prior evidence suggesting its presence is a sign of end-tidal overdistension. The presence of ‘paradox’ should prompt re-evaluation of modifiable determinants of end-tidal overdistension, including VT, PEEP, and body position.

Chest wall loading in the ICU: pushes, weights, and positions

Clinicians monitor mechanical ventilatory support using airway pressures—primarily the plateau and driving pressure, which are considered by many to determine the safety of the applied tidal volume. These airway pressures are influenced not only by the ventilator prescription, but also by the mechanical properties of the respiratory system, which consists of the series-coupled lung and chest wall. Actively limiting chest wall expansion through external compression of the rib cage or abdomen is seldom performed in the ICU. Recent literature describing the respiratory mechanics of patients with late-stage, unresolving, ARDS, however, has raised awareness of the potential diagnostic (and perhaps therapeutic) value of this unfamiliar and somewhat counterintuitive practice. In these patients, interventions that reduce resting lung volume, such as loading the chest wall through application of external weights or manual pressure, or placing the torso in a more horizontal position, have unexpectedly improved tidal compliance of the lung and integrated respiratory system by reducing previously undetected end-tidal hyperinflation. In this interpretive review, we first describe underappreciated lung and chest wall interactions that are clinically relevant to both normal individuals and to the acutely ill who receive ventilatory support. We then apply these physiologic principles, in addition to published clinical observation, to illustrate the utility of chest wall modification for the purposes of detecting end-tidal hyperinflation in everyday practice.

Heterogeneity of Ventilation/Perfusion Mismatch at Different Levels of PEEP and in Mechanical Phenotypes of COVID-19 ARDS

BACKGROUND

COVID-19-related ARDS is characterized by severe hypoxemia with initially preserved lung compliance and impaired ventilation/perfusion (V̇/Q̇) matching. PEEP can increase end-expiratory lung volume, but its effect on V̇/Q̇ mismatch in COVID-19-related ARDS is not clear.

METHODS

We enrolled intubated and mechanically ventilated subjects with COVID-19 ARDS and used the automatic lung parameter estimator (ALPE) to measure V̇/Q̇. Respiratory mechanics measurements, shunt, and V̇/Q̇ mismatch (low V̇/Q̇ and high V̇/Q̇) were collected at 3 PEEP levels (clinical PEEP = intermediate PEEP, low PEEP [clinical - 50%], and high PEEP [clinical + 50%]). A mixed-effect model was used to evaluate the impact of PEEP on V̇/Q̇. We also investigated if PEEP might have a different effect on V̇/Q̇ mismatch in 2 different respiratory mechanics phenotypes, that is, high elastance/low compliance (phenotype H) and low elastance/high compliance (phenotype L).

RESULTS

Seventeen subjects with COVID-related ARDS age 66 [60-71] y with a PaO2 /FIO2 of 141 ± 74 mm Hg were studied at low PEEP = 5.6 ± 2.2 cm H2O, intermediate PEEP = 10.6 ± 3.8 cm H2O, and high PEEP = 15 ± 5 cm H2O. Shunt, low V̇/Q̇, high V̇/Q̇, and alveolar dead space were not significantly influenced, on average, by PEEP. Respiratory system compliance decreased significantly when increasing PEEP without significant variation of PaO2 /FIO2 (P = .26). In the 2 phenotypes, PEEP had opposite effects on shunt, with a decrease in the phenotype L and an increase in phenotype H (P = .048).

CONCLUSIONS

In subjects with COVID-related ARDS placed on invasive mechanical ventilation for > 48 h, PEEP had a heterogeneous effect on V̇/Q̇ mismatch and, on average, higher levels were not able to reduce shunt. The subject's compliance could influence the effect of PEEP on V̇/Q̇ mismatch since an increased shunt was observed in subjects with lower compliance, whereas the opposite occurred in those with higher compliance.

Paradoxical Positioning: Does "Head Up" Always Improve Mechanics and Lung Protection?

OBJECTIVES

Head-elevated body positioning, a default clinical practice, predictably increases end-expiratory transpulmonary pressure and aerated lung volume. In acute respiratory distress syndrome (ARDS), however, the net effect of such vertical inclination on tidal mechanics depends upon whether lung recruitment or overdistension predominates. We hypothesized that in moderate to severe ARDS, bed inclination toward vertical unloads the chest wall but adversely affects overall respiratory system compliance (C rs ).

DESIGN

Prospective physiologic study.

SETTING

Two medical ICUs in the United States.

PATIENTS

Seventeen patients with ARDS, predominantly moderate to severe.

INTERVENTION

Patients were ventilated passively by volume control. We measured airway pressures at baseline (noninclined) and following bed inclination toward vertical by an additional 15°. At baseline and following inclination, we manually loaded the chest wall to determine if C rs increased or paradoxically declined, suggestive of end-tidal overdistension.

MEASUREMENTS AND MAIN RESULTS

Inclination resulted in a higher plateau pressure (supineΔ: 2.8 ± 3.3 cm H 2 O [ p = 0.01]; proneΔ: 3.3 ± 2.5 cm H 2 O [ p = 0.004]), higher driving pressure (supineΔ: 2.9 ± 3.3 cm H 2 O [ p = 0.01]; proneΔ: 3.3 ± 2.8 cm H 2 O [ p = 0.007]), and lower C rs (supine Δ: 3.4 ± 3.7 mL/cm H 2 O [ p = 0.01]; proneΔ: 3.1 ± 3.2 mL/cm H 2 O [ p = 0.02]). Following inclination, manual loading of the chest wall restored C rs and driving pressure to baseline (preinclination) values.

CONCLUSIONS

In advanced ARDS, bed inclination toward vertical adversely affects C rs and therefore affects the numerical values for plateau and driving tidal pressures commonly targeted in lung protective strategies. These changes are fully reversed with manual loading of the chest wall, suggestive of end-tidal overdistension in the upright position. Body inclination should be considered a modifiable determinant of transpulmonary pressure and lung protection, directionally similar to tidal volume and positive end-expiratory pressure.

Paradoxical response to chest wall loading predicts a favorable mechanical response to reduction in tidal volume or PEEP

BACKGROUND

Chest wall loading has been shown to paradoxically improve respiratory system compliance (C_RS) in patients with moderate to severe acute respiratory distress syndrome (ARDS). The most likely, albeit unconfirmed, mechanism is relief of end-tidal overdistension in 'baby lungs' of low-capacity. The purpose of this study was to define how small changes of tidal volume (V_T) and positive end-expiratory pressure (PEEP) affect C_RS (and its associated airway pressures) in patients with ARDS who demonstrate a paradoxical response to chest wall loading. We hypothesized that small reductions of V_T or PEEP would alleviate overdistension and favorably affect C_RS and conversely, that small increases of V_T or PEEP would worsen C_RS.

METHODS

Prospective, multi-center physiologic study of seventeen patients with moderate to severe ARDS who demonstrated paradoxical responses to chest wall loading. All patients received mechanical ventilation in volume control mode and were passively ventilated. Airway pressures were measured before and after decreasing/increasing V_T by 1 ml/kg predicted body weight and decreasing/increasing PEEP by 2.5 cmH₂O.

RESULTS

Decreasing either V_T or PEEP improved C_RS in all patients. Driving pressure (DP) decreased by a mean of 4.9 cmH₂O (supine) and by 4.3 cmH₂O (prone) after decreasing V_T, and by a mean of 2.9 cmH₂O (supine) and 2.2 cmH₂O (prone) after decreasing PEEP. C_RS increased by a mean of 3.1 ml/cmH₂O (supine) and by 2.5 ml/cmH₂O (prone) after decreasing V_T. C_RS increased by a mean of 5.2 ml/cmH₂O (supine) and 3.6 ml/cmH₂O (prone) after decreasing PEEP (P < 0.01 for all). Small increments of either V_T or PEEP worsened C_RS in the majority of patients.

CONCLUSION

Patients with a paradoxical response to chest wall loading demonstrate uniform improvement in both DP and C_RS following a reduction in either V_T or PEEP, findings in keeping with prior evidence suggesting its presence is a sign of end-tidal overdistension. The presence of 'paradox' should prompt re-evaluation of modifiable determinants of end-tidal overdistension, including V_T, PEEP, and body position.

Monitoring Lung Injury Severity and Ventilation Intensity during Mechanical Ventilation

Acute respiratory distress syndrome (ARDS) is a severe form of respiratory failure burden by high hospital mortality. No specific pharmacologic treatment is currently available and its ventilatory management is a key strategy to allow reparative and regenerative lung tissue processes. Unfortunately, a poor management of mechanical ventilation can induce ventilation induced lung injury (VILI) caused by physical and biological forces which are at play. Different parameters have been described over the years to assess lung injury severity and facilitate optimization of mechanical ventilation. Indices of lung injury severity include variables related to gas exchange abnormalities, ventilatory setting and respiratory mechanics, ventilation intensity, and the presence of lung hyperinflation versus derecruitment. Recently, specific indexes have been proposed to quantify the stress and the strain released over time using more comprehensive algorithms of calculation such as the mechanical power, and the interaction between driving pressure (DP) and respiratory rate (RR) in the novel DP multiplied by four plus RR [(4 × DP) + RR] index. These new parameters introduce the concept of ventilation intensity as contributing factor of VILI. Ventilation intensity should be taken into account to optimize protective mechanical ventilation strategies, with the aim to reduce intensity to the lowest level required to maintain gas exchange to reduce the potential for VILI. This is further gaining relevance in the current era of phenotyping and enrichment strategies in ARDS.