Hemidiaphragm work in large pleural effusion and its insignificant impact on blood gases: a new insight based on in silico study

Abstract

Objective: Computer simulations, enabling observations of variables inaccessible in living patients, provide a powerful approach to studying complex physiological phenomena. This in silico study presents the use of a virtual patient to investigate the impact of large pleural effusion (PE) and therapeutic thoracentesis (TT) on hemidiaphragm function and arterial blood gases.

Methods: Inspired by unexpected phenomena observed in living patients undergoing large-volume TT, we formulated four questions regarding this impact. To answer these questions, we simulated right-sided PE in our virtual patient and studied changes in the pleural pressure in the ipsilateral hemithorax (Ppli) and lung volume during the respiratory cycle (exemplified by Ppli-V loops, where V is the volume of both lungs), airflows in the main bronchi, and alveolar O2 (PAO2) and CO2 (PACO2) partial pressures.

Results: Simulations highlighted that: (a) mediastinal compliance critically affects hemidiaphragm work; (b) the 8-shaped Ppli-V loops are associated with hemidiaphragm inversion, where exhalation from the ipsilateral lung occurs during a part of both the inspiratory and expiratory phases, and vice versa; (c) pre-TT PAO2 may be elevated due to reduction of the tidal volume to end-expiratory lung volume ratio; and (d) pre-TT Ppli amplitudes during respiration can exceed post-TT values when mediastinal compliance is high.

Conclusion: Our findings emphasize the significance of mediastinal compliance in pleural effusion physiology and suggest insignificant influence of the ipsilateral hemidiaphragm inverted due to large PE on arterial gas tensions. This study underscores the utility of virtual patient models for elucidating unexpected physiological behaviors and optimizing clinical interventions.

Keywords: arterial blood gases; hemidiaphragm function; hemidiaphragm inversion; in silico study; large pleural effusion; pendulum breathing; therapeutic thoracentesis; virtual patient.

FIGURE 1

The P-V loops: living patients. P – the pleural pressure in the ipsilateral hemithorax, V – the change in the total lung volume during inspiration (green/light) and expiration (red/dark). 1,2 and 3 - the loops from measurements before pleural fluid withdrawal, after a withdrawal of approximately 1.9 L and after withdrawal of approximately 3.8 L, respectively. p1 … p8 – living patients from Table 1.

FIGURE 2

Changes in selected parameters during therapeutic thoracentesis in living patients. P_tcO₂ and P_tcCO₂- transcutaneous oxygen and carbon dioxide pressure, respectively; V_E – minute ventilation; _∼CO – the product of heart rate and pulse pressure (an estimation of cardiac output); P_{pl_ampl} – the median value of the pleural pressure amplitude form intervals recorded between aspiration of subsequent pleural fluid portions; lines - linear regression of a parameter on time (A–D) or volume (E) calculated for the first and the second stages of therapeutic thoracentesis. p1 … p7 – living patients from Table A1 (P_tcO₂ and P_tcCO₂ were not measured in the patient p8 due to technical problems).

FIGURE 3

An example of P-V loop changes during therapeutic thoracentesis. Although pleural pressure at functional residual capacity (FRC) moderately fell as pleural fluid was withdrawn, the pleural pressure amplitude increased dramatically, particularly after withdrawal of 1,000 mL of the fluid (which was the reason for therapeutic thoracentesis termination) despite that tidal volume did not change. Presumably, this is the case of patient with the nonexpendable lung and very compliant mediastinum.

FIGURE 4

Examples of simulated P-V loops for compliant (A) and stiff (B) mediastinum. P–the pleural pressure in the ipsilateral hemithorax, V–the change in the total lung volume during inspiration (green) and expiration (red).

FIGURE 5

Explanation of the 8-shaped P-V loop. (a–d) an illustration of hemidiaphragms, rib cage and mediastinum movements and flows in main bronchi during (a) the inspiration beginning, (b) the second inspiration phase, (c) the expiration beginning, and (d) the second expiration phase (green/red arrows–fresh/processed air; black arrows–movement of the structures; P↓/P↑ – decrease/increase in P_pl and corresponding change in P_A); (e) the simulated 8-shaped P-V loop from Figure 1B; the letters a-d correspond to (a–d) (see the text for details).

FIGURE 6

Examples of simulated airflows in the main bronchi and alveolar O₂ and CO₂ partial pressures: the virtual patient with PE in the right hemithorax. (a, c) inflow to the left bronchus (blue) and the right bronchus (black and in one respiratory cycle: light/dark green–inflow/outflow during inspiration, light/dark red–inflow/outflow during expiration; these colors correspond to the colors in Figure 2E); (b, d) the mean values of the alveolar oxygen partial pressure (P_AO₂) and the alveolar carbon dioxide partial pressure (P_ACO₂), respectively.