Respiratory Variations of Central Venous Pressure as Indices of Pleural Pressure Swings: A Narrative Review

Abstract

The measurement of pleural (or intrathoracic) pressure is a key element for a proper setting of mechanical ventilator assistance as both under- and over-assistance may cause detrimental effects on both the lungs and the diaphragm. Esophageal pressure (Pes) is the gold standard tool for such measurements; however, it is invasive and seldom used in daily practice, and easier, bedside-available tools that allow for rapid and continuous monitoring are greatly needed. The tidal swing of central venous pressure (CVP) has long been proposed as a surrogate for pleural pressure (Ppl); however, despite the wide availability of central venous catheters, this variable is very often overlooked in critically ill patients. In the present narrative review, the physiological basis for the use of CVP waveforms to estimate Ppl is presented; the findings of previous and recent papers that addressed this topic are systematically reviewed, and the studies are divided into those reporting positive findings (i.e., CVP was found to be a reliable estimate of Pes or Ppl) and those reporting negative findings. Both the strength and pitfalls of this approach are highlighted, and the current knowledge gaps and direction for future research are delineated.

Keywords: central venous pressure; critically ill patients; esophageal pressure; inspiratory effort; lung mechanics; pleural pressure.

Figure 1

Chest X-ray mediastinum view. The tip of the central venous catheter (asterisk) and the tip of the esophageal balloon (arrow) are both closely located within the mediastinum (yellow area).

Figure 2

Upper panel: Model of the respiratory system and transmission of pleural pressure during inspiration. The respiratory system is composed of the lungs and the chest wall in series. The figure shows the different pressures within the system and the relative distending forces (in black). The difference between the alveolar and atmospheric pressure, i.e., the transthoracic pressure, is the pressure that distends both the lungs and the chest wall; the transpleural pressure (i.e., pleural minus atmospheric) is the pressure needed to distend the chest wall, whereas the transpulmonary pressure (i.e., airway minus pleural) is the pressure that distends the lungs. The thick, orange arrows depict the downward displacement of the diaphragm during inspiration, which lowers the pleural pressure (orange minus signs). This negative pleural pressure swing (ΔPpl) diffuses into the intrathoracic space and is transmitted through the esophagus to the balloon-tipped esophageal catheter (ΔPes) and through the superior vena cava to the central venous catheter (ΔCVP). Pes, esophageal pressure; Ppl, pleural pressure; CVP, central venous pressure. Lower panel: Pressure waveforms for CVP, Pes and Paw. Central venous pressure swings (CVP, blue wave), esophageal pressure swings (Pes, orange wave), airway pressure swings (Paw, yellow wave) and flow (Faw, green wave) during assisted mechanical ventilation.