static and dynamic compliance

Static compliance more than 20 is good. It’s tidal volume over driving pressure.

TvPplat−PEEP\frac{T_v}{P_{plat}-PEEP}

Dynamic compliance measures airway resistance (e.g. small ET tube):

TvPpeak−PEEP\frac{T_v}{P_{peak}-PEEP}

Airway resistance less than 10 H2O/L/S is good.

Ppeak−Ppleateauflow\frac{P_{peak}-P_{pleateau}}{flow}

Dynamic compliance is the volume change divided by the peak inspiratory transthoracic pressure. Static compliance is the volume change divided by the plateau inspiratory pressure minus the PEEP. With the initiation of an inspiratory breath, the transthoracic pressure gradient increases to a peak value. This increase is a function of elastic resistance of the lung and chest wall as well as airway resistance. The pressure then falls to a plateau level as the gas redistributes in alveoli. Consequently, dynamic compliance is always lower than static compliance.12345

The static compliance of the respiratory system is defined as the change in volume that results from the driving pressure: C = ∆V/∆P, where C is the compliance, ∆V is the inhaled tidal volume, and ∆P is the difference between the plateau pressure and PEEP; the plateau pressure is the pressure in the lungs at the end of full inspiration. In conditions that increase the inward recoil of the lung parenchyma, decrease the outward pull of the chest wall, or limit diaphragm mobility, a larger pressure is required in order to maintain lung inflation at a set tidal volume at the end of inspiration. In this patient, the measured compliance is 450 / (35 – 5) = 15 mL/cm H2O.

Lung parenchymal conditions that increase inward recoil of the respiratory system include fibrotic or inflammatory interstitial lung diseases, interstitial edema from inflammatory exudates (ARDS, infection), hydrostatic edema, or loss of pulmonary surfactant. Other intrathoracic conditions that increase inward recoil of the respiratory system include pleural effusions, positive intrapleural pressure (pneumothorax), and cardiomegaly. Extrathoracic variables may exert pressure on the chest wall and diaphragm to limit lung expansion. Pressure on the chest wall from obesity and weakness or stiffness of the chest wall from neuromuscular disease are common conditions that limit chest wall expansion. Similarly, increased pressure in the abdomen from tense ascites, abdominal compartment syndrome, and abdominal obesity also limit the downward motion of the diaphragm into the abdominal cavity, thus impairing lung expansion. In this patient, large-volume paracentesis would likely allow for downward movement of the diaphragm and reduce the external pressure on the lungs, allowing the lungs to hold the same tidal volume at a lower end-inspiratory pressure (choice C is correct).

Footnotes

  1. SEEK Questionnaires ↩

  2. Loring SH, Topulos GP, Hubmayr RD. Transpulmonary pressure: the importance of precise definitions and limiting assumptions. Am J Respir Crit Care Med. 2016;194(12):1452-1457. PubMed ↩

  3. Mauri T, Lazzeri M, Bellani G, et al. Respiratory mechanics to understand ARDS and guide mechanical ventilation. Physiol Meas. 2017;38(12):R280-R303. PubMed ↩

  4. Shapiro MB, Bartlett RH. Pulmonary compliance and mechanical ventilation. Arch Surg. 1992;127(4):485-486. PubMed ↩

  5. Walter JM, Corbridge TC, Singer BD. Invasive mechanical ventilation. South Med J. 2018;111(12):746-753. PubMed ↩