Airway Resistance Calculator

Static Compliance Calculator

Respiratory System Compliance 

Respiratory System Compliance (C) is an important measure of the stiffness of the lungs. Compliance calculations should be routinely monitored for all ventilated patients. To attain an accurate assessment of compliance, two maneuvers must be performed because the alveolar pressures at end inspiration and end expiration are not available under dynamic conditions. An end inspiratory hold maneuver allows reading of a plateau pressure (P_plat). Because compliance (C) is calculated with C= ∆V/∆P, tidal volume (corrected for tubing compliance) is ∆V and (P_plat – PEEP) is ∆P. if auto-PEEP is present, it must be accounted for by adding the auto-PEEP level to the applied PEEP to attain a PEEP_T value. Normal compliance ranges between 60 and 100 mL/cm H₂O_. Diseases of the lung parenchyma, such as pneumonia, pulmonary edema, and any chronic disease causing fibrosis cause decreased effective compliance. Acute changes, such as Atelectasis, pulmonary edema, ARDS, and lung compression, caused by tension Pneumothorax cause a rapid decrease in compliance. Compliance can be less than 25 to 30 mL/cm H₂O in ARDS.  

Resistance 

Depending on the driving pressure measured, various resistances can be calculated, including airway, pulmonary, chest wall, and total respiratory resistance. Airway resistance (Raw) is determined dynamically from simultaneous measurements of airflow and the pressure differences and between the airway opening (P_ao) and the alveoli (P_alv), R = (P_ao - P_alv)/flow. Because resistance changes throughout inspiration and expiration with, generally, expiratory resistance greater than inspiratory resistance, instantaneous measurement of resistance usually is not performed. Alveolar pressure can be instantaneously measured with an interrupter method that allows such measurements, but this estimate of resistance usually is reserved for research protocols. Inspiratory resistance can be calculated simply during constant flow ventilation for monitoring of airway status time or after the effects of bronchodilator therapy occur by dividing the pressure change (∆P) by the flow change (∆F) [Raw = ∆P/∆F = (P_peak – P_plat)/V(flow)]. Automated methods of measuring early expiratory resistance have been integrated into some ventilators.

            In ventilated patients, a significant component of the total flow resistance may be added with endotracheal tubes, which have highly curvilinear flow-resistive properties. In healthy persons, flow is laminar during tidal ventilation and becomes turbulent only with increasing ventilatory demands. The flow resistance offered by the endotracheal tube increases markedly with increasing flow and varies with the size of the tube. Normal Raw is approximately 1 to 2 cm H₂O/L per second; however, intubated patients receiving mechanical ventilatory support typically have a Raw of 5 to 10 cm H₂O/L per second or more. Tube compensation modes have been added to mechanical ventilators to adjust flow to account for the added resistance of the endotracheal tube.