Tissue Doppler Imaging of the Diaphragm: A New Kid on the Block?

The diaphragm is the primary constituent of the respiratory pump. Like the cardiac pump, diaphragm performance can be best characterized in terms of the force and velocity generated at a given muscle length. In the respiratory system, these parameters can be assessed by measuring the pressure and flow achieved at a given lung volume. However, methods used to assess maximal transdiaphragmatic pressure are relatively invasive and not practical for widespread clinical use. To make diaphragm evaluation more assessable to the clinician, there is a growing interest in applying cardiac ultrasound methods to the diaphragm. Over the past four decades, M-mode (time based) and B-mode (2 dimensional) ultrasound have been used to image the diaphragm dome and the diaphragm muscle in the zone of apposition (ZOA) of the diaphragm to the rib cage. Ultrasound measures of the diaphragm dome evaluate its caudal motion, whereas ultrasound measures of the diaphragm in the ZOA allow the clinician to directly assess diaphragm musculature.