American Society of Regional Anesthesia and Pain Medicine February 2018 - 14
Figure 5: Sniff test (M-Mode).
from the cephalad aspect. As the transducer is moved caudally, the
diaphragm thickens during inspiration and is evaluated caudal to
the pleural line so that the diaphragm is not obscured by pleura.
During unforced inspiration, the intercostal muscles remain still,
and the diaphragmatic pleura descend caudally. The presence of
pleural movement (sliding lung sign) does not equate to diaphragm
contraction as the accessory muscles of respiration and the
contralateral diaphragm can cause pleural motion, despite the
presence of ipsilateral hemidiaphragm paralysis/abdominal paradox.
Patients with acute hemidiaphragm paralysis from interscalene
nerve block will have normal diaphragm thickness, but the degree
of thickening (TR or TF) would be diminished.
TF serves as a measure of the efficiency of diaphragmatic
contractility and can be calculated by using the B-mode.
Phrenic Nerve Block and Abdominal Paradox. Unilateral
diaphragm paralysis that typically occurs after interscalene block
can be diagnosed using M-mode at the ZOA. After successful
phrenic nerve block, forceful inhalation or sniffing (Figures 4 and 5)
will cause the contralateral hemidiaphragm to increase the intraabdominal pressure, which then passively shifts the paralyzed/
flaccid diaphragm cephalad, resulting in an abdominal paradox.
The abdomen moves inward, and the rib cage expands in response
to the increased negative intrapleural pressure. Additionally, the
mediastinum shifts to the contralateral side. However, observing this
phenomenon can be difficult, and a false-positive finding may occur
in the absence of diaphragm paralysis. False-negative results can
also occur because accessory muscle activation can cause rib cage
expansion, displacing the diaphragm caudally. These errors can be
eliminated by diaphragm ultrasound at the ZOA, using B-mode, to
evaluate for the presence of normal diaphragm thickening.
TF = thickness at end inspiration - thickness at end expiration/
thickness at end expiration. Lower limit of normal TF is 0.2.
The supine position provides the most accurate measurement
of diaphragm excursion because the abdominal viscera move
Both the thickening ratio (TR) and thickening fraction (TF) can be
calculated to quantify the degree of thickening. TR quantifies the
degree of thickening, by comparing the differences between the
two measurements, and is calculated as follows:
TR = thickness at maximal inspiration/thickness at end expiration,
which is normally greater than 1.2.
14
American Society of Regional Anesthesia and Pain Medicine
2018
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