= Apical impulse
The \b[apical impulse] is a localized movement of the <chest wall> associated with the <heart>'s mechanical activity, assessed by inspection or <palpation>. It is a surface observation rather than the anatomical <cardiac apex> itself. Its position, area, duration and timing within the <cardiac cycle> describe different features of the observation.
A primary study compared physical examination with two-dimensional <echocardiography> in 41 patients. The investigators found that impulse location relative to selected chest landmarks did not reliably identify increased left ventricular <end-diastolic volume>.<Accuracy of precordial palpation for detecting increased left ventricular volume by S. D. Eilen and others>[Abstract, examination/echocardiography comparison and conclusion]{ref} The observation illustrates why an external landmark should be tested against an independent measurement instead of being treated as a direct cavity-volume reading. It does not establish a universal diagnostic rule for all patients.
A simple original measurement model explains one source of ambiguity. Let $u(t)$ represent an internal mechanical displacement and let the measured local surface displacement be
$$
y(t)=g\,u(t)+b(t).
$$
The coefficient $g$ summarizes an idealized mechanical coupling and viewing direction; $b(t)$ represents other contributions. If the same internal motion $u=2$ mm is observed with $g=0.5$ or $g=0.25$ and zero background, the surface amplitudes are 1 mm and 0.5 mm. Changing the coupling can change the observed amplitude without changing the internal motion.
Real <chest wall> mechanics are more complicated than a single gain. The model nevertheless explains why body position, contact and the chosen observation site belong to the measurement description. Neither a surface displacement amplitude nor an apparent lateral position alone specifies <ejection fraction> or ventricular volume.
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