Objectives and execution technique
The main objective of comparative percussion is to identify pathological changes in the physical properties of chest organs (the appearance of dense, cavitary, or mixed formations). The main diagnostic principle consists of alternating percussion over strictly symmetrical zones on the right and left. The only exception is the cardiac projection area, where anatomical symmetry is disrupted.
The study is conducted with the patient sitting or standing. When evaluating the posterior surface of the chest, the patient is asked to cross their arms over the chest, hugging their shoulders. This technique allows the scapulae to be displaced laterally and exposes the interscapular areas of the lungs for percussion, which are covered by bone tissue in a standard posture.
Strikes with the pleximeter finger are delivered along the intercostal spaces, moving sequentially from top to bottom. There is an anatomical feature: when evaluating the lung apices anteriorly, the finger is placed vertically. Loud percussion is used in the comparative method. The physical justification lies in the fact that the force of the strike is directly proportional to the depth of the sound wave. The response sound is formed by the entire tissue layer. However, if the strike is too strong, deep-lying healthy tissues may resonate so strongly that they mask the sound from small superficial consolidation foci.
Normal sound and physiological asymmetry
The clear pulmonary sound is considered the standard sound when examining a healthy chest. Physically, it is characterized as loud, prolonged, and low-pitched. Its acoustic tint is influenced by the patient's body habitus:
- In asthenics (with poor muscle development), the sound may acquire a box-like tint, becoming louder and lower-pitched.
- In athletes (with strongly developed musculature), the sound, by contrast, sounds slightly muffled due to the thickness of the chest wall.
In the projection of the lung apices (supraclavicular and infraclavicular spaces), natural physiological asymmetry is observed. On the right, the sound is usually shorter and more muffled. This is due to the stronger muscle development of the right half of the body and the anatomy of the bronchial tree (the right upper lobar bronchus is shorter and wider than the left). Historically, enormous attention was paid specifically to the percussion of the apices, as this is the typical localization for early stages of tuberculosis.
Pathological changes of the percussion sound
When the physical properties of the lung tissue or pleural cavity change, the physician identifies three main types of pathological sounds:
- Dull (muffled) sound. Appears with a decrease in aeration and tissue consolidation. The denser the area, the quieter and shorter the sound vibrations become. It is characteristic of inflammatory infiltration (pneumonia), tumors, extensive atelectasis, pulmonary infarction, and thickening of the pleural leaves.
- Tympanic sound. A loud, prolonged sound of medium or high pitch with a musical tint. Normally, it is detected only over hollow organs of the gastrointestinal tract. In the chest, tympanites occurs when a cavity with air is formed (accumulation of gas in pneumothorax, tuberculous cavity, large cysts, or an emptied abscess).
- Box-like sound. A low and loud tone acoustically resembling a strike on a pillow or an empty box. Occurs with a pathological increase in tissue aeration against the background of the loss of elasticity of the alveolar septa. This is a classic sign of pulmonary emphysema.
Diagnostics of fluid accumulation in the pleura
The appearance of transudate, exudate, pus, or blood in the pleural cavity causes a characteristic physical examination picture. The fluid compresses the lung, reduces the respiratory surface, and provokes dyspnea. Upon inspection, the affected half of the chest lags behind in the act of respiration, the intercostal spaces are smoothed out, and vocal fremitus is sharply weakened or not conducted at all, as the fluid dampens the sound.
During comparative percussion, an absolutely dull (femoral) sound is detected over the effusion zone. The threshold volume of fluid at which explicit dullness is determined is at least 500 ml. Smaller accumulations in the lower sinuses are searched for using quiet (weak) percussion.
It is interesting how the sound changes above the fluid level zone: a muffled-tympanic tint is determined there. This is due to the fact that the compressed lung (compression atelectasis) loses its elasticity, the walls of the bronchi relax, but the air inside is preserved.
The upper border of dullness assists in differential diagnosis. In inflammatory exudative pleurisy, it forms an obliquely ascending line with a peak at the posterior axillary line (Ellis–Damoiseau–Sokolov line). If air also enters the pleura (hydropneumothorax), the fluid boundary, obeying the laws of physics, becomes strictly horizontal.
To memorize the cause of a dull sound, imagine a dense barrier (water, tumor, inflammation) that mechanically dampens the sound wave, making it short and quiet.
Class and exam questions
Why are patients asked to cross their arms over their chest during back percussion?
This posture allows the scapulae to be moved apart (laterally). This exposes the interscapular areas of the lungs for examination, which are covered by bone in the usual posture.
How does chest wall thickness affect the percussion sound?
In patients with developed musculature, the sound may be slightly muffled. In asthenics with poor muscle development, the sound becomes louder and lower-pitched, acquiring a box-like tint.
When does a tympanic sound appear over the lungs?
Tympanites indicates the presence of a cavity with air. This may be an accumulation of free gas in pneumothorax or intrapulmonary cavities (cavity, emptied abscess, large cyst).
What is the minimum volume of fluid that can be detected by percussion?
An explicit dull percussion sound is determined if at least 500 ml of fluid has accumulated in the pleural cavity. Smaller volumes in the sinuses can only be found using the quiet percussion method.
What else to study on this topic
- Methodology of topographic percussion of the lungs
- Formation and significance of the Ellis-Damoiseau-Sokolov line
- Garland's triangle: localization and mechanism of formation
- Rauchfuss-Grocco triangle on the healthy side
- Etiology and differential diagnosis of pleural effusions
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