Diastolic sound

Mechanism of the second heart sound (S2)

• Phase of the cardiac cycle: Occurs at the beginning of diastole.
• Mechanism: Caused by the closure of the semilunar valve cusps — the aortic valve and the pulmonary valve.

Additional diastolic sounds

In addition to the primary second sound ($S_{II}$), an extra sound may be auscultated during diastole — the tricuspid opening snap.

• Timing: Occurs in diastole after the second heart sound ($S_{II}$).
• Characteristics: It is the analogue of the mitral opening snap.
• Auscultation point: Best heard over the lower sternum.
• Frequency: Observed quite rarely.

Do not confuse: other meanings of the abbreviation S2

Since the abbreviation S2 is widely used in medicine, it denotes completely different concepts in other fields:

• Sacral spinal cord segments (S2–S4):
• Parasympathetic innervation: Neurons of the lateral horns of segments S2–S4 send preganglionic fibers as part of the pelvic splanchnic nerves (nn. splanchnici pelvini). They mediate the contraction of the detrusor muscle of the urinary bladder, stimulate rectal peristalsis, relax the internal sphincters for evacuation, and cause penile erection.
• Somatic innervation: $\alpha$-motor neurons of the anterior horns of S2–S4 form Onuf's nucleus. Via the pudendal nerve (n. pudendus), they control the striated external sphincters of the urethra and rectum.
• Afferent pathway: Segments S2–S4 receive impulses from stretch receptors in the walls of the urinary bladder and rectum.
• Lower limb innervation: Segment S2 contributes to the root supply of the inferior gluteal nerve (n. gluteus inferior), the posterior cutaneous nerve of the thigh (n. cutaneus femoris posterior), and the sciatic nerve (n. ischiadicus). Segment S2 is also included in the innervation ranges of the triceps surae, the flexor digitorum longus and brevis, the flexor hallucis longus, and the plantar muscles of the foot. These muscles participate in plantar flexion of the foot with supination, as well as in flexion, adduction, and abduction of the toes.
• CHA2DS2-VASc score criterion: The letter S2 (Stroke / TIA) stands for a history of stroke, transient ischemic attack, or thromboembolism. It is scored 2 points when clinically assessing the risk of ischemic stroke in patients with nonvalvular atrial fibrillation to decide on anticoagulant therapy.
• Ulcer scarring stage: In endoscopy, S2 refers to the scarring stage (Scarring) when assessing the regression (healing) of a gastric ulcer.
• Tumor marker stage: In male germ cell tumors, the S2 criterion corresponds to stage IIIB with pN1–3 and M0, or with any N and M1a. This is a combination with intermediate levels of tumor markers.

Class and exam questions

What else to study on this topic

• Phases of the cardiac cycle
• Sacral plexus (Plexus sacralis)
• Autonomic innervation of pelvic organs
• CHA2DS2-VASc score

Other topics in "Physiology concepts"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.