Mechanism of the Second Heart Sound (S2)
- Cardiac Cycle Phase: Occurs at the beginning of diastole.
- Mechanism: Caused by the snapping shut of the semilunar valve cusps — the aortic valve and the pulmonary valve.
Additional Diastolic Sounds
In addition to the primary second heart sound ($S_{II}$), an extra sound may be auscultated during diastole — the tricuspid valve opening snap.
- Timing: Occurs in diastole after the second heart sound ($S_{II}$).
- Characteristics: The equivalent of the mitral valve opening snap.
- Auscultation Point: Heard near the lower sternum.
- Frequency: Relatively rare.
Do Not Confuse: Other Meanings of the Abbreviation S2
Because 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 in the lateral horns of segments S2–S4 send preganglionic fibers as part of the pelvic splanchnic nerves (nn. splanchnici pelvini). They mediate contraction of the detrusor smooth muscle of the urinary bladder, stimulation of rectal peristalsis, relaxation of internal sphincters for elimination, and penile erection.
- Somatic Innervation: Alpha motor neurons in the anterior horns of S2–S4 form Onuf's nucleus. Through the pudendal nerve (n. pudendus), they control the skeletal striated external sphincters of the urethra and rectum.
- Afferent Pathway: Segments S2–S4 receive sensory input from stretch receptors in the walls of the bladder and rectum.
- Lower Limb Innervation: Segment S2 contributes to the root supply of the inferior gluteal nerve (n. gluteus inferior), posterior cutaneous nerve of the thigh (n. cutaneus femoris posterior), and sciatic nerve (n. ischiadicus). Segment S2 also participates in the innervation range of the triceps surae, flexor digitorum longus and brevis, flexor hallucis longus, and plantar foot muscles. These muscles participate in plantar flexion of the foot during supination, as well as flexion, adduction, and abduction of the toes.
- CHA2DS2-VASc Score Criterion: The letter S2 (Stroke / TIA) denotes a history of stroke, transient ischemic attack, or thromboembolism. It is assigned 2 points when clinically assessing the risk of ischemic stroke in patients with nonvalvular atrial fibrillation to guide anticoagulant therapy decisions.
- Ulcer Healing Stage: In endoscopy, S2 refers to the scarring stage (Scarring) when assessing the regression (healing) of a gastric ulcer.
- Tumor Staging Marker: For testicular germ cell tumors in men, the S2 criterion corresponds to stage IIIB with pN1–3 and M0, or with any N and M1a, in combination with intermediate tumor marker levels.