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Parasternal line

The parasternal line is a vertical imaginary line on the anterior surface of the thorax, positioned midway between the sternal line (along the lateral border of the sternum) and the midclavicular line (passing through the midpoint of the clavicle), and running parallel to the sternum on both the right and left sides.^[1]^[2] In anatomical terminology, the parasternal line serves as a key surface landmark for dividing the anterior chest wall into regions, facilitating precise localization of underlying structures such as the heart, lungs, and great vessels; it lies lateral to the sternal line and medial to the midclavicular line, contributing to a standardized framework alongside other vertical lines like the mammary (midmammary) line and anterior axillary line.^[2]^[3] Clinically, the parasternal line is essential in cardiovascular examinations, where palpation along its path—particularly the left parasternal border—detects abnormalities such as a parasternal heave, a sustained lift indicating right ventricular hypertrophy or enlargement due to conditions like pulmonary hypertension.^[4]^[5] It also guides auscultation of the tricuspid valve area at the fourth intercostal space along the left parasternal line and defines probe placement for parasternal echocardiographic views, which visualize cardiac structures like the left ventricle, aortic root, and mitral valve in long- and short-axis orientations.^[6]^[7] Additionally, it aids in thoracic percussion and lung auscultation during respiratory assessments to identify dullness or abnormal breath sounds overlying the heart or mediastinum.^[8]

Anatomy

Definition and Location

The parasternal line is defined as a vertical imaginary line on the anterior thoracic wall, running parallel to the sternum on both the right and left sides.^[1] It is positioned midway between the lateral sternal line, which follows the edge of the sternum, and the midclavicular line, which passes through the midpoint of the clavicle.^[9] The line extends superiorly from the sternal notch to the costal margin inferiorly, providing a consistent longitudinal reference across the thorax.^[1] As part of the anterior thoracic surface anatomy, the parasternal line serves as a key landmark for standardizing anatomical descriptions and measurements.^[9] It aids in delineating the boundaries of thoracic structures and facilitates precise localization in educational and clinical contexts, such as mapping surface projections of internal organs.^[2]

Relations to Other Thoracic Lines

The parasternal line relates closely to other standard vertical lines on the anterior thorax, which collectively provide a framework for surface anatomy mapping. The sternal line, also known as the lateral sternal line, runs vertically along the lateral margin of the sternum on both sides, serving as the immediate medial boundary for thoracic orientations.^[10] The midclavicular line, positioned laterally, is a vertical line passing through the midpoint of the clavicle, approximately aligning with the nipple in adults and extending downward.^[11] These lines intersect with horizontal landmarks such as the intercostal spaces, which are the gaps between ribs, allowing precise localization of underlying structures in two dimensions.^[10] Positionally, the parasternal line is defined as approximately halfway between the sternal line and the midclavicular line, representing a consistent relational geometry for anatomical reference.^[1] This midway positioning facilitates standardized descriptions across the thorax, avoiding variability from direct sternal measurements. The parasternal line exists bilaterally, mirroring the symmetry of the sternum on both the right and left sides, though the left parasternal line receives greater clinical emphasis owing to its proximity to cardiac structures.^[1] This bilateral configuration ensures comprehensive coverage in surface projections, with the lines running parallel from the level of the clavicles to the costal margin.

Surface Anatomy Applications

Markings for Cardiac Structures

The parasternal line serves as a key reference in surface anatomy for projecting the positions of cardiac valves and other heart structures onto the anterior thoracic wall. This line, running parallel to the sternum midway between the sternal line and the midclavicular line, facilitates the mapping of deep cardiac features for educational diagrams, clinical palpation, and preliminary assessments. Specifically, the aortic valve area is projected to the second right intercostal space at the right sternal border, medial to the right parasternal line, where sounds from the aortic valve are optimally transmitted due to the proximity of the ascending aorta to the chest wall.^[12] Similarly, the pulmonic valve area corresponds to the second left intercostal space at the left sternal border, medial to the left parasternal line, reflecting the valve's position at the junction of the sternum and the left third costal cartilage.^[13] The tricuspid valve area is mapped to the fourth or fifth left intercostal space at the left sternal border, medial to the left parasternal line, aligning with the valve's posterior projection behind the right half of the lower sternum.^[14] In surface anatomy diagrams, the left sternal border, medial to the lower left parasternal line, approximates the location of the right ventricle and tricuspid valve, aiding in the visualization of the heart's right-sided structures relative to the thoracic cage. This marking is particularly useful for delineating the right ventricular outline, which extends along the sternal border from the third to the sixth intercostal spaces. The inferior border of the heart intersects the left parasternal line at approximately the fifth intercostal space, providing a reference for precordial mapping in anatomical illustrations and helping to outline the diaphragmatic surface of the heart.^[15] These surface markings are approximations of deeper cardiac structures and exhibit variations based on individual anatomy, such as body habitus and diaphragmatic position. For instance, in obesity, projections may shift laterally due to increased subcutaneous tissue and altered thoracic geometry, reducing the precision of these landmarks.^[14] Such limitations underscore the need for complementary imaging in clinical practice, though these points guide initial auscultation efforts.^[12]

Markings for Pulmonary Structures

The anterior border of the right lung follows the right sternal border closely, medial to the parasternal line, extending from just lateral to the second costal cartilage superiorly to the sixth costal cartilage inferiorly, demarcating the extent of the lung fields over this region.^[9] On the left side, the anterior border deviates laterally around the fourth costal cartilage due to the cardiac notch, but the parasternal line approximates the medial extent of the upper lobe from the second to the fourth costal cartilages, before the border shifts outward and then returns toward the midline at the sixth costal cartilage.^[9] These markings reflect the lung's position relative to the mediastinum, with the right lung maintaining a more medial projection throughout.^[16] The major fissures of the lungs also project near the parasternal line, aiding in delineating lobe boundaries on the surface. The horizontal fissure of the right lung, separating the upper and middle lobes, begins at the fourth costal cartilage at the sternal border (medial to the parasternal line) and extends laterally across the fourth intercostal space.^[9] On the left, the oblique fissure, which divides the upper and lower lobes, crosses the left parasternal line at approximately the fifth intercostal space before descending to the sixth rib in the midclavicular line.^[17] These projections highlight the lobar divisions, with the right lung's three lobes (upper, middle, lower) and the left's two lobes (upper, lower) influencing auscultatory and imaging interpretations along the parasternal region.^[18] Pleural reflections extend beyond the lung borders, providing a buffer zone critical for procedural safety. Along the parasternal line, the inferior limit of the costodiaphragmatic recess—the deepest part of the pleural cavity—reaches the sixth to eighth intercostal spaces, where the costal and diaphragmatic pleura meet, allowing lung expansion during deep inspiration without parietal breach.^[9] This recess's anterior projection along the parasternal line is shallower than laterally, emphasizing the need for precise localization to avoid pleural injury in interventions like thoracentesis.^[19] In diagnostic contexts, parasternal line markings facilitate percussion to distinguish pulmonary from non-pulmonary areas, yielding hyperresonance over aerated lung fields superiorly and laterally, while lower segments may show dullness where lung borders overlap cardiac projections.^[20] This technique helps identify shifts in resonance patterns indicative of conditions like pneumothorax or consolidation, with the right parasternal line particularly useful for confirming normal lung extension medial to the heart.^[21]

Clinical Applications

Physical Examination Techniques

In physical examination of the cardiovascular system, palpation along the left parasternal line, particularly at the third and fourth intercostal spaces, is used to detect precordial thrills or heaves indicative of right ventricular hypertrophy or dilation.^[22]^[4] A palpable lift or heave in this region suggests increased right ventricular pressure, often due to conditions such as pulmonary hypertension or congenital heart defects.^[23] The examiner places the heel of the hand flat against the chest wall in this area while the patient is supine, noting any sustained outward thrust during systole.^[24] Auscultation at the parasternal line focuses on specific valvular areas to identify heart sounds and murmurs. The tricuspid valve is best assessed at the lower left parasternal border in the fourth or fifth intercostal space, where the first heart sound (S1) may be prominent due to delayed tricuspid closure, and holosystolic murmurs of tricuspid regurgitation can be heard, often radiating inferiorly along the line and augmenting with inspiration.^[25]^[26] The pulmonic valve is auscultated at the upper left parasternal border in the second intercostal space, where the second heart sound (S2) splitting—normal physiological separation of aortic (A2) and pulmonic (P2) components during inspiration—is most clearly appreciated.^[27]^[28] Murmurs such as pulmonic stenosis may also be detected here, presenting as an ejection systolic sound radiating to the neck.^[25] Percussion along the parasternal line helps delineate the borders of cardiac dullness from adjacent lung resonance, providing an estimate of heart size. Starting medially from the sternum and moving laterally in the third to fifth intercostal spaces, the examiner percusses to identify the transition from the dull note over the heart to the resonant lung fields, with the right border typically along the right sternal border or slightly lateral to it (about 1 cm from the sternal line) and the left extending 8-10 cm from the midline.^[29] Abnormal widening of this dullness may indicate cardiomegaly or pericardial effusion.^[30] Optimal patient positioning enhances detection of findings along the parasternal line. The supine position with the head elevated 30-45 degrees is standard for initial palpation and auscultation, while rolling the patient into the left lateral decubitus position brings the heart closer to the chest wall, amplifying low-frequency sounds like S3 or S4 at the tricuspid area and improving palpation of parasternal impulses.^[24]^[6] These techniques align with surface anatomy projections of the right heart structures along the parasternal line.^[22]

Echocardiography Views

The parasternal line provides a critical acoustic window for transthoracic echocardiography due to its proximity to the heart's anterior structures. For the parasternal long-axis (PLAX) view, the ultrasound probe is positioned along the left parasternal line in the 3rd or 4th intercostal space, with the transducer indicator directed toward the patient's right shoulder at approximately the 10-12 o'clock position. This orientation captures the long axis of the left ventricle (LV), interventricular septum, mitral and aortic valves, portions of the left atrium (LA), and proximal ascending aorta. The patient is typically placed in the left lateral decubitus position to optimize heart proximity to the chest wall and improve image quality. From the PLAX position, the parasternal short-axis (PSAX) view is obtained by rotating the probe 90 degrees clockwise, directing the indicator toward the patient's left shoulder. This adjustment yields cross-sectional images of the right ventricular outflow tract (RVOT), aortic valve, papillary muscles, and LV at various levels (basal, mid-papillary, and apical). The PLAX view is commonly used to measure left ventricular ejection fraction via end-diastolic and end-systolic dimensions, while the PSAX enables circumferential evaluation of ventricular wall motion and chamber symmetry. Technical considerations include maintaining the probe perpendicular to the chest wall initially, then fine-tuning with slight heel-toe or sliding adjustments to center the image. Common artifacts, such as acoustic shadowing from ribs, are mitigated by angling the probe within the intercostal space to align the ultrasound beam between bony structures and avoid lung interference. These views are integral to focused cardiac ultrasound (FoCUS) protocols for rapid bedside assessment. Clinically, the parasternal views facilitate evaluation of wall motion abnormalities indicative of ischemia or cardiomyopathy, assessment of mitral and aortic valve morphology and function (e.g., stenosis or regurgitation), and detection of pericardial effusions by identifying fluid accumulation anterior to the descending aorta or around the LV. They are standard in protocols for hemodynamic instability, chest pain, and routine cardiac screening, providing essential diagnostic insights into structural and functional cardiac pathology.

Procedural and Surgical Uses

The parasternal line serves as an important medial landmark for thoracostomy procedures, particularly in guiding chest tube insertion to minimize risks to adjacent structures. In standard placement, the tube is inserted in the fourth or fifth intercostal space along the anterior to midaxillary line, lateral to the parasternal line to avoid injury to the heart on the left side and the internal mammary artery and vein, which course along the parasternal border.^[31] This positioning leverages the "safe triangle" bounded by the latissimus dorsi, pectoralis major, and a horizontal line at the fifth intercostal space, ensuring effective drainage of pleural air or fluid while reducing complications such as cardiac laceration.^[31] In mediastinoscopy, particularly the parasternal mediastinotomy (Chamberlain procedure), an incision is made along the left parasternal line in the second intercostal space to provide direct access to the anterior mediastinum, including the aortopulmonary window and subaortic lymph nodes.^[32] This approach, first described by McNeill and Chamberlain in 1966, is commonly employed for staging non-small cell lung cancer by sampling mediastinal lymph nodes, allowing assessment of resectability without the need for more invasive thoracotomy.^[33] The procedure involves retracting the lung and thymus to biopsy suspicious nodes, with the parasternal location minimizing disruption to major vessels while enabling histologic confirmation of metastasis.^[32] For cardiac surgery, during median sternotomy planning and execution, the parasternal lines help indicate the adjacent location of the internal mammary arteries along the lateral sternal borders, guiding the midline incision and subsequent retraction to expose the heart safely while avoiding injury to these vessels. The skin incision runs from the sternal notch to the xiphoid process, with the sternum divided precisely between these lines using a saw, followed by placement of a retractor on the sternal edges to facilitate access for procedures like coronary artery bypass grafting or valve replacement.^[34] This landmark ensures avoidance of excessive lateral dissection that could injure the internal mammary arteries, which are harvested in many cases and lie adjacent to the parasternal lines.^[34]^[35] The parasternal line is also referenced in pericardiocentesis, particularly for ultrasound-guided approaches that complement the more common subxiphoid method. In the parasternal technique, a linear probe is positioned along the left parasternal line in the third or fourth intercostal space to identify the cardiac notch and pericardial effusion, allowing needle insertion at a 45° angle under real-time visualization to aspirate fluid safely.^[36] This echo-guided confirmation reduces risks such as coronary or chamber puncture, though deviation from the line may lead to pneumothorax or vascular injury due to proximity to pleural reflections and the internal thoracic vessels.^[36] The approach achieves high success rates, with studies reporting 100% procedural efficacy and no major complications when properly executed.^[36]

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The parasternal line is a vertical reference line on the anterior chest wall, running parallel and lateral to the sternal line, midway between it and the ...
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