Chapter 3: Thorax. Thorax

Transcription

1 Chapter 3: Thorax Thorax

2 Thoracic Cage I. Thoracic Cage Osteology A. Thoracic Vertebrae Basic structure: vertebral body, pedicles, laminae, spinous processes and transverse processes Natural kyphotic shape, which becomes exaggerated in patients with osteoporosis and often results in a hump back hunched over look Vertebral bodies and transverse processes articulate with the ribs, serving as an anchoring point Costovertebral joint consists of: Vertebral body superior to the rib Intervertebral disc Head of the rib (AKA crest of rib) Body of the vertebra that is the same number as rib Rib is the same number as the lower vertebra in joint Example: T5T6 costovertebral joint contains the sixth rib Costotransverse joint consists of the transverse process of vertebra of same number as the rib and the tubercle of the rib B. Sternum Ribs complete the thoracic cage by articulating with the sternum Comprised of the three separate bones: manubrium, sternal body and the xiphoid process Sternal angle: junction between the manubrium and the sternal body Sternal angle serves as an anatomical landmark for: Articulation point of rib 2 Intervertebral disc between T4 and T5 Beginning of aortic arch Bifurcation of trachea Xiphoid process lies at the same level as the T10 vertebra and the T6 dermatome Used as a landmark for a pericardiocentesis, which is a procedure involving insertion of a needle into the pericardial space around the heart to drain fluid or blood Sternum is often used by hematologists as a bone marrow biopsy site because it is subcutaneous, easily accessible and its marrow maintains hematopoietic activity into late adulthood Figure Fig. 3.1: Anterior View of Rib Cage Figure 3.2: Posterior View of Rib Cage Figure 3.3: Oblique View of Rib Cage

3 C. Ribs 12 pairs of ribs, which are classified into three categories: true ribs, false ribs and floating ribs True ribs (ribs 17): articulate with the sternum individually via costal cartilage for each rib False ribs (ribs 812): articulate with a common costal cartilage, thus each rib does not individually articulate with the sternum Costal margin is a palpable inferior border of the thoracic cage formed by ribs 710 Floating ribs (ribs 11 and 12): do not articulate with the sternum and project outward II. Neuromuscular Structures of Thoracic Cage Figure 3.4: Axial View of an Intercostal Nerve Figure 3.5: Sagittal View of Posterior Intercostal Arteries A. Neurovascular Bundle Travels within the costal groove of the inferior surface of the ribs Contents from superior to inferior: intercostal Vein, Artery and Nerve (VAN) Intercostal nerves are the anterior primary rami of the first 11 thoracic spinal nerves and run in the costal groove of the ribs (inferior surface) Terminate as lateral and anterior cutaneous branches and muscular branches Innervate the intercostal, subcostalis and transversus thoracis muscles Provide cutaneous innervation to the skin of the anterior and lateral chest Intercostal arteries run in the costal groove and supply muscles of the intercostal space and adjacent structures Anterior intercostal arteries branch off of either the internal thoracic artery or the musculophrenic artery Posterior intercostal arteries branch directly off the thoracic aorta Anastomosis between the anterior and posterior intercostals exist to provide collateral circulation for the viscera and muscles of the thorax Intercostal veins drain the intercostal spaces and follow the same course as the intercostal arteries B. Muscles of the Thoracic Cage Intercostal, diaphragm, transversus thoracis, subcostalis and levator costarum muscles are the primary muscles responsible for movement of the thoracic cage Diaphragm is a dome shaped skeletal muscle at the inferior thoracic aperture Major muscle responsible for inspiration

4 Innervation: phrenic nerves (two nerves one innervating each hemidiaphragm) Blood supply: pericardiacophrenic, musculophrenic and inferior phrenic arteries Has three main hiatuses where structures from the thorax pass through the diaphragm to enter the abdomen T8 Inferior Vena Cava (IVC) T10 esophagus T12 aorta Dome shape in resting position contraction drives inspiration by flattening the diaphragm, which leads to increased volume decreased intrathoracic pressure drives air into lungs Note: the diaphragm is not involved in quiet expiration, which is driven by elastic recoil of lung tissue pushing air out of the lungs V P Resting Diaphragm P atm = P IT Contracting Diaphragm P atm > P IT Figure 3.6: Diaphragm Movement Driving Respiration Loss Phrenic Nerve Abdominal Cavity Figure 3.7: Paradoxical Movement of the Diaphragm Clinical Pearls Paradoxical movement of the diaphragm occurs when one hemidiaphragm is denervated resulting in paralysis for that side of the muscle Phrenic nerve drives the normal side which descends decrease abdominal cavity volume increase abdominal cavity pressure pushes paralyzed hemidiaphragm superiorly During inspiration, the intact side of the muscle descends, while the paralyzed side of the muscle simultaneously rises Diaphragmatic hernias are either congenital or acquired Involves abdominal contents protruding into the thoracic cavity via openings in the diaphragm

5 Figure 3.8: Intercostal Muscles Figure 3.9: AP Xray of a Congenital Diaphragmatic Hernia Published with permission from LearningRadiology.com Table 3.1: Intercostal Muscles Intercostal Origin Insertion Innervation Action Muscle External Lower border of ribs Upper border of rib below Intercostal nerves Elevate ribs expand cavity (inspiration) Internal Lower border of ribs Upper border of rib below Intercostal nerves 1. Costal part: depress and retract ribs compress thoracic cavity expel air (expiration) 2. Interchondral part: elevate ribs Innermost Lower border of ribs Upper border of rib below Intercostal nerves expand cavity (inspiration) Elevate ribs expand cavity (inspiration) Accessory muscles of respiration assist with breathing, but do not play a primary role Neck muscles, such as the sternocleidomastoid and the scalene muscles assist in elevating the rib cage depending on the effort of the person Muscles of abdominal wall can assist with breathing Along with the diaphragm, aids in coughing, vomiting, defecation and urination because they directly affect the volume of the abdominal cavity, thus changing pressure within the abdominal cavity III. Thoracic Cage Function A. Thoracic Cage Dimensions Two apertures which are superior and inferior outlets that serve as passageways for structures into and out of the thoracic cage Superior thoracic aperture borders consist of the T1 vertebra, first ribs and manubrium Note: the clavicle is not part of the borders (often used as a trick on exams) Inferior thoracic aperture borders consist of the xiphoid process, costal margin, ribs 11 and 12 and T12 vertebra Thoracic cage has three axes: vertical, anteriorposterior and the transverse B. Thoracic Cage Movement Ventilation is the movement of air into (inspiration) and out of (expiration) the lungs Inspiration is driven by movement of the thoracic cage along its three axes to increase the volume of the thoracic cavity producing a negative air pressure in the lungs, which drives air movement into the respiratory tract

6 Figure 3.10: Pump Handle Movement of Thoracic Cage Figure 3.11: Bucket Handle Movement of Thoracic Cage Thoracic cage has two motions which drive inspiration: Pump Handle Movement: sternum is thought of as the handle of a pump handle that during inspiration pulls the thoracic cage anterior along its anteriorposterior axis to expand intrathoracic volume Bucket Handle Movement: a rib in its entirety is thought of as a bucket handle and its articulating points (vertebrae and sternum) are the fixed bucket Movement of the ribs outward (laterally) increases the volume of the thoracic cage along the transverse axis C. Mechanics of Respiration Quiet inspiration: diaphragm contracting and dropping inferiorly to increase the volume of the thoracic cage along the vertical axis only Quiet expiration: passive and solely driven by the recoil of the elastic fibers in lung tissue Active inspiration Muscles of the thoracic cage carrying out both the pump handle and bucket handle movements working simultaneously with the diaphragm Thoracic cavity volume increases along the vertical, transverse and anteriorposterior axes Active expiration Contraction of the abdominal muscles to decrease the volume of the abdominal cavity in order to increase its pressure Increased abdominal pressure helps push up on the diaphragm to decrease the thoracic cavity volume and increase its pressure Results in a pressure gradient with the atmospheric pressure that drives air out of the lungs Flail chest is a medical emergency that occurs when multiple adjacent ribs are broken causing a segment of the thoracic cage to move independently during respiration Affected region of the thoracic cage is unsupported and moves paradoxically during respiration inward on inspiration, outward on expiration Common in victims of motor vehicle accidents Symptoms: chest pain and shortness of breath Treatment: intubation, positive pressure ventilation, chest tubes, pain control to make breathing easier and sometimes surgical fixation of ribs Table 3.2: Thoracic Cage Axes Increased During Types of Breathing Quiet Inspiration Quiet Expiration Active Inspiration Type of Breathing Axes Increased Vertical Vertical Vertical, Transverse, AP