Anatomy for anaesthetists

Transcription

1 VOL 17 NO 1 ANESTHESIA JANUARY 1962 Anatomy for anaesthetists (4) The thoracic inlet and the first rib HAROLD ELLIS, MCh, FRCS Senior Lecturer in Surgery, Westminster Hospital Illustrated by MISS MARGARET MCLARTY The Radcliffe Infirmary, Oxford The thoracic inlet, viewed either on the skeleton or in the dissecting room, is a surprisingly small space into which are packed the apices of the lungs, the trachea and esophagus and the great vascular trunks - the innominate artery, the innominate veins, the left carotid and the left subclavian artery - together with the vagi, the cervical sympathetic chains, the phrenic nerves and the thoracic duct. As a back-cloth to the inlet on either side lies the brachial plexus, sandwiched between the scalenus anterior and medius. It is the brachial plexus, of course, which renders this area of intense practical importance to the anasthetist (FIG. 1). OUTLINES AND BOUNDARIES The thoracic inlet is kidney-shaped because of the forward projection into it of the body of the first thoracic vertebra and measures some four inches in transverse diameter and two inches anteroposteriorly. Its boundaries are the body of the first thoracic vertebra, the first ribs, their costal cartilages and the upper border of the manubrium sterni. The inlet slopes downwards sharply from behind forwards, forming an angle of about 60" with the horizontal. There is, in fact, a one and a half inch difference in height between the anterior and posterior extremities of the inlet, the upper border of the manubrium lying opposite the disc between the second and third thoracic vertebra. During quiet respiration this level hardly varies, but in forced inspiration and expiration the upper border of the manubrium moves about the length of a vertebral body in each direction. 69

2 70 ANESTHESIA FIG. 1 The thoracic inlet, its boundaries and background. A segment of clavicle has been removed to show more clearly the three important structures which cross the first rib THE FIRST RIB (FIGS. 1 AND 2) The first rib is the key to the important neurovascular relationships of this region. It is unique in being the shortest, flattest and most curvaceous of the ribs. Its extreme flattening and curvature gives it broad upper and lower surfaces and sharp outer and inner margins, the latter bearing the scalene tubercle (or tubercle of Lisfranc). The first rib has a rounded head, with a single facet for the body of the first thoracic vertebra, a long neck and a prominent tubercle which articulates with the transverse process of the first thoracic vertebra. Crossing the neck extrapleurally are, medially, the sympathetic trunk, laterally, the large branch of the anterior primary ramus of the first thoracic nerve passing to the brachial plexus and, between them, the superior intercostal artery with its accompanying vein.

3 ANBSTHESIA 71 FIG. 2 The first rib, viewed from above The scalene tubercle provides the insertion for the tendon of scalenus anterior. Immediately in front of this tubercle, the upper surface of the rib bears a groove for the subclavian vein; because of the obliquity of the thoracic inlet this vessel lies well down (and safely behind) the clavicle. Behind the scalene tubercle lies a second groove which is for the subclavian artery and the lower trunk (C8, T1) of the brachial plexus. This groove is particularly well marked when the subject has a post-fixed brachial plexus with a large contribution from T2. Immediately behind this groove is the area of insertion of scalenus medius. To the inner margin of the first rib is attached the suprapleural membrane, better known to clinicians as Sibson s fascia. This is a tough sheet of fibrous tissue which spreads out like a tent from its origin, the transverse process of C7, to form a protective covering over the cervical pleura. Of less practical importance to the anasthetist, but to complete this list of first rib relations and attachments, the small subclavius muscle arises from the anterior extremity of the upper surface of the rib to become inserted into the under aspect of the clavicle - it is usually able to prevent fragments of a comminuted fracture of the clavicle from piercing the subclavian artery. Serratus anterior and the intercostal muscles of the first space find attachment to the lateral margin of the rib, and its inferior aspect lies against the cervical pleura. THE CERVICAL RIB About 0.5 per cent of individuals have a cervical rib, which may comprise merely an enlarged costal process of C7, continuing as a fibrous strand to the fist rib just beyond the scalene tubercle (the commonest

4 72 ANESTHESIA manifestation), or a true rib with articular facets which articulate with the body and transverse process of C7, again with a fibrous connection with the first rib, or, finally, a complete rib which articulates or fuses with the front of the first rib and which has the scalene muscles attached to it. A cervical rib is usually symptomless, but it may sometimes be associated with either neurological or vascular disturbances. It may seem rather strange, but pressure effects on the brachialplexus are rare in the presence of a completely developed cervical rib. This is because, in such cases, the plexus is usually pre-fixed (its main components being derived from C4-C8), and lies clear of the extra rib. Conversely the plexus may be post-fixed, from C6-T2 and this may be associated with an anomalous first thoracic rib, which is rudimentary and replaced by a fibrous strand. The lower cord of the normally constituted brachial plexus may be snared over the fibrous prolongation of an incomplete cervical rib with resultant parasthesia over the distribution of C8 and T1 (the ulnar border of the forearm and hand), together with the weakness and wasting of the small muscles of the hand, especially those of the thenar eminence. The subclavian artery must, of necessity, arch over a complete cervical rib; it is then unusually prominent in the supraclavicular fossa and is often mistaken for an aneurysm. The artery may be narrowed at this site, and develop a post-stenotic dilatation in which thrombosis may occur. It is emboli arising from this source which result in the cold, cyanosed upper limb, with absent pulses, claudication pain and perhaps digital gangrene which is sometimes seen in association with a cervical ribl. SURFACE MARKINGS The first rib is the anasthetist s key to the brachialplexus. In the supraclavicular fossa, the trunks of the plexus are closely clumped together between the rib and the overlying skin and, in fact, can be rolled under the finger at this site in the thin subject. A barrage of local anasthetic placed superficially to the rib where it is crossed by the nerve trunks must, of necessity, block the plexus. Unfortunately the first rib is completely impalpable in the great majority of people and we must rely on other landmarks to find the zone where the plexus crosses the rib. There are three useful guides which lead to the same place immediately above the clavicle where the anasthetist s needle must be inserted; these are, the mid-point of the clavicle, the spot where the line of the external jugular vein, continued downwards, cuts this bone and finally (and best) the point immediately lateral to a finger pressing on the pulsations of the subclavian

5 ANESTHESIA 13 artery. This pulse is felt where the artery, having emerged from between the scalenes, crosses the first rib immediately in front of the plexus (FIG. 1). A needle inserted at the spot defined by these landmarks, passed backwards, inwards and downwards (BID) will strike the first rib in the region of the brachial plexus2. FIG. 3 The stellate ganglion viewed in relationship to the roots of the brachial plexus and the vertebrse THE STELLATE GANGLION (FIG. 3) The inferior cervical sympathetic ganglion is fused with the first thoracic ganglion in about 80 per cent of subjects; this combined structure is termed the stellate ganglion (although the term is often used for the two ganglia even when they are separate). The inferior cervical ganglion itself lies in front of the anterior ramus of the eighth cervical nerve and immediately posterior to the vertebral vessels or to the upper border of the subclavian artery, if this is rather highly arched; it is at the level of the disc space between C7 and T1 vertebra. The first thoracic ganglion lies against the head of the first rib behind the pleura. The inferior ganglion is connected with the middle cervical ganglion both by the cervical sympathetic chain itself and by the ansa subclavia (or ansa Vieussens) which loops around the inferior margin of the subclavian artery then passes upwards to join the middle ganglion.

6 74 ANESTHESIA Preganglionic sympathetic fibres from segments T1 and 2 pass upwards along the sympathetic chain to relay mainly in the superior cervical ganglion for distribution to the head and neck. Fibres from spinal segments T2-7 relay mainly in the ganglia of T1 and 2 and in the inferior and middle cervical ganglia. Grey rami stream thence to the roots of the brachial plexus and are thus distributed to the upper limb. The close relationship of the stellate ganglion and lower cervical sympathetic chain to the brachial plexus means that diffusion of local anaesthetic solution will often produce a sympathetic blockade of the head and neck after a brachial plexus infiltration is performed. The clinical manifestation of this is a Horner s syndrome; the pupil is small (paralysis of the dilator pupillz), there is ptosis (paralysis of the sympathetic supply to levator palpebrze) and there is unilateral vasodilatation and anhidrosis due to blockage of the sympathetic outflow to the skin of the face; the vasodilatation also causes unilateral nasal blockage. There is doubt whether the enophthalmos described in this syndrome actually occurs. In performing a sympathetic denervation of the upper limb (upper dorsal sympathectomy), the surgeon divides the sympathetic chain immediately below the T3 ganglion, then dissects up the chain, dividing all its connections, but carefully preserves the stellate ganglion with its white ramus from TI. In this way, the sympathetic outflow to the upper limb (T2-7) is completely cut off by preganglionic section, but the supply to the head and neck via T1 is preserved; by this manceuvre the rather unsightly Horner s syndrome is avoided3. References ~ROSS, J. PATERSON. (1958). Surgery of the sympathetic nervous system, 3rd edition, Bailliere, Tindall and Cox, London. *MACINTOSH, R. R. and MUSHIN, w. w. (1954). Local analgesia, brachial plexus, 3rd edition, E. and S. Livingstone Ltd, Edinburgh. 3ROSS, J. PATERSON. (1959). Annals Of surgery 150~340 The Vascular COmplications of cervical rib.