The collateral intercostal artery ( CIA ) anastomosis with the internal thoracic artery ( ITA ), forming an angle of approxi- mately 90 ° 

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... intercostal spaces are of considerable importance both clinically and surgically; however, descriptions of their anatomy in the classic texts are of limited value, particularly with respect to size of the collateral branches of the posterior intercostal artery. The aim of the present study was, therefore to determine the detailed anatomy of the collateral intercostal arteries. The importance of having such detailed information is re- lated to the fact that there are reports of intercostal artery injuries during traditional thoracocentesis [6, 12, 15]. This study also aims to provide support for routine procedures in general clinical practice which require access to this area. The collateral intercostal artery originated from the posterior intercostal artery in all cases examined, forming an acute angle of 45–60 ° with it. The point of origin was always located between the vertebral body and the costal angle on the inferior border of the rib (Fig. 1). The artery then crossed the intercostal space to reach the superior border of the inferior rib, running parallel and in close association with it (Fig. 2). On reaching the anterior chest wall it anastomosed with the internal thoracic artery, forming an angle of approxi- mately 90 ° (Fig. 3). As the collateral intercostal artery descended across the intercostal space, it occasionally gave a recurrent posterior branch that reached the region of the rib head. In most intercostal spaces, vertical and/or oblique branches were observed between the intercostal arteries and their collateral branches. The size of the posterior intercostal artery and its collateral branch diminished from posterior to anterior, being on average across all intercostal spaces 2.2 and 1.0 mm posterior to point A and 1.5 and 0.6 mm at point B. From the midaxillary line to the parasternal region, an increase in size was observed; however, it did not exceed 0.5 mm. Considering the vascular bundle as a whole the values were respectively: 3.7 and 1.6 mm and 2.9 and 1.0 mm. At the rib angle the collateral intercostal artery showed a significant decrease in mean size from 2.3 mm to 1.7 mm from the 2nd to the 5th intercostal spaces; however, at the same point the posterior intercostal artery showed an increase from 2.5 mm to 3.6 mm (Tables 1, 2). In the 1st intercostal space the difference in size between the intercostal artery and its collateral branch was minimal, with the branch sometimes being the greater; this was usually observed in the 2nd intercostal space. Below the 4th intercostal space this difference became progressively larger. The mean values for the intercostal artery and its collateral branch in the 8th intercostal space were: at point A 4.3 mm and 1.0 mm, at point B 3.8 mm and 0.5 mm, and at point C 2.0 and 1.5 mm, respectively. The distances between the vascular bundles in the intercostal spaces were 7.0 mm at point A, 5.0 mm at point B and 7.0 mm at point D. The posterior and anterior intercostal arteries are direct branches of the aorta and internal thoracic artery, respectively [1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 13, 16, 17, 18, 19, 20, 22, 23, 24, 25, 26, 27]. The collateral intercostal artery originates from the intercostal artery at the costal angle and descends to the upper border of the lower rib to anastomose with the anterior intercostal artery from the internal thoracic artery [3, 4, 8, 11, 17, 18, 20, 22, 27]. The branches diverge at various angles, anastomosing with branches of the intercostal artery in the same or an adjacent space. The caliber of the collateral vessel may be larger or smaller than the intercostal artery with the branches being transverse, longitudinal or plexiform [23]. In the lower two intercostal spaces, the posterior intercostal artery and its collateral branch pass directly into the anterior abdominal wall, there being no anastomosis with the anterior intercostal artery [4]. The collateral branch is usually accompanied by a collateral branch of the intercostal nerve [13]. There appears to be no data in the literature on the size of the collateral intercostal artery. From the present study the posterior intercostal artery and its collateral branch do not show significant differences in caliber between the 1st and 4th intercostal spaces. The increase in size that occurs in the intercostal artery from the 4th intercostal space is not, however, accompanied by an increase in the size of its collateral. On the con- trary, there is a progressive decrease in size of the order of 10% from branch to branch at point A. On the basis of the data presented here, the procedure for performing thoracocentesis through the intercostal space is recommended to be as follows. The patient should be seated or lying in the lateral decubitus posi- tion. The arm on the side to be punctured should be elevated and the hand placed on the back of the neck, thereby causing rotation of the scapula and enlargement of the intercostal spaces [2]. Thoracocentesis should be performed in the 8th or 9th intercostal spaces immediately below the scapula [10]. However, for hypertensive pneumothorax, the 2nd or 3rd spaces should be used, when draining blood or pus the 6th or 7th intercostal spaces, and when inserting chest drains the 5th, 7th or 8th intercostal spaces using the midclavicular or midaxillary lines as a reference [19]. Access to the thoracic cavity through the intercostal space is performed as follows. The upper margin of the inferior rib is located using the thumb or index finger and the needle introduced immediately above its upper border, taking care to avoid damaging the neurovascular bundle that runs under the superior rib [2]. When placing chest drains, the scissors must advance along the superior margin of the inferior rib to avoid the intercostal vessels [19]. Due to the presence of the collateral branch and its significant size, a puncture close to the costal margin, as recommended in the literature, may lead to laceration of the collateral intercostal artery. Such a laceration could cause consequent hemorrhage, especially with punctures of the 3rd intercostal space for hypertensive pneumothorax, where the collateral intercostal artery can be of a similar size or larger than the intercostal artery. The same may also be said about chest drainage, in which the advancing instruments (scissors or Kelly) may damage the structures in question. Thoracoscopy is increasingly being utilized, with access to the thorax also being via the intercostal space: this is usually along the anterior axillary line in the 7th or 8th intercostal space [14]. One of the most common complications associated with the technique is hemor- rhaging, which may be severe [14, 21]. The size of the intercostal and collateral vessels decreases towards the midaxillary line, after which it increases again to the parasternal region. In view of this it is considered that the midaxillary line is the safest place to implement techniques that use the intercostal space to access the thoracic cavity. The present study has verified that the present techniques of thoracocentesis do not adequately take into account the size of the collateral branches of the intercostal artery. In some intercostal spaces the size of collateral branch was found to be equal to or greater than that of the intercostal artery. Consequently, care must be taken to avoid damage. From an anatomical viewpoint it is recommended that the following criteria for procedures to avoid lacerations to the intercostal and collateral arteries are adopted: 1. Use the lowest intercostal spaces possible, preferably the 6th, 7th or 8th intercostal spaces. 2. Make the initial incision as close to the midaxillary line as possible. 3. Do not go immediately above the inferior costal border, but aim for the center of the intercostal space. These criteria should be applied within any technical limitations and need to be considered on a case-by-case ...