Surgical Anatomy

The thoracic cavity is a cage formed by the 13 pairs of ribs laterally, the thoracic vertebrae dorsally and the sternum and costal cartilages ventrally. The ribs which articulate with the sternum, via a costal cartilage, are called the sternal ribs and are typically ribs 1–9; ribs 10–12 are called the costal ribs and form the costal arch. The thirteenth rib is a floating rib. The superficial thoracic wall consists of combined muscles: the latissimus dosrsi, the serrates, the scalene and the external abdominal oblique. The deep thoracic wall consists of the ribs and the external and internal intercostal muscles. The intercostal muscles receive segmental innervation and blood supply from the spinal cord and aorta respectively. These nerves and vessels run down the caudal aspect of each rib. The thoracic cavity is separated from the abdomen by a musculotendinous sheet, the diaphragm. The diaphragm receives its innervation from the phrenic nerve which leaves the spinal cord at C5 and travels through the chest closely associated with the pericardium.

Physiology of the Thoracic Wall

The thoracic wall has a combination of active muscular and passive elastic structures. The elastic nature of the thoracic wall can cause an increase or a decrease in the intrathoracic volume depending on the position of the ribs at the time of relaxation. The action of the intercostal muscles is to draw the ribs in a cranial direction, which, because of the 'bucket handle' effect results in an increase in thoracic volume. In addition, contraction of the muscular diaphragm flattens it, causing a further increase in thoracic volume. The net effect of the increase in thoracic volume is a reduction in the intrathoracic pressure which creates a pressure gradient down which air flows, via the airway, to fill the lungs and equalise the pressure. The act of expiration, unlike inspiration, is passive. The thoracic wall (and lung tissue) has elastic properties: once the active muscular contractions subside they cause the thoracic volume to return to its resting level.

Effects of Thoracotomy

Opening the chest allows air to enter the pleural space, preventing the pressure changes required for pulmonary ventilation. Positive-pressure ventilation restores pulmonary function but may diminish pulmonary circulation. Intraoperative mechanical restrictions to pulmonary function include atelectasis (collapse) of the lung that is lowermost (lateral thoracotomy) and packing off of the lungs to increase the surgical exposure. The resultant hypoventilation may cause hypoxaemia and hypotension. Stimulation of the vagus nerve during surgery may also induce a bradycardia, further contributing to the respiratory and cardiovascular compromise.

Surgical Approaches

 Lateral intercostal

 Lateral - rib resection

 Midline sternotomy

 Transdiaphragmatic

 Minimally invasive - endosurgical

See Figures 1 and 2.  

Figure 1. Surgical approaches. Diagram showing potential sites for lateral intercostal incisions and cranial and caudal sternotomy incisions.

Figure 2. Location of intrathoracic structures relative to position of intercostal thoracotomy.

Thoracocentesis

Indications include:

 Unstable pneumothorax (spontaneous, traumatic or tension)

 Removal of free pleural fluid, either for investigative or therapeutic purposes (blood, pus, chyle, chylous, transudate)

Usually use a 25–50 mm butterfly needle or intravenous catheter in the dorsal or ventral seventh or eighth intercostal space on the affected side. Clip the hair from the selected area and prepare in a routine aseptic manner. Insert the needle at a 45-degree angle to reduce the risk of damage to the lung. Fluid is most easily collected by ventral thoracocentesis in the standing or ventrally recumbent animal, vice versa for air. The use of a three-way tap prevents inadvertently creating or worsening a pneumothorax.

Thoracic Drainage (Tube Thoracostomy)

Indications include:

 Routine postoperative care

 Long-term drainage of cases with pleural disease:

 Pyothorax

 Pneumothorax

 Chylothorax

 Chylous thorax

 Other pleural fluid accumulation

In the emergency situation this can be carried out in the conscious or lightly sedated animal. If performed as an elective procedure in a stable patient general anaesthesia is preferred. A selection of different size thoracic drains is advisable, trocar drains can be very useful if rapid placement is required. It may be necessary to make extra holes in the end of the drain using Mayo scissors.

Tube placement through the dorsal third of the seventh or eighth intercostal space is usually the most safe. A generous area of skin is clipped over one or both sides of the chest while the animal is still conscious. Under either sedation or anaesthesia, following preparation, a small stab incision is made over the 11th or 12th rib. Using either a trocar tube or a large artery forceps (Carmalt) with the chest tube held between its jaws, a subcutaneous tunnel is made extending cranially to the seventh or eighth intercostal space. The tube or forceps are elevated so as to point at the opposite shoulder and with controlled force of short duration the intercostal muscles are penetrated. The tube is then directed cranially and ventrally until its end is level with the first rib. The tube is secured to the skin using a Chinese finger-trap suture; a second purse-string suture is placed around the stab incision and tied loosely.

Placement of drains following thoracotomy is a little easier as long as a few principles are followed. The tube should not enter the chest through the thoracotomy incision but caudal to it. A two to three rib space subcutaneous tunnel is ideal (Figure 3). Ensure the drain does not have holes in it which are within the subcutaneous tunnel and remember to place it while the chest is still open so the lung tissue can be protected.

Figure 3. Correct position for thoracostomy tube placement.

Postoperative Patient Care for the Thoracic Patient

Thoracotomy patients need postoperative fluid therapy to replace insensible losses as well as losses into the pleural space. A balanced electrolyte solution at a rate of 10 ml/ kg/hr is appropriate initially, providing the animal is not in heart failure. The rate and composition of fluid should be adjusted according to the nature of the underlying disease(s) and the results of the postoperative patient evaluations. In addition, careful attention should be given to respiratory function, cardiac function, analgesia and inter-pleural accumulations of air or fluid. These can all be assessed by the frequent collection of a combination of subjective and objective data.

 Respiratory function:

 Respiratory rate and effort

 Mucous membrane colour

 Thoracic auscultation

 Arterial blood gas

 Pulse oximetry

 Thoracic radiographs

 Cardiovascular function:

 Heart rate and rhythm

 Mucous membrane colour and capillary refill time

 Pulse rate and quality

 Direct or indirect arterial blood pressure

 Central venous pressure

 Electrocardiogram (ECG)

 Packed cell volume/total solids (PCV/TS)/azostix estimate of azotaemia/dextrometer evaluation of blood glucose level

 Pleural disease (fluid/air)

 Auscultation

 Amount of fluid/air removed via thoracostomy tube(s)

 Thoracic radiography

 Thoracic ultrasonography

The intensity of postoperative care required varies for each patient and depends, again, on the age of the animal, nature of the underlying disease, presence of concurrent disease and the surgical procedure performed. In general any animal with a thoracostomy tube needs constant supervision until such time that the tube can be removed. The parameters mentioned above should be monitored as frequently as is necessary to make well informed decisions about therapeutic alterations. As parameters stabilise the intensity of observations and acquisition of data should be gradually reduced.

Analgesia for the Thoracic Patient

Most thoracotomy patients with normal renal function should receive a non-steroidal anti-inflammatory drug, an opioid (e.g., methadone, morphine) and local analgesia in the form of an intercostal nerve block or sternal incision block with bupivacaine or ropivacaine perioperatively. After surgery, maintaining adequate analgesia is essential to a good recovery. Interpleural local anaesthetic can be 'topped up' by placing the dose into the thoracostomy tube and flushing it into the chest with saline or through a wound soaker catheter. Opioids alone or opioid combinations (methadone, lidocaine and ketamine - MLK) should be administered intravenously for the first 12–24 hours after surgery. Non-steroidal drugs should be continued immediately after surgery and can be continued for 1–2 weeks, as needed. The level of analgesia provided must be sufficient to make the animal comfortable but should not keep the animal recumbent beyond the first 12 hours postoperatively. Adjustment of the analgesic protocol following removal of the chest drain, and based on the response to opioid analgesia and subjective assessment of patient comfort, will be required.

Care of the Thoracostomy Tube

Animals with thoracostomy tube(s) in place require continuous monitoring. Removal of the end of such a tube by the patient could result in a rapidly fatal pneumothorax.

 The tube should have a three-way tap on the end which is wrapped in antiseptic-soaked swab when not in use.

 The tube should also be clamped separately using an artery forceps/gate clamp for added security.

 The whole tube can then be incorporated in a lightly applied chest bandage.

 If a continuous drainage system is used (pleurovac, water seal collection system or Heimlich valve) all the connections should be kept clean by wrapping them in antiseptic-soaked swabs.

 The entry incision can also be kept clean in a similar manner, using a sterile dressing that is changed daily.

 Frequency of aspiration of the chest tubes will depend on the disease process for which they are being used. Usually every 2 hours is sufficient for moderate fluid or gas accumulations. If more frequent drainage is required, a continuous system may be more advisable.

 For animals that have had a tube placed as routine postoperative management and in which prolonged pleural fluid accumulation is not expected, once the pleural space is empty (i.e., very little to no fluid or air on two consecutive drainage attempts) the tube can be removed.

 The timing of tube removal, in other circumstances, will depend on the volume and character of fluid being removed. The author's rules of thumb here are: tube removal when less than 10 ml/kg/day of fluid is being produced and no tube should stay in for longer than 7 days.

 Cytological examination of the fluid being removed can be helpful in this decision making.

Pulmonary Function

As previously mentioned, thoracotomy can result in atelectasis of lung and this, in turn, will result in reduced pulmonary function as a consequence of ventilation:perfusion mismatching. In addition, if the original disease process had caused aspiration pneumonia (e.g., vascular ring anomaly), ongoing support of pulmonary function is needed. Postoperative care practices can have an extremely beneficial influence over recovery:

 Appropriate analgesic protocol: this will promote a normal breathing pattern with deep breaths and sighs, which are essential to keeping all lungs functioning.

 Oxygen supplementation: all thoracic surgical patients benefit from oxygen supplementation in the immediate postoperative period. This can be provided by nasal cannulae, nasal prongs, flow-by oxygen or the use of an oxygen cage.

 Regular patient turning will prevent persistent collapse of dependent lung lobes secondary to the pressure of adjacent organs.

 Removal of thoracostomy tube will make the patient more comfortable and encourage deeper breathing and facilitate movement.

 Physical therapy:

 Early ambulation: this will also encourage deep breaths and lead to more rapid resolution of atelectasis.

 Coupage: gentle coupage, usually coupled with nebulisation therapy, can help loosen thick tracheobronchial secretions and exudate and facilitate their clearance and thereby hasten recovery from atelectasis or pneumonia.

Nutrition

As with any surgical patient, how the nutritional needs of the animal will be met after surgery, must be considered. Ensuring appropriate nutrition in the postoperative period is essential to a rapid recovery from surgery. For a patient with ongoing needs (e.g., dog with vascular ring) provision of a feeding tube (in this example gastrostomy tube would be most appropriate) should be part of the surgical plan. For the majority of patients, provision of a feeding tube is not necessary but early oral alimentation should be encouraged and will be facilitated by ensuring analgesia is appropriate and other aspects of patient care (fluid and electrolyte balance etc.) are under control.

References

1.  Bennett A, Orton EC, et al. Cardiopulmonary changes in conscious dogs with induced progressive pneumothorax. American Journal of Veterinary Research 1989;50:280–284.

2.  Conzemius MG, Brockman DJ, et al. Analgesia in dogs after intercostal thoracotomy: a clinical trial comparing intravenous buprenorphine and interpleural bupivacaine. Veterinary Surgery 1994;23:291–298.

3.  Evans HE, Christensen GC. The respiratory apparatus. In: Miller's Anatomy of the Dog. 2nd ed. Philadelphia: WB Saunders, 1979.