Small animal thoracic surgery: approaches and techniques

Transcription

1 Vet Times The website for the veterinary profession Small animal thoracic surgery: approaches and techniques Author : Tim Charlesworth Categories : RVNs Date : December 1, 2011 Tim Charlesworth MA VetMB CertSAS MRCVS, discusses the essentials of this high-risk surgery THERE are many indications for operating within the thoracic cavity of small animals. Thoracic surgery inevitably carries a significant risk to the patient, irrespective of the underlying disease process and type of surgical intervention being performed. The veterinary nurse has an essential role in ensuring that this risk is minimised through careful preoperative preparation and planning, intraoperative assistance and anaesthesia, and postoperative monitoring and care. Surgical approaches Surgical approaches to the thorax include: intercostal thoracotomy, rib resection thoracotomy, median sternotomy, transsternal thoracotomy and transdiaphragmatic thoracotomy. The choice of surgical approach is normally dictated by the location and type of underlying disease process, the size of any lesion to be excised and the status of the patient. The intercostal approach is most often used, although the field of view obtained can be quite limited, and some lesions for example, very large lung tumours may not be suitable for removal through this type of incision. The rib space selected is dictated by the target organ see Table 1. Intercostal thoracotomy also restricts the surgeon to one hemithorax. Median sternotomies are therefore preferred when large masses need to be removed or when bilateral disease is present. It is also the approach of choice for an exploratory thoracotomy. Median sternotomies are inherently more invasive procedures and thoracoscopy has started to be used as a minimally invasive alternative in certain procedures or to identify and localise a target lesion that can then be accessed through the relevant intercostal thoracotomy. 1 / 6

2 Details of these surgical approaches can be found elsewhere (Orton, 2003). Surgical instrumentation Both surgeon and assistant should be familiar with the surgical instruments commonly used in thoracic surgery. Many instruments are the same as for abdominal surgery, although commonly much longer to allow for dissection of structures deep inside the thoracic cavity ( Figure 1 ). Additional instrumentation includes Satinsky cardiovascular clamps, Galibans sutureholding forceps and stapling equipment ( Figures 2 and 3 ). Gelpi retractors may be adequate to retract the edges of the thoracotomy wound in small dogs and cats, but Finochietto retractors are commonly used in larger animals ( Figure 4 ). All instruments should be checked as they are laid out on the trolley to ensure they are in good working order and free of roughened or irregular areas for example, in the jaws of forceps that could lead to accidental tissue trauma when used. Chest drains and connectors should be available. Patient positioning For a median sternotomy patients are positioned in dorsal recumbency with their forelimbs tied in extension. Patients are in lateral recumbency for intercostal thoracotomy. It is useful to place a sandbag under the chest at the level of the proposed incision site as this helps spread the uppermost ribs and improve exposure once thoracotomy has been achieved. The sandbag should be removed prior to closure. Anaesthetic considerations Careful thought should be given to the choice of anaesthetic regime employed for thoracic patients. Patients may be well and asymptomatic, such as for patent ductus arteriosus (PDA), or dyspnoeic and debilitated, such as for pyothorax, in which case preoperative stabilisation is required. Appropriate neuroleptanalgesic premedication should be given to all but the most compromised of patients. Low doses of acepromazine ( mg/kg) coupled with an opioid (for example, morphine, methadone or pethidine) usually suffice. Care should be taken to not adversely affect ventilation with the choice and dose of opioid used. Intravenous access should be achieved before induction. NSAIDs can be given perioperatively if there are no contra-indications to their use. All patients should be pre-oxygenated (for example, by face mask) unless this causes stress, in which case it can be counterproductive. Anaesthesia may be induced with injectable agents, such as propofol, alfaxalone or ketamine, by mask, or a combination of the two. Maintenance may be within normal inhalational agents alone or in combination with intravenous infusions (for example, fentanyl, alfentanil or analgesic constant-rate infusions), which will lower the amount of inhalation agent needed. It is most important, however, that the anaesthetist uses an anaesthesia regime with which he or she is comfortable and experienced, rather than trialling unfamiliar drugs during a high- 2 / 6

3 risk surgery. A pneumothorax will be created as soon as the thorax is entered and the normal relations of the lungs to the thoracic wall will be disrupted. The lungs will collapse inwards through their inherent elastic recoil and the ribs will spring outwards. Patients will continue to breathe unaided, but the lung on the open side of a unilateral thoracotomy will expand on expiration (paradoxical respiration) and take in carbon dioxide-rich (and oxygen-depleted) air from the contracting contralateral lung. Ventilation is required to abolish these effects and also to prevent atelectasis. Any atelectatic lung contributes to ventilation/perfusion mismatch through physiological shunting of blood (flowing between the pulmonary and systemic circulations without being oxygenated by the alveoli). It is essential the patient is ventilated until the pleural cavity has been evacuated at the end of surgery. Patients may be ventilated either manually (a second anaesthetist is required) or by using a mechanical ventilator. Advantages and disadvantages are associated with both methods. Neuromuscular blockade is not necessary. All normal patient parameters should be monitored during anaesthesia, including: heart rate and rhythm, capillary refill time, pulse quality, ECG, pulse oximetry, capnography and blood pressure. Arterial catheters are useful for serial blood gas analysis and invasive blood pressure monitoring, but they can be difficult to place and maintain. Non-invasive blood pressure monitoring is usually adequate. The Doppler probe can be secured in place to give an audible warning of any intraoperative haemodynamic changes. Temperature should be monitor closely and warming blankets, such as Bair Huggers, used whenever possible. Extremities can be wrapped in bubble wrap and intravenous fluids should be warmed when necessary. Irrigation of the thoracic cavity with warmed fluids will rapidly raise the core temperature and may be performed intermittently during surgery. Irrigation with cool fluids will rapidly cause hypothermia and should be avoided (unless planned). Patients are frequently hypotensive, which can lead to renal hypoperfusion. It is therefore helpful to place a urinary catheter and record urine output. Intravenous fluids should be administered as needed to maintain normal blood pressure, tissue perfusion and urine output. It is vital for the surgical team and anaesthetists to communicate effectively during thoracic surgery ventilation should be timed so that entry to the thoracic cavity is on expiration to minimise the risk of traumatising underlying lung tissue. The surgeon should time surgery so that sharps are not in contact with the lungs during inflation. The surgeon must advise the anaesthetist of exactly what is being done at each stage of the surgery so that any adverse physiological effects can be anticipated and minimised for example, manipulation of the vagus nerve ( Figure 5 ) can cause a sudden and profound bradycardia, retraction of the mediastinum can occlude the caudal vena cava, and handling of the heart can cause arrhythmias. The surgeon should be asked to stop any non-essential manoeuvre that causes a sudden 3 / 6

4 unexpected change to any of the vital parameters. Similarly, if the lungs stop expanding, the anaesthetist should be alerted immediately. If there is no break in the anaesthetic circuit, the trachea/ bronchi may need to be suctioned to remove any plugs of mucus/foreign material that may be occluding the airway. The scrubbed assistant Surgery involving the thoracic cavity can appear daunting, but this is usually due to unfamiliarity with the regional anatomy. It is very useful for an assistant to familiarise him or herself with the anatomy and the details of the procedure prior to surgery. It is good practice to run through precise surgical details with the lead surgeon and anaesthetist on the day of surgery so that everybody knows exactly what is planned, what could go wrong and how this may be resolved. Tissues should be handled with care as lungs can be very friable and easily damaged in various disease states. Retracting the heart and/or great vessels will inevitably lead to pronounced cardiovascular effects and it may be necessary to release the heart occasionally to allow the patient s cardiovascular status to recover. Details of each surgical procedure will vary, but there are certain common steps, as described below. Local anaesthetic (such as bupivacaine) is used to place an intercostal block at the surgical site and two rib spaces cranial and caudal prior to incision (intercostal approach) or at the junction of each rib with the sternebrae (median sternotomy). The surgical approach is made, the wound edges are protected with moistened laparotomy swabs and retractors placed. Lung tissue that interferes with the surgeon s view can be packed away with moistened swabs ( Figure 5 ). The surgical intervention is performed. The thorax is lavaged. If the surgery has involved the respiratory tract, such as lung lobectomy, the lavage fluids are examined for the formation of air bubbles during inspiration that may reflect a leaking bronchus, etc. If no such leak occurs, the fluid is removed by suction. All swabs are removed (swab count performed) and the lungs visually inspected to ensure that reinflation is occurring (this may take several breaths) and that no lung lobes have torsed. A chest drain (thoracostomy tube) is placed through a separate stab incision, tunnelled under the skin and into the thorax, taking great care not to penetrate any organs in the process. The surgeon starts to close the wound. Once the wound appears airtight, the assistant evacuates 4 / 6

5 air from the pleural cavity using a syringe and a three-way tap, or one-way centesis valve, connected to the chest tube. This continues until negative pressure is obtained. The patient is allowed to resume spontaneous respiration at this point and oxygenation levels are monitored closely. The wounds are closed, a Chinese finger trap suture is placed around the chest tube and a purse string suture can be pre-placed and then tied when the chest tube is removed. The patient is allowed to recover ( Figure 6 ). Postoperative care Postoperative care will vary according to the procedure(s) performed, the preoperative status of the patient and the presence or absence of any significant intraoperative complications. The immediate priorities are to maintain respiratory function and to provide sufficient analgesia. Airway the airway may need to be suctioned on recovery if there has been fluid or mucus released into the airways by intraoperative manipulation. Postoperative oxygen should be provided via nasal prongs or an oxygen cage, or be easily to hand. The patient s respiratory rate and character, oxygenation and general status should be monitored continually until recovery from anaesthesia is complete. Patients should be recovered on deep beds and maintained in sternal recumbency to allow maximum inflation of both lungs. Great care must be given to patients undergoing major lung lobectomy or pneumonectomy as recovery in lateral recumbency may prove fatal (for example, if the left lung is removed and the animal is recovered in right lateral recumbency then it may not adequately oxygenate its remaining lung). Thoracostomy tube normally checked when the patient is in recovery and then at least hourly for the first few hours postoperatively. The frequency of chest drainage will depend on the surgery performed and the volume and character of air/fluid retrieved at the previous drainage. The drain is secured to the patient s body with a stockinette bandage. An Elizabethan collar may be used to prevent patient interference. The chest drain can be removed after only a few hours if no air/fluid is produced. In the presence of ongoing pleural effusion, the drain can be removed when the level of fluid is less than 2ml/kg/24 hours. Sterile gloves should be worn by anyone handling the chest drain and only personnel confident in using three-way taps and gate clamps, etc should perform thoracic drainage. A sudden and dramatic increase in the amount of air obtained via a chest drain may indicate a sudden leak from the intrathoracic respiratory tract, dislodgement or a leak from the chest drain itself or operator error in using the three-way tap. Analgesia this is essential as any pain associated with movement of the thoracic wall may further compromise respiratory function. A multimodal approach is appropriate, using combinations of opioids, NSAIDs, constant-rate infusions and local anaesthetics (for example, instilling bupivacaine down the chest tube). 5 / 6

6 Powered by TCPDF ( Thoracic surgery requires good planning and input from all the team members at all stages to ensure a good outcome. References and further reading Baines S J and Neiger-Aeschbacher G N (2005). Principles of head, neck and thoracic surgery. In BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery. McMillan M (2009). How to breathe easy during anaesthesia, VN Times CPD supplement, 3 (6): pp 1-4. Orton E C (2003). Thoracic Wall. In: Slatter D (ed) Textbook of Small Animal Surgery. Volume 1 (3rd edn). Saunders, Philadelphia. Pascoe P J (2007). Thoracic Surgery. In BSAVA Manual of Canine and Feline Anaesthesia and Analgesia (2nd edn). 6 / 6