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Thoughts on Anaesthesia for a Splenectomy - A Tea Time Teaser

Ana del Alamo Foster MS DipACVAA MRCVS

Sam is a 12 year old male labrador, with a body condition score (BCS) of 7/9, who has presented with a large, pendulous abdomen. He is very lethargic and painful. His mucus membranes are very pale, with capillary refill time (CRT) of less than 2 seconds, moderately weak femoral pulse and a heart rate of 112 bpm.

Abdominal radiography confirmed the presence of a very large mass on the spleen that needs to be surgically removed and an ultrasound showed large amounts of fresh blood in the abdominal cavity.

1.  From the point of view of anaesthesia, what are the immediate concerns with Sam?

Sam is not an ideal candidate for anaesthesia at this point and, ideally, efforts should be made to stabilise his vital signs before surgery.  However, in some splenic tumor rupture cases in which hemorrhage is severe, emergency surgery is necessary before completely stabilizing his condition.

Hypovolemia, due to hemorrhage, is the main concern with Sam. This will result in low blood pressure, inadequate perfusion of organs, tissue hypoxia, changes in body fluid composition and electrolytes, centralization of blood (resulting in cold limbs), changes in heart rate and contractility, and decrease number of red blood cells in the bloodstream. Efforts should be aimed at replenishing blood volume (with isotonic fluids, colloids, fresh blood or packed red blood cells) as soon as possible.

While getting an IV catheter and placing Sam on fluids (begin with administering a 20 ml/kg fluid bolus of a balanced isotonic solution, such as Hartmann’s) a blood sample should be obtained to, at the very least, determine Sam’s packed cell volume (PCV) or hematocrit.

The PCV will help determine the need for blood transfusion or not. The cutoff hematocrit at which a transfusion is warranted has been set at 20% for dogs and 15% for cats. Below this level, tissue oxygenation will be severely compromised. However, these values are not absolute; the decision to begin a transfusion should depend on the clinical signs and status of the patient, the chronicity of the bleed (on one side, lower hematocrits are tolerated with chronic anaemia and, on the other, hematocrit may not accurately reflect the severity of the blood loss when hemorrhage is very recent and fast), and the availability of blood products, along with the hematocrit value.

In Sam’s case, there are clinical signs clearly indicating compensated hypovolemia and spleen tumor rupture indicates the process is acute. Due to this, Sam would hugely benefit from a blood transfusion before surgery.

2.   Should a blood transfusion be done before or after removing the spleen?

After all, the spleen is actively bleeding and the blood transfused will be wasted, since it will end up in the abdomen.

This is true. If the spleen is actively bleeding, the patient will continue to loose blood as you are administering. And yes, this blood is quite valuable and expensive. So why waste it?

Because this same patient still needs to survive general anaesthesia and surgery and, for that, a blood transfusion may be vital. In some cases, a blood transfusion only buys the patient enough time to make it to surgery and further transfusions may be necessary to replenish red blood cells during the post-operative period.

In order to best assess the need for a blood transfusion, one should look at the disease process and how acute it is, the progression of clinical signs, and the co-morbidities already present in the patient (i.e. diabetes, liver or kidney disease etc.).

3.   What other concern is associated with this case?

The vast majority of dogs that come to us with splenic tumors are more than 10 years old.

An animal is said to be geriatric when it has reached 75% of its expected lifespan thus, all these dogs would be considered geriatric and should not be treated in the same way as a sick three-year-old patient.

Although age in itself is not a disease, it does mean that the reserve capacity of the major organs in the body is reduced.

When, as in this example, there is a situation of hypovolemia, the heart may not be able to increase the force of contractility effectively enough to keep distributing blood to the body, aggravating the problem. At the same time, the organs that are now hypoxic, may not be able to recover their full health when adequate perfusion in restored.

Geriatric patients tend to be overweight. In Sam’s case, his cardiovascular system is compromised not only because of the loss of blood and the anaemia, but also because of his age and possible long term compensation from being overweight.

Moreover, during an exploratory laparotomy patients are positioned in dorsal recumbency, which will throw the weight of the abdominal organs over the diaphragm, diminishing the vital capacity of the lungs. This is more severe in overweight and obese patients than in those with healthy weights.

4.   What anaesthetic protocol is the best?

As you know, there is not one magical protocol that works for every patient so it is difficult to suggest a specific combination of drugs.

Choice of drugs will vary greatly between patients with this disease but, generally, I would advise using adequate analgesia (i.e. methadone) in combination with very small doses of other sedatives, such as midazolam or dexmedetomidine.

That said, there are two interesting points to be made here about the sedatives available for premedication:

  • Acepromazine: remember that, though it is a very useful sedative, acepromazine’s main side effect is hypotension caused by vasodilation (it is an antagonist of alpha-1 receptors in the peripheral vasculature, which mediate vascular tone). This hypotension is problematic for two reasons. First, it is difficult to treat and, second, acepromazine cannot be antagonized and is long acting. Due to this, it should be used with caution in patients with compromised perfusion. If needed at all, I would recommend using 0.0025-0.005 mg/kg IV. Such a small dose will provide sufficient sedation in combination with an opioid.
  • Midazolam: typically used only as a co-induction agent with propofol, midazolam can also be used as a premedication drug. In adult companion animals, as opposed to people, it has negligible sedative effects. However, when used in combination with other premedication drugs (i.e. methadone or dexmedetomidine), it potentiates their sedative effects. This is especially useful in geriatric patients and those that are too sick to tolerate more potent sedatives. I would recommend a dose of 0.1-0.2 mg/kg, administered IV or IM.

For induction, a combination or propofol and midazolam (or diazepam), titrated slowly to effect should be acceptable.

Occasionally, in cases that need to be rushed into surgery and have lost so much blood that perfusion to organs is severely compromised and mentation is poor, induction can be achieved, without using premedication, with a combination of fentanyl and midazolam titrated to effect. Propofol, despite its high safety margin in dogs and cats, can cause pronounced hypotension in sick animals, which is why it is good idea to substitute it with safer agents in high risk patients (ASA status of 5). Do remember that the analgesic effects of fentanyl don’t go beyond 20 minutes of duration and you will need to supplement analgesia after that time.

5.   Speaking of analgesia, what pain relief is adequate for a splenectomy?

An exploratory laparotomy can be classified as a moderately painful procedure. Because of this, it is certainly important to provide adequate analgesia.

A full mu agonist should be provided, such as methadone or morphine (or fentanyl). Ideally, it should be given before surgery.

I would suggest supplementing the analgesic effect of the opioid with either a lidocaine or fentanyl constant rate infusion (CRI) during surgery. Fentanyl is also a full mu agonist and lidocaine is a local anaesthetic that provides good visceral analgesia. Both of them, when given intraoperatively, will have MAC sparing effects, which will help reduce the hypotensive effects from the inhalant anaesthetic (isoflurane).

Before administering a CRI to a patient, remember to administer a bolus dose first to bring drugs plasma levels up to a useful concentration! Otherwise, the CRI will be pointless.

Another option for analgesia is to perform an epidural with morphine and bupivacaine, which will also have a MAC sparing effect. However, the main side effect that we see with epidural drug administration is arterial hypotension due to sympathetic outflow blockade (causing vasodilation). The advantage of an epidural with morphine is that it can provide good analgesia for up to 24 hours!

Non-steroidal anti-inflammatory drugs (NSAIDs) are often a subject of controversy in this topic. The main concern when using these drugs is the potential for renal damage, which is why their use is avoided when there is the potential for renal disease… and during general anaesthesia. However, there is a large body of research investigating the use of different NSAIDs, such as carprofen, during general anaesthesia in dogs and cats, during normo and hypotension and, in general, results have shown that they are safe to administer before general anaesthesia. In practice, most anaesthetists are cautious and tend to administer them intra or post-operatively, except in some of the healthy cases anaesthetized for routine or orthopedic procedures.

Unfortunately, there is no perfect answer yet to the question of when is best to administer NSAIDs.

In Sam’s case, I would choose to administer meloxicam intraoperatively, provided that blood pressure had been maintained at a mean arterial pressure above 60 mmHg after induction and during the first few minutes of general anaesthesia.   

6.   What can you expect to happen during anaesthesia?

There are many things that can wrong when anaesthetizing an old, overweight, anaemic dog for a splenectomy but I believe that there are a few of these complications that are worth mentioning as a reminder: ventricular premature contractions and complications derived from positioning of the patient.

VPC´s are a common finding in dogs with splenic and liver tumors or that present with GDV (gastric dilatation and volvulus). It is suspected that these VPC´s are a consequence of myocardial hypoxia or myocyte irritation from toxins circulating in the bloodstream but the exact cause, in dogs with these pathologies, is still not clear. Regardless of the reason, it is important to monitor the ECG of these patients but, in my experience, VPC´s have rarely posed a problem and have usually resolved on their own a few days after surgery.

I would recommend placing an ECG before induction of general anaesthesia to quantify their frequency and to assess their morphology, and I would also measure blood pressure, at the same time. If blood pressure is low and the VPC´s are more frequent than one every 6-10 normal QRS complexes, or if they are multifocal (different shapes will mean that there are several foci in the ventricles generating an electrical impulse, which increases the risk of ventricular fibrillation), I would suggest administering a slow bolus of 2 mg/kg of lidocaine and beginning a CRI at 50-100 mcg/kg/min. The aim will be to increase cardiac output and blood pressure, by reducing the frequency of VPC´s, instead of abolishing them completely.

However, remember to keep the whole clinical picture in mind. Low blood pressure may, most likely, be due to hypovolemia instead of VPC´s.

Speed is of essence when a patient like Sam is anaesthetised and often times the solution for hypotension is to surgically stop the bleeding and administer fluids and blood products.

Positioning for an exploratory laparotomy is a major issue in these patients because they need to be in dorsal recumbency in order to perform surgery on them.

The main issue with this is that, when placing an animal on its back, the weight of the abdominal organs, including the large splenic tumor, falls on caudal vena cava, partially occluding it, impeding venous return to a heart that may very well be already struggling to pump an inadequate low blood volume.

The second complication from this is that the weight of the organs will also fall on the diaphragm, reducing the ability of the lungs to expand (reducing both functional residual capacity and tidal volume). So not only will you find yourself with an anaemic patient that is hypovolemic, with compromised venous return to the heart and hypotension, but now they may also struggle to ventilate properly and may need to be assisted with IPPV (mechanical ventilation) or manual ventilation.

On top of this, as mentioned before, these patients are often geriatric, which means their organs reserve capacity is already stretched to the limit.

You will notice that Sam may not be able to withstand dorsal recumbency, despite your efforts to ventilate and treat hypotension. This is why it is often useful to clip the abdomen before general anaesthesia or even in lateral recumbency, as he will be able to compensate better in this position.

Lastly, remember that you may see severe hypotension when the surgeon lifts the tumor off the abdominal cavity. This is short lived, but intense, and is usually due to the sudden reperfusion of the tissues underneath the tumor and the release of waste toxins from hypoxic cells that were trapped there, being underperfused.

It is sometimes useful for the surgeon to communicate with the anaesthetist when they are ready to remove the tumor and to do it at a relatively low pace, to give the patient’s cardiovascular system a chance to compensate.

A few final thoughts:

As with many other emergencies, it is not the actual anaesthetic drugs chosen that makes a difference. It is far more important to assess each patient separately, treat hypovolemia and hypotension aggressively and know what to expect during general anaesthesia and how to manage the problems that arise from positioning and surgery.

Lastly, efficiency and reducing the amount of time under general anaesthesia are paramount for a successful outcome.

 

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