Anesthesia of the "sick"patient
By David Liss, RVT, VTS (ECC, SAIM)
How often are veterinary technicians faced with patients that are not really “stable!?” The older dog with heart disease who needs a dental, the patient who has diabetes or another endocrinopathy who needs some orthopedic procedure. Anesthesia can be a scary thing to do on a patient with sever underlying disease. A discussion of the approach to a patient other than a young OVH or neuter is warranted.
General considerations
Anesthesia is an art and there are many different approaches to a sick patient. The first step in creating the safest anesthetic plan possible is to…plan! Discuss the case with the veterinarian and understand or review the underlying pathophysiology. Then discuss the procedure, estimated anesthesia time and specific concerns for the patient. Once these steps are covered, it will be clearer how to develop the anesthetic protocol. For example, if a patient has heart disease and you are worried about a weak flabby heart (in dilated cardiomyopathy) drugs that cause vasodilation or decreased contractility should be avoided. After the concerns for the patient are discussed and compiled the anesthetic plan can be formulated. The technician should look at what the hospital has for anesthetic drugs and equipment and develop a pro/con list for each.
Some hospitals may be limited with what tools they have available. General rules of thumb for dealing with sick patients include:
- Minimize stress: Use pre-medications to calm the patient and start treating for any potential pain
- Always have an IV catheter in place
- Avoid mask/box inductions
- Pre-oxygenate the patient for at least 5 minutes
- Ensure all equipment is setup and ready to go prior to anesthetic induction
- Reduce anesthetic times by clipping/prepping before the procedure if possible
- Have multiple staff members available to assist in surgical nursing, or anesthetic monitoring so it is not all up to one person
Specific Cases
Dystocia
Dystocia is most common in dogs. During gestation and parturition, the bitch has an increased metabolic demand and blood volume increases. Maternal anemia is also common. An increased metabolic rate increases cardiac contractility, which increases stroke volume and blood pressure. In opposition, systemic vascular resistance (SVR) is decreased to prevent hypertension from increased volume. Thus, sympathetic stimulation of SVR in times of crisis (hemorrhage, etc) is reduced. Additionally, increased abdominal contents and volume decreases lung capacity andfunctional residual capacity (FRC). There is also an increased risk of aspiration/regurgitation from hormonal changes.
Anesthetic concerns involve decreased ability to ventilate, decreased ability to compensate for shock/hypovolemia, and risk of aspiration. Note: Pre-medications are beneficial to the bitch but can be detrimental to the fetus. Debate exists in using these drugs.
Anesthetic procedure: Avoid alpha-2 agonists and phenothiazines. Opioids are recommended and can be reversed with naloxone. Atropine and glycopyrrolate can be used to treat bradycardia. Atropine crosses the placental barrier; glycopyrrolate doesn’t. Benzodiazepines can cause respiratory depression but can be reversed. The goal of the procedure should be a quick, smooth induction. Prepare the patient prior to the OR and induction as best as possible (clip, scrub, etc). Pre-oxygenate the patient on 100% O2 for at least 5 minutes. Do not use an inhalant induction.
Options for induction include; ketamine and diazepam, Propofol or Etomidate. Ketamine can cause severe fetal depression. Propofol is rapidly metabolized and so it tends to be the drug of choice. Inhalant anesthetics can be used for maintenance as they can be rapidly metabolized during respiration. Propofol/Iso combinations provide the most favorable outcomes.
Cardiac patients
Cardiac patients can be comprised on many levels. They often have an overactive renin- angiotensin-aldosterone system and sympathetic output. They may have secondary or primary kidney failure or hypertension. They may also have primary valvular or heart muscle disease. Recommendations for anesthesia in cardiac patients include: administration of pre-anesthetic drugs, induction drugs that are cardiovascularly sparing, minimizing anesthesia times, and supporting physiologic systems while the patient is under.
Pre-anesthetic drugs can help reduce stress and sympathetic output and are very important in providing anesthesia to a cardiac patient. Examples include: benzodiazepines, opioids, Avoid the phenothiazines and alpha-2 agonists.
Pre-oxygenate the patient before induction with 100% O2 for at least 5 minute. Avoid Alpha 2 agonists. They have significant cardiovascular side effects (significant hypertension and reflex bradycardia). Opioids are good choice because they tend to have minimal cardiovascular side effects. Administration of a pure mu opioid (Morphine, Hydromorphone, Oxymorphone, or Fentanyl) is recommended because these drugs can be reversed by the mu antagonist, Naloxone. Opioids may cause excitement by themselves, so combining them with a benzodiazepine is a very safe. Using anticholinergics may be indicated either pre-operatively or intra-operatively.
But remember that atropine is very arrhythmogenic (tachyarrhythmias), and tachycardia increases the myocardial oxygen demand which could lead to ischemia. If an anticholinergic is needed glycopyrrolate is preferred as it causes less tachycardia. For induction there are several choices. Etomidate is an expensive induction drug but it causes virtually no CV effects. It really is the drug of choice for severe heart failure. Avoid phenothiazines as they have a propensity to cause hypotension. Propofol can be used but efforts should be made to reduce the dose required for induction as it has fairly significant cardiorespiratory effects (vasodilation, negative inotrop,e, apnea). Ketamine is a sympathomimetic and increases HR and BP. This can be detrimental in constrictive heart disease (HCM) but maybe beneficial in DCM. It should also be used with caution. Inhalants cause severe CV effects (decreased inotropy and vasodilation) but can be used.
Sample protocol for DCM: Opioid/benzo premed. Induction: ketamine/diazepam or etomidate. iso/sevo, opioid CRI intra-operatively to reduce MAC. If hypotension occurs, use dobutamine to provide inotropy vs. dopamine/norepinephrine. Reduce fluid rates in surgery.
Sample protocol for HCM: Premed similar to above, avoid stress. Induction: ketamine/diazepam, or propofol + diazepam. Avoid mask/boxing inductions. Avoid NSAID’s. Maintain BP.
Renal
The kidney has the ability to autoregulate BP at MAP of 60-180mmHg. Below that mean pressure, arterioles lose ability to control blood flow. So in patients that either have severe renal disease, or suffer bouts of hypotension, the perfusion to the kidney is compromised. Renal patients may be: dehydrated, have metabolic acidemia, anemic, uremic, oliguric/anuric, hypovolemic, hypo/hyperkalemic, hyperphosphatemic. Pre-operative blood work and blood pressures are important to get a physiologic baseline for your patient. These patients should not be considered stable. Patients with renal disease often have concurrent illness, so knowing the patients health status and pre-existing medical problems is incredibly important.
General considerations: Avoid phenothiazines as they can cause hypotension. Can use benzodiazepines, and opioids. It is important to note that ketamine is excreted unchanged as active drug in the urine of cats, meaning there may be reduced metabolism if renal diseaes is present. Propofol and inhalant anesthetics have the potential ability to cause hypotension and further exacerbate renal injury. Attempt to lower MAC and reduce propofol doses of these are going to be used.
Head trauma
Patients often present after trauma and may need emergency surgery, or even minor surgery such as laceration repairs. However, they may have some degree of head trauma if they present with oral or facial trauma, or have neurologic signs (nystagmus, anisocoria, miosis or mydriasis, seizures or ataxia/paralysis). The biggest anesthetic concern is increasing intracranial pressure which will put additional pressure on the brain causing ischemia and potentially herniation. The goal is to maintain cerebral blood flow and avoid excessive vasodilation or vasoconstriction in the vessels perfusing or draining the brain (cerebral/carotid arteries, jugular veins).
As cerebral perfusion is important, the cerebral perfusion pressure (CPP) is calculated by taking the MAP and subtracting it from the itnracranial pressure (ICP). Thus, reductions in MAP must be avoided and normotension should be maintained appropriately. Patient positioning can play a large role in ensuring reduced ICP so elevate head slightly, approximately 15-30 degrees. Avoid jugular catheterization or blood draws as occluding the jugular vein can temporarily increase ICP. End- tidal CO2 monitoring is incredibly important in dealing with brain cases. The cerebral blood flow, talked about earlier, is essentially determined by the CO2 concentration.
Hypercapnia (hypoventilation) causes vasodilation in an effort to rid the brain of CO2, but can inadvertently increase cerebral blood flow and increase ICP. This should be avoided. In addition, hyperventilation, or hypocapnia should also be avoided as it causes vasoconstriction and a reduction in cerebral blood flow and a decrease in cerebral perfusion pressure. Maintain end-tidalCO2 (with ventilation) around 30 mmHg. Another drug to avoid is ketamine; it has been shown to increases cerebral blood flow (CBF) and cerebral metabolic rate (CMR). Drugs to use include: opioids, although beware of respiratory depression, benzodiazepines and Propofol.
GDV
Patients with gastric dilitation-volvulus typically have a deranged venous volume, FRC (increased intraabdominal pressure) and organ perfusion. Anesthetic concerns include: excessive release of endotoxins. fluid therapy and stabilization prior to surgery, arrhythmias, and consequences of shock (organ failure, coagulopathy). These patients often have ventricular arrhythmias and should be monitored for them pre, intra, and post-operatively.
Pre-oxygenate these patients with 100% O2 prior to induction. Pain can be a significant part of this disease so opioids are a good choice for pre-medications and even used in inductions. Benzodiazepines are also a good choice as they are cardiovascularly sparing drugs. Lidocaine also has its place in the GDV anesthetic protocol as it is a CNS depressant, is anti-arrhythmic, and has free-radical scavenging properties. Ketamine is a sympathomimetic but can also be used in these patients. One can also use propofol but it has profound CV effects. If patient is already tachycardic use propofol instead of ketamine, as the ketamine may exacerbate the tachycardia. Hypotension is common in these patients - treat as needed.
References are available upon request.This information was presented by David Liss at the CenCan Veterinary Conference in Winnipeg, MB. CVT
