Paula Moon, DVM, DACVA
Anesthesia & Analgesia
Because our pets are living longer, it is now almost commonplace to be asked to anesthetize a dog or cat that has reached the extraordinarily old age of 18 to 22 plus years. The normal aging process these animals undergo is associated with physiologic changes that may influence the success of an anesthetic event. Some adjustments in anesthetic protocols are often necessary for such elderly patients.
IMPORTANT ANESTHETIC CONSEQUENCES
The reserve strength for every organ system, particularly the cardiopulmonary system, is diminished due to a smaller functional mass. Consequently, there is less room for error in drug decisions (type, dose, frequency of administration) and even minor misjudgments may result in patient decompensation; in a young patient, such effects may go unnoticed.
The frequency of concurrent disease greatly increases with age, and these disease processes may influence the anesthetic protocol. The incidence of secondary diseases in elderly humans that also may affect the canine and feline population includes:
- Renal disease (31 percent)
- Chronic obstructive pulmonary disease (14 percent)
- Cardiomegaly (13 percent)
- Diabetes (9 percent)
- Liver disease (8 percent)
- Congestive heart failure (8 percent)
PRE-OP EVALUATION
Determining the pre-operative “medical fitness” of a patient, and not focusing on the chronological age of a patient, is a primary factor in determining the post-operative outcome of elderly human surgical patients. Even in the absence of clinical signs, decreased reserve and concurrent disease are positively associated with increasing age and are thought to account directly for the higher morbidity and mortality in human geriatric patients undergoing general anesthesia. Underlying disorders need to be clearly identified by: a complete history, thorough physical exam, and a broad minimum data base of laboratory tests (Table 1), and potentially to be treated prior to anesthesia. Unfortunately, there is no objective method of detecting “medical fitness” in our patients, nor are there veterinary studies that specifically evaluate different anesthetic protocols for geriatric pets. Hence, the superiority of one anesthetic technique over another remains undetermined. However, information is emerging on how geriatric pets differ from young adults. A summary of the anesthetic considerations for geriatric patients is provided (Table 2) and a more detailed discussion is provided below.
DETERMINING DOSAGE
It is not sufficient to conclude from this information that geriatric patients simply require a smaller anesthetic dose. In truth, the dose response curve for geriatrics for a particular drug response is broader than a younger population while only the average for the geriatric population is centered on a lower dose. Furthermore, there is an overlap in the minimum dose required to produce a response in these two populations and thus an equally important conclusion is that predicting the dose of a drug for any particular geriatric patient is more difficult than in a younger patient (because of the inhomogeneity of the aging process and the variability in severity of underlying disease).
ANESTHETIC CONSIDERATIONS
CARDIOVASCULAR
There are a variety of changes that occur as pets age that affect the cardiovascular system. These changes must be taken into consideration when anesthetizing elderly patients. When anesthetizing geriatrics, even one that is asymptomatic for heart disease, anesthetics should be chosen that minimally alter contractility or heart rate and are not arrhythmogenic. Such drugs are the least likely to cause cardiac decompensation in an older patient when cardiac function cannot be completely assessed.
Potential complications
The heart is less compliant, ventricular hypertrophy or dilation may exist, and arterial elasticity decreases with increasing age.
An impaired autonomic reflex and a reduction in adrenergic receptor responsiveness to sympathomimetics (endogenous or exogenous) add to a decrease in maximal cardiac output.
Endocardiosis with valvular regurgitation affects up to 58 percent of dogs greater than nine years old, especially small to medium-sized breeds. These changes further decrease cardiac output and organ perfusion.
OXYGEN REQUIREMENTS AND VENTILATION
As an animal passes through adulthood, there is a continued decrease in metabolic rate, which, in turn, decreases oxygen consumption, heat production and cardiac output. There is a reduction in the alveolar surface area and increased stiffness in the lungs, causing impaired efficiency of gas exchange and an increased work of breathing.
Potential complications
- Older toy breeds are at risk for tracheal collapse and chronic pulmonary disease such as bronchitis.
- When obesity, dorsal recumbency, or both are superimposed upon the depressant effects of anesthesia and the fragility of old age, the work of breathing is even greater and hypoventilation commonly occurs.
- Hypoxemia may also occur, especially if the patient is not breathing an enriched oxygen concentration.
- Assessment of respiratory function with a capnometer and oxygenation with a pulse oximeter are useful in these patients. Other recommendations include:
- Intubation and assisting ventilation may be necessary if hypoventilation or hypoxemia develops.
For more complex surgical procedures, arterial blood gas analysis from an acute care bedside monitor may be a more appropriate method of evaluating respiratory function that also provides information on metabolic and acid/base status.
METABOLIC FUNCTIONS AND BODY COMPOSITION
Fat stores increase and muscle mass decreases as animals get older. Blood volume and protein-binding of drugs are lessened in the elderly. Consequently, there is both an increased volume of distribution for lipid-soluble drugs and the initial blood concentrations of a drug may be higher than expected in the elderly, the latter resulting in an exaggerated initial response.
Furthermore, in the geriatric brain, there is a decrease in the number of neurons and in neurotransmitter activity, so anesthetic requirements are decreased and autonomic homeostasis is impaired. Potential complications
Given the greater fat stores, lipid-soluble drugs can accumulate and cause a prolongation of anesthesia and a delay in recovery times.
Reductions in hepatic and renal mass also cause reduced drug clearance and promote further prolongation of drug effects.
Normal, healthy geriatrics have a tendency for lower concentrations of thyroid hormone and higher basal concentrations of cortisol. In stressful situations, such as surgery, neither of these hormones responds as fully as they would in the young adult, and this depressed response results in an inability of elderly patients to cope with the stress of surgery.
Age-related cardiovascular changes, combined with the fact that most volatile anesthetics and sedative-hypnotics decrease sympathetic outflow, vascular tone, and myocardial contractility, are likely to produce a net effect of more frequent and significant hypotension during geriatric anesthesia.
Many patients also may be volume-depleted because of reduced fluid intake (NPO orders) and their diminished renal ability to conserve fluids. This puts geriatric patients further at risk for intraoperative hypotension as well as post-operative renal dysfunction related to intraoperative hypoperfusion. Monitoring and support of the cardiopulmonary system are of paramount importance
When anesthetizing elderly patients, a less potent, reversible anesthetic or anesthetics of short duration (i.e., butorphanol or morphine) should be used instead of buprenorphine or oxymorphone, and supportive intraoperative measures, such as heating units, intravenous fluids, etc., should be optimized.
SPECIFIC ANESTHETIC PROTOCOLS
The superiority of one specific anesthetic technique has not been demonstrated in geriatric patients. In some cases, a local or regional anesthetic technique plus sedation for minor procedures works well and minimizes systemic depression.
Techniques
Sedation with morphine (0.5 to 1.0 mg/kg SC or IM) or hydromorphone (0.1 to 0.3 mg/kg IM in dogs) plus a quiet environment, cotton balls in the ears, and a local or regional block is an effective protocol for procedures such as excisional biopsy of cutaneous masses and even digit amputations. However, this type of protocol is often not feasible for a particular procedure or for patients with a hostile temperament
In healthy geriatrics, all common general anesthetic combinations have been used safely, including using halothane, isoflurane or sevoflurane. To reduce stress and anxiety level, patients benefits from tactics such as:
- A quiet environment, calm handling
- Verbal reassurances
- A sedative-hypnotic premedicant drug (Table 3)
Selection of the induction drug (Table 4) should take into account the decreased cardiac reserve of the patient and the delay in clearance of the drug. There may be a delay in drug onset but also a more profound or prolonged response as well
Poor compensatory reflexes may make cardiovascular collapse more likely with rapid intravenous drug boluses and slower drug titration is important.
If patient cardiac output is reduced, then inhalational uptake and induction times will be faster.
The selection of the induction anesthetic drugs may be based on the type and severity of the patient’s concurrent disease. In most situations, continuing medications for such disorders through the perioperative procedure is suggested, but drug interactions should be considered and doses may need to be adjusted (e.g. diabetics should continue with their insulin doses). Older dogs frequently have valvular cardiac disease, pulmonary fibrosis or some type of endocrinopathy while older cats frequently have concurrent renal disease, cardiomyopathy, hyperthyroidism or neoplasia. Depending on the concurrent disease, some injectable drugs may hold advantages over other options. For example:
In patients with a history of epilepsy, the barbiturates and benzodiazepines are useful in suppressing seizures.
In patients with cardiac compromise, ketamine-benzodiazepine combinations, etomidate or opioid-benzodiazepine combinations have cardiovascular advantages.
In patients with concurrent diseases, sevoflurane or isoflurane may be a better inhalant than halothane due to less metabolism and being more cardiovascular sparing.
Numerous anesthesia books provide more detailed descriptions of anesthesia protocols for patients with such concomitant diseases.
MONITORING
Because of potential concurrent disease and more limited physiologic reserve, monitoring should be more intense for elderly patients. Non-invasive monitoring should be standard with these patients and include:
- ECG
- Monitoring respiratory rate and depth (tidal volume)
- Pulse oximeter
- Temperature probe
- Measurement of indirect blood pressure
In some situations, invasive monitoring (central venous pressure or direct systemic blood pressure measurements) will provide earlier recognition and a better ability to treat intraoperative complications, thus contributing to a lower morbidity and mortality rate. However, not every geriatric patient requires extensive monitoring and this should be evaluated on an individual basis.
CONCLUSION
Postoperative priorities should focus on maintaining cardiopulmonary function, adequacy of oxygenation, fluid balance, analgesia and thermoregulation.
If one remains uncertain how to adjust the anesthetic protocol for a geriatric patient, even after reviewing the described anesthetic considerations, the most effective overall plan is to administer smaller doses, administer each dose over a longer period of time and wait longer between doses. In addition, the careful titration of short-acting anesthetics and vigilant intraoperative monitoring will provide the best care for any patient.
Table 1. Initial history and pre-anesthetic workup
| History |
|
| Physical exam with emphasis on |
|
| Laboratory tests |
|
| Specific diagnostic procedures or consultations |
|
Table 2. Anesthesia overview for geriatric patients
| Action | Reason | Mechanism |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
- MAC=minimum alveolar concentration
- Vd= volume of distribution
Table 3. Acceptable anesthetic drugs for premedication in healthy geriatric patients.
Drug
Advantages
Disadvantages
Acepromazine 0.005 to 0.20 mg/kg SC, IM, IV
Potent sedation, Lowest doses may be adequate, can combine with opioids
Prolonged duration Not reversible Vasodilation promotes both hypothermia and hypotension
Atropine 0.02 to 0.04 mg/kg SC, IM
Prevent bradycardia Decrease salivations
Crosses blood brain barrier Potential for tachycardia
Butorphanol 0.2 to 0.4 mg/kg SC, IM, IV
Same as meperidine Reversible with naloxone
Same as meperidine
Diazepam or midazolam 0.05 to 0.10 mg/kg IM, IV
Decreases dose of other drugs Muscle relaxation
Only mild sedation
Glycopyrrolate 0.01 to 0.02 mg/kg SC, IM
Like atropine but longer acting Not cross blood brain barrier
Less likely to increase heart rate than atropine
Hydromorphone 0.1 to 0.3 mg/kg IM (dogs) 0.1 to 0.2 mg/kg IM (cats)
Same as meperidine but longer acting (4 to 5 hrs) Reversible with naloxone
Same as meperidine
Ketamine (cats only) 3 to 8 mg/kg Im
Supports heart rate and contractility
Hypersalivation, some recoveries excitable
Meperidine 2 to 5 mg/kg SC, IM Or Morphine 0.5 to 1.0 mg/kg SC, IM
Analgesic & short acting (20 to 40 min); cardiovascular stable Reversible with naloxone Partial reversal with butorphanol
Mild sedation may not be adequate Do not give iv Promotes bradycardia (give anticholinergic) Respiratory depressant (give oxygen)
Oxymorphone 0.04 to 0.06 mg/kg SC, IM, IV
Same as meperidine but longer acting (2 to 3 hrs)
Same as meperidine Prolonged duration of effect
Table 4. Acceptable induction agents for healthy geriatric patients.
Drugs
Advantages
Disadvantages
Etomidate 2 mg/kg Iv
Short acting Cardiovascular sparing
Expensive Transient apnea
Fentanyl (with diazepam or midazolam) 30 to 50 mcg/kg Iv
Potent analgesic Cardiovascular sparing
Bradycardia (treat with anticholinergics) Profound respiratory depression (Intubate and assist ventilation)
Ketamine (with diazepam or midazolam) 8 to 10 mg/kg Iv
Maintains heart rate
Some recoveries excitable Poor muscle relaxation Hypersalivation
Methohexitol 5 to 8 mg/kg IV
Maintains heart rate Rapid recovery Rapid redistribution to fat
Initial apnea Cardiac depressant
Propofol 6 to 8 mg/kg Iv
Short acting Extrahepatic metabolism Less cumulative effect
Transient hypotension & bradycardia Transient apnea & Hypoxia on room air
Telazol 3 mg/kg IV
Same as ketamine
More prolonged recovery
Thiopental 10 mg/kg IV
Maintains heart rate Rapid recovery Rapid redistribution to fat
Initial apnea Cardiac depressant & arrhythmogenic
References:
- Aucoin DP. Drug Therapy in the Geriatric Animal: The Effect of Aging on Drug Disposition. Vet Clin North Amer: Sm Ani Prac 19(1),41-47; 1989.
- Baker AB. Physiology and Pharmacology of Aging. 1994 International Anesthesia Research Society Review Course, P106-109, 1994.
- Eger II EI, Bahlman SH, and Munson ES. The Effect of Age on the Rate of Increase of Alveolar Anesthetic Concentrations. Anesthesiology. 35(4);365-372;1971.
- King LG, Anderson JG, Rhodes WH, et al. “Arterial Blood Gas Tensions in Healthy Aged Dogs” Am J Vet Res 1992; 53:1744-1748.
- Maher EE And Rush JE. Cardiovascular Changes in The Geriatric Dog. The Compendium, 12(7), 921-930; 1990.
Initial dose more “potent” Drug t1/2 increased Slower onset time
