Let's Talk About Geriatric Anesthesia!
Article by Elana Rybak, DVM, cVMA
The patient:
A calm, sweet 19 year old, 4kg MN Daschund, presenting for being unable to walk in all four limbs. The patient also has a recent history of 'staring off into space' at times. The neurologist evaluated the patient as non-ambulatory tetraparetic, and determined that a C-Spine and possible brain MRI was indicated.
On exam, this patient has a grade 2/6 systolic heart murmur, heard on both sides of the thorax. The patient also has generalized decreased muscle mass.
On bloodwork, the patient is mildly anemic, with some changes indicative of earlier stage chronic renal disease. Thoracic radiographs are generally unremarkable.
The patient already has an IV catheter in place.
Geriatric anesthesia considerations:
While we always say 'age is not a disease', geriatric patients may often have age-related physiologic changes that should be considered.
Some more common aging changes that may have an impact during anesthesia can include:
Respiratory system: Geriatric pets can have decreased chest wall compliance and decreased lung elasticity.
Cardiovascular system: Decreased cardiac output and blood pressure is possible, in part due to increased vagal tone and cardiac degenerative changes.
Central nervous system: Senility can decrease the minimum alveolar concentration (MAC) of inhalant anesthetics (so it may take less anesthetic gas to maintain anesthesia). Older patients may be more anxious. Increased blood-brain barrier permeability can increase drug sensitivity.
GI tract: Decreased lower esophageal sphincter tone can increase risk of reflux/regurgitation.
Decreased hepatic blood flow can delay drug metabolism.
Geriatric pets are prone to hypothermia.
Geriatric pets may be prone to emergence delirium due to altered drug metabolism, potential 'senility' and increased anxiety.
Geriatric pets are more likely to have other pre-existing medical conditions.
Geriatric pets may have arthritis, decreased muscle mass, or obesity - these all have their own considerations for anesthesia including gentle handling and positioning, thermoregulation, and drug distribution and dose requirements.
So, how do we safely anesthetize our patient considering all of his conditions?
Judicious use of IV fluids was elected due to the heart murmur and anemia, while also keeping in mind the patient's renal status: 3.5ml/kg/hr was chosen as the initial fluid rate.
In MRI, injectable contrast is often used which rarely may cause renal damage, therefore, a half-dose of contrast was used in this patient due to his renal disease.
Ventilator tidal volume settings, if needed, are usually lower to decrease the risk of pulmonary barotrauma.
Efforts to keep the patient warm are important.
In general, lower-range dosages are used for each drug due to the age of the patient.
Drug selection:
Due to potential for a painful underlying condition (such as a ruptured disc), a pure Mu opioid was selected. Fentanyl (2ug/kg) was used as a premedication in this patient for rapid onset, pain management properties, some sedative properties, as well as short half-life. If needed, additional doses could be given during anesthesia, but the drug would also be eliminated more quickly, allowing for a potentially smoother recovery. It is also easily reversible with Naloxone if adverse effects were noted.
The Fentanyl offered a nice level of sedation for our patient and allowed us to easily pre-oxygenate him.
Anesthesia was then induced with Midazolam (0.15mg/kg) and Propofol to effect (10mg needed for induction). Co-induction using these two drugs allows lower doses of Propofol to be used, which reduces the cardiovascular and respiratory depression that may occur with higher doses. Midazolam is also very cardiovascularly safe, induces muscle relaxation, and can be reversed with Flumazenil if adverse effects are noted.
(Alfaxalone could have also been used for induction instead of Propofol in this patient.)
The patient was then placed on inhaled Isoflurane for anesthetic maintenance.
Events during anesthesia:
During the beginning of the anesthesia, it was noted that the patient seemed light - higher heart rate (150s-160's), and slightly increased jaw tone. A 5mg propofol bolus was given and the Isoflurane was increased by 0.2%, which put the patient in a better anesthetic plane, but then caused apnea. The patient was put on the ventilator at this time. After a short while, the patient began breathing over the ventilator and was taken off of it. It was noted that the heart rate was still elevated and blood pressure measurement was able to be taken reliably on the tail which read a Mean Arterial Pressure (MAP) of around 80-90mmHg (normal). At this time, due to the suspicion that the patient was still light and the fact that the Propofol appeared to make the patient apneic and not last very long, a dose of Alfaxalone (0.5mg/kg IV) was given. We elected to give an injectable medication instead of increasing the Isoflurane to try to prevent hypotension, which is more likely to occur with increasing doses of inhalants.
After the Alfaxalone was given, the patient's HR came down to the 120-130's and the patient continued breathing on his own. The remainder of the anesthesia was unremarkable, though he did become hypothermic despite warm blankets and insulated pads being used.
The patient recovered unremarkably with no reversals needed.
What we can learn:
This case highlights some of the challenges with geriatric patients. Lower drug dosages are usually needed and these patients can have variable responses to medications. Hypothermia is a concern especially during longer anesthetic events. Additionally, care to choose lower initial tidal volume settings on ventilators is important, and intravenous fluid rates may need to be adjusted to account for cardiac or renal impairment.
The use of both Propofol and Alfaxalone in this case is safe, and demonstrates how a patient can react very differently to two drugs that are used for the same purpose.