Constant Rate Infusions in Small Animal Anesthesia: A Guide to the Practitioner
Constant Rate Infusions (CRIs) for Hypotension
Vasopressors and inotropes
|
Drug
|
Contractility
|
Heart rate
|
Vascular tone
|
IV dose
|
|
Dobutamine
|
|
|
-
|
5–20 mcg/kg/min
|
|
Dopamine
|
|
|
|
5–20 mcg/kg/min
|
|
Epinephrine
|
|
|
|
0.1–1 mcg/kg/min
|
|
Norepinephrine
|
0
|
No change
|
|
0.2–2 mcg/kg/min
|
|
Phenylephrine
|
0
|
-
|
|
1–5 mcg/kg/min
|
|
Vasopressin
|
0
|
-
|
|
0.5 units/kg
|
Vasopressor and inotrope dilution volumes
|
Drug
|
60 ml syringe
|
150 ml bag
|
250 ml bag
|
500 ml bag
|
|
Dobutamine 12.5 mg/ml
|
1.5
|
3.75
|
6.25
|
12.5
|
|
Phenylephrine 10 mg/ml
|
0.36
|
0.9
|
1.5
|
3
|
|
Ephedrine 50 mg/ml
|
0.36
|
0.9
|
1.5
|
3
|
|
Vasopressin 20 U/ml
|
0.36
|
0.9
|
1.5
|
3
|
|
Epinephrine 1 mg/ml
|
0.72
|
1.8
|
3
|
6
|
|
Dopamine 40 mg/ml
|
0.3
|
0.75
|
1.25
|
2.5
|
|
Norepinephrine 1 mg/ml
|
1.45
|
3.6
|
6
|
12
|
Analgesic CRIs
Fentanyl is very effective when administered as a CRI. After a loading dose of 1 to 4 mcg/kg slowly IV, start the CRI at 1 to 4 mcg/kg/h IV. The rate should be adjusted up or down based upon patient needs.
Another readily recognized analgesic CRI is morphine-lidocaine-ketamine (MLK). There are many useful variations of the MLK CRI. Several different opioids can be substituted for the morphine. The addition of dexmedetomidine, or detomidine, can provide greater comfort if the opioid-lidocaine-ketamine CRI is not sufficient on its own. If appropriate pain relief is not achieved within 10 minutes (or sooner) of administering IV loading doses and commencing the CRI at 1 ml/kg/h, the rate is adjusted in small increments up to 2 ml/kg/h until relief is obtained. If appropriate relief is not obtained, additional analgesics may be necessary until adequate analgesia can be obtained. If patients become too obtunded, rates should be decreased proportionally to degree of patient sedation.
Morphine/Lidocaine/Ketamine (MLK)
Morphine 0.1 to 0.4 mg/kg/h, loading dose 0.2 to 0.4 mg/kg IV
Lidocaine 1 to 2 mg/kg/h, loading dose 2 mg/kg IV
Ketamine 0.12 to 0.6 mg/kg/h, loading dose 0.5 to 2 mg/kg IV
MLK Dilutions
Amount of fluid to remove (ml), and drug volumes to add (ml), to different size bags of 0.9% NaCl. Start at 1 ml/kg/h and adjust up or down based upon patient comfort (see table below).
|
|
60 ml syringe
|
150 ml
|
250 ml
|
500 ml
|
1000 ml
|
|
Remove
|
4.6
|
11.6
|
19.3
|
38.6
|
77
|
|
Morphine 15 mg/ml
|
0.8
|
2
|
3.3
|
6.6
|
13.2
|
|
Lidocaine 20 mg/ml
|
3.7
|
9.4
|
15.6
|
31
|
62
|
|
Ketamine 100 mg/ml
|
0.09
|
0.22
|
0.38
|
0.76
|
1.5
|
Hydromorphone/Lidocaine/Ketamine (HLK)
Hydromorphone 0.03 to 0.05 mg/kg/h, loading dose 0.03 to 0.075 mg/kg IV
Lidocaine 1 to 2 mg/kg/h, loading dose 2 mg/kg IV
Ketamine 0.12 to 0.6 mg/kg/h, loading dose 0.5 to 2 mg/kg IV
HLK Dilutions
Amount of fluid to remove (ml), and drug volumes to add (ml), to different size bags of 0.9% NaCl. Start at 1 ml/kg/h and adjust up or down based upon patient comfort (see table below).
|
|
60 ml syringe
|
150 ml
|
250 ml
|
500 ml
|
1000 ml
|
|
Remove
|
4.7
|
2.5
|
19.8
|
39
|
78.5
|
|
Hydromorphone 2 mg/ml
|
0.9
|
2.2
|
3.8
|
7.5
|
15
|
|
Lidocaine 20 mg/ml
|
3.7
|
9.4
|
15.6
|
31
|
62
|
|
Ketamine 100 mg/ml
|
0.09
|
0.22
|
0.38
|
0.76
|
1.5
|
Oxymorphone/Lidocaine/Ketamine (OLK)
Oxymorphone 0.03 to 0.05 mg/kg/h, loading dose 0.03 to 0.075 mg/kg IV
Lidocaine 1 to 2 mg/kg/h, loading dose 2 mg/kg IV
Ketamine 0.12 to 0.6 mg/kg/h, loading dose 0.5 to 2 mg/kg IV
OLK Dilutions
Amount of fluid to remove (ml), and drug volumes to add (ml), to different size bags of 0.9% NaCl. Start at 1 ml/kg/h and adjust up or down based upon patient comfort (see table below).
|
|
60 ml syringe
|
150 ml
|
250 ml
|
500 ml
|
1000 ml
|
|
Remove
|
5.6
|
14
|
23.5
|
46.8
|
93.5
|
|
Oxymorphone 1 mg/ml
|
1.8
|
4.5
|
7.5
|
15
|
30
|
|
Lidocaine 20 mg/ml
|
3.7
|
9.4
|
15.6
|
31
|
62
|
|
Ketamine 100 mg/ml
|
0.09
|
0.22
|
0.38
|
0.76
|
1.5
|
Fentanyl/Lidocaine/Ketamine (FLK)
Fentanyl 0.5 to 3 mcg/kg/h (0.0005 to 0.003 mg/kg/h), loading dose 0.5 to 3 mcg/kg IV
Lidocaine 1 to 2 mg/kg/h, loading dose 2 mg/kg IV
Ketamine 0.12 to 0.6 mg/kg/h, loading dose 0.5 to 2 mg/kg IV
FLK Dilutions
Amount of fluid to remove (ml), and drug volumes to add (ml), to different size bags of 0.9% NaCl. Start at 1 ml/kg/h and adjust up or down based upon patient comfort (see table below).
|
|
60 ml syringe
|
150 ml
|
250 ml
|
500 ml
|
1000 ml
|
|
Remove
|
5.6
|
14
|
23.5
|
46.8
|
93.5
|
|
Fentanyl 50 mcg/ml
|
1.8
|
4.5
|
7.5
|
15
|
30
|
|
Lidocaine 20 mg/ml
|
3.7
|
9.4
|
15.6
|
31
|
62
|
|
Ketamine 100 mg/ml
|
0.09
|
0.22
|
0.38
|
0.76
|
1.5
|
Buprenorphine/Lidocaine/Ketamine (BLK)
Buprenorphine 0.003 to 0.005 mg/kg/h, loading dose 0.03 to 0.05 mg/kg IV
Lidocaine 0.5 to 2 mg/kg/h, loading dose 2 mg/kg
Ketamine 0.12 to 0.6 mg/kg/h, loading dose 0.5 to 2 mg/kg IV
BLK Dilutions
Amount of fluid to remove (ml), and drug volumes to add (ml), to different size bags of 0.9% NaCl. Start at 1 ml/kg/h and adjust up or down based upon patient comfort (see table below).
|
|
60 ml syringe
|
150 ml
|
250 ml
|
500 ml
|
1000 ml
|
|
Remove
|
4.4
|
11
|
18.5
|
36.8
|
73.5
|
|
Buprenorphine 0.3 mg/ml
|
0.6
|
1.5
|
2.5
|
5
|
10
|
|
Lidocaine 20 mg/ml
|
3.7
|
9.4
|
15.6
|
31
|
62
|
|
Ketamine 100 mg/ml
|
0.09
|
0.22
|
0.38
|
0.76
|
1.5
|
Methadone/Lidocaine/Ketamine (MethLK)
Methadone 0.1 to 0.3 mg/kg/h, loading dose 0.1 mg/kg IV
Lidocaine 1 to 2 mg/kg/h, loading dose 2 mg/kg IV
Ketamine 0.12 to 0.6 mg/kg/h, loading dose 0.5 to 2 mg/kg IV
MethLK Dilutions
Amount of fluid to remove (ml), and drug volumes to add (ml), to different size bags of 0.9% NaCl. Start at 1 ml/kg/h and adjust up or down based upon patient comfort (see table below).
|
|
60 ml syringe
|
150 ml
|
250 ml
|
500 ml
|
1000 ml
|
|
Remove
|
4.4
|
11
|
18.5
|
36.8
|
73.5
|
|
Methadone 10 mg/ml
|
0.6
|
1.5
|
2.5
|
5
|
10
|
|
Lidocaine 20 mg/ml
|
3.7
|
9.4
|
15.6
|
31
|
62
|
|
Ketamine 100 mg/ml
|
0.09
|
0.22
|
0.38
|
0.76
|
1.5
|
Adding Dexmedetomidine or Detomidine (DMLK, DHLK, DOLK, DFLK, DBLK, DMethLK)
Patients with stable cardiorespiratory function requiring additional analgesia or mild sedation may benefit from the addition of dexmedetomidine at 0.00025 to 0.0005 mg/kg/h (0.25 to 0.5 mcg/kg/h) or detomidine at 0.0005 to 0.001 mg/kg/h (0.5 to 1 mcg/kg/h). Dexmedetomidine and detomidine should only be used in patients with stable cardiac and respiratory function.
Dexmedetomidine 0.25 to 0.5 mcg/kg/h (0.00025 to 0.0005 mg/kg/h)
Load with 0.00025 mg/kg IV (insulin syringe)
Medetomidine 0.5 to 1 mcg/kg/h (0.0005 to 0.001 mg/kg/h)
Load with 0.0005 mg/kg IV (insulin syringe)
Drug volumes to add (ml), to different size bags of 0.9% NaCl. Use an insulin syringe for accurate dosing. Start at 1 ml/kg/h and adjust up or down based upon patient comfort (see table below).
|
|
60 ml syringe
|
150 ml
|
250 ml
|
500 ml
|
1000 ml
|
|
Dexmedetomidine 0.5 mg/ml
|
0.03
|
0.075
|
0.13
|
0.25
|
0.5
|
|
Medetomidine 1 mg/ml
|
0.03
|
0.075
|
0.13
|
0.25
|
0.5
|
If adding either of these drugs to a CRI that has been running for a while, the drug volume added must be calculated based upon the remaining volume in the bag.
Monitoring During Any Analgesic CRI
Comfort
Temperature
Heart rate
Arterial blood pressure
Urination
Mentation
Unwanted Side Effects
Treat the unwanted side effects of the CRI while still maintaining patient comfort. It is desirable to administer the CRI at the lowest dose necessary to maintain patient comfort. Apply eye lubricant frequently if necessary. Express the bladder if necessary.
Weaning from the Opioid-L-K CRI
It will take approximately 6 to 12 hours to wean patients from analgesic CRIs to oral medications or combination injectable/oral protocols. Wean the infusion rate approximately 10% to 20% every one to two hours. Commence oral or oral/injectable protocols about half way through the weaning period.
Midazolam CRI
Midazolam, a benzodiazepine, may be administered as a CRI to produce muscle relaxation or mild sedation. It is usually administered in combination with fentanyl or any of the above -LK CRIs. The loading dose of midazolam is 0.2 to 0.4 mg/kg IV followed by a CRI of 0.2 to 0.4 mg/kg/h.
Midazolam volume to add (ml), to different size bags of 0.9% NaCl when adding to any -LK CRI
|
|
60 ml syringe
|
150 ml
|
250 ml
|
500 ml
|
1000 ml
|
|
Additional amount to remove
|
2.4
|
6
|
10
|
20
|
40
|
|
Midazolam 5 mg/ml
|
2.4
|
6
|
10
|
20
|
40
|
Butorphanol CRI
A CRI of butorphanol may be useful to sedate patients and provide very mild analgesia. Butorphanol is administered at 0.1 to 0.4 mg/kg/h IV. Butorphanol can be combined with midazolam CRI for greater relaxation.
Maropitant (Cerenia®) CRI
Maropitant is a neurokinin (NK1) receptor antagonist that blocks the action of substance P in the central nervous system. Maropitant has been shown to decrease sevoflurane MAC in dogs undergoing ovariohysterectomy by 24% after IV injection of 1 mg/kg followed by 0.03 mg/kg/h CRI. Another study in dogs showed sevoflurane MAC was reduced by 30% after 5 mg/kg IV followed by 0.15 mg/kg/h CRI. In cats undergoing ovariohysterectomy, a single injection of 1 mg/kg IV reduced sevoflurane MAC by 16%. The same study showed no further decrease in MAC with a single injection of 5 mg/kg IV. A maropitant CRI was not administered in the feline study.