P
Pacemaker: Transthoracic Cardiac Pacing
INDICATIONS
• Support of heart rate and BP during general anesthesia for dogs with medically refractory bradyarrhythmias undergoing permanent pacemaker implantation or another surgery unrelated to pacemaker implantation
• Emergency treatment of hemodynamically unstable, medically refractory, complete atrioventricular block until more definitive therapy can be instituted
EQUIPMENT, ANESTHESIA
• Continuous electrocardiogram (ECG) and transthoracic pacing system required, including disposable adhesive transthoracic patch electrodes. Newer versions of transthoracic patch electrodes are available that allow not only noninvasive cardiac pacing but can be used for defibrillation, synchronized cardioversion and ECG monitoring.
• A No. 10 clipper blade (to shave the hair from the left and right precordia (area of the thorax overlying the heart)
PREPARATION: IMPORTANT CHECKPOINTS
• Have pacing system prepared (patch electrodes on chest and ECG monitoring ongoing) at induction of general anesthesia in dogs undergoing permanent pacemaker implantation or other dogs at risk for bradyarrhythmic complications with general anesthesia. This is a precaution in the event of bradyasystole during induction.
POSSIBLE COMPLICATIONS AND COMMON ERRORS TO AVOID
• Pain or distress experienced in awake dogs requires rapid induction of general anesthesia for continued use of the temporary pacemaker. Analgesia (opiate or nonsteroidal antiinflammatory drug [NSAID]) is also recommended.
• Skeletal muscle twitching, more pronounced in smaller dogs, may require neuromuscular blockade to lessen the jerking of forelimbs and chest during surgical procedure.
• Most common error is suboptimal placement of patch electrodes, causing inconsistent pacing. Ensure placement directly over the palpable cardiac impulse beats. Readjustment of patch electrodes may be necessary.
PROCEDURE
• Patch electrodes are placed directly over cardiac impulse on left and right hemithorax. The fur on the chest is shaved, and a small dollop of ECG paste is placed on the surface of the patch electrode facing the skin. Dogs ideally should be placed in lateral recumbency.
• Ensure accurate ECG sensing of dog’s intrinsic cardiac rhythm by adjusting ECG gain and lead selection. A sensing marker will appear on the animal’s QRS complex, signaling accurate sensing.
• After setting the pacing rate (range 40-170 beats per minute), the pacing current (range 0-200 mA) is gradually increased until ventricular capture is confirmed on the ECG monitor and by palpation of a corresponding arterial pulse. Ventricular capture is recognized by a wide QRS complex and T wave following a pacing spike on the ECG. The pacing current is maintained just above the capture threshold.
ALTERNATIVES AND THEIR RELATIVE MERITS
• Can be associated with infection, bleeding, lead wire displacement, lead wire-induced arrhythmia, cardiac perforation, and thromboembolism.
• Same merits as with transvenous pacing in terms of no pain or twitching associated with pacing stimulus and less expensive pacing system
• However, based on limited clinical experience in dogs, only the atria can be paced. No ventricular pacing is possible.
(Courtesy DeFrancesco TC, et al: Noninvasive transthoracic temporary cardiac pacing in dogs. J Vet Intern Med 17:663–667, 2003.)
Parenteral Nutrition
INDICATIONS
Animals in need of nutritional support when:
• Enteral nutrition is contraindicated (e.g., gastrointestinal [GI] dysfunction or risk of pulmonary aspiration).
CONTRAINDICATIONS
• Animals that are at risk with catheter placement:
○ Central venous catheters in animals at high risk of thromboembolic disease (e.g., protein-losing nephropathy or enteropathy, hyperadrenocorticism, disseminated intravascular coagulopathy)
EQUIPMENT, ANESTHESIA
• Venous access:
○ A dedicated catheter is required for delivery of the nutrient solution to avoid septic complications and drug-nutrient interactions.
○ Treat catheter placement as a surgical procedure (surgical prep, draping, and the use of sterile gloves) (see p. 1293).
○ Place a central venous catheter to deliver TPN, because the nutrient solution is hyperosmolar. Solutions that are more dilute can be prepared, but the resulting large volume that must be infused can become a limiting factor.
○ Catheters made of nonthrombogenic materials (polyurethane or silicone) are preferred, particularly for peripheral infusion.
• Nutrient admixture:
○ Amino acid solutions (3%-10%) are used for providing protein. These solutions come with and without added electrolytes.
○ Nonprotein calories can be provided by a combination of lipid emulsions (10% or 20%) and dextrose (10%-50%) or dextrose alone.
○ Electrolytes can be added to the nutrient solution as needed or provided separately in the animal’s crystalloid fluid therapy. The latter allows greater flexibility. Alternatively, a combination amino acid and electrolyte solution can be used.
○ Special parenteral vitamin and mineral preparations are available. However, because most companion animals receive PN for relatively short periods of time (<2 weeks), only certain vitamins and minerals are commonly added to the nutrient admixture (B complex, ± potassium phosphate, magnesium sulfate, and trace elements [zinc, copper, manganese, and chromium]).
PREPARATION: IMPORTANT CHECKPOINTS
POSSIBLE COMPLICATIONS AND COMMON ERRORS TO AVOID
• Common metabolic complications:
○ When addressing electrolyte abnormalities, delivering supplements in the IV fluids rather than in the PN solution allows greater flexibility. Later, when the amount of supplementation for maintenance is established, it can be included in the PN formulation.
○ Hyperglycemia: if present, reduce the percentage of nonprotein calories from dextrose, or give regular insulin.
○ Hypokalemia: if present, supplement IV fluids with potassium chloride. Monitor for hypomagnesemia (see online chapter: Refeeding Syndrome).
○ Hypophosphatemia: if present, supplement IV fluids with potassium phosphate, or add potassium phosphate to PN formulation.
○ Hypomagnesemia: if present, supplement IV fluids with magnesium sulfate or add magnesium sulfate to PN formulations.
PROCEDURE
• Choose between central or peripheral venous administration.
○ Central venous administration is preferred because smaller volumes of PN solution are required, and thrombophlebitis is less likely to occur.
○ Peripheral venous administration is possible when central venous access is not available, but the animal must be fluid tolerant because required lower tonicity means larger volume of infusate.
• Calculating the caloric goal:
○ Calculate the resting energy requirement (RER) using the animal’s current body weight (or the animal’s estimated ideal weight if overweight).
○ Formula: RER = 70 + 30 × (body weight in kilograms) for animals weighing 2-30 kg; use 70 × (body weight in kilograms)0.75 for animals that weigh outside this range.
• Decide what percentage of calories will be delivered as protein depending on the animal’s level of protein tolerance or deficiency.
• Decide what percentage of nonprotein calories will be delivered as dextrose with the remainder to be delivered as lipid:
○ 100% of the nonprotein calories can be delivered as dextrose, but hyperglycemia is a common sequela; 50% dextrose provides 1.7 kcal/mL.
○ Alternatively, lipid emulsions can be used for delivering 50%-70% of the nonprotein calories; 20% lipid emulsions provide 2 kcal/mL.
• Decide whether electrolytes and minerals will be added.
○ Supplementation may not be necessary if amino acid solutions containing additional electrolytes are used.
Sample PN Worksheet for a 17.5 kg (39 lb) Dog: Centrally Administered PN Calculation
| CANINE HIGH-PROTEIN REGIMEN | |
|---|---|
| Weight: 17.5 kg | Resting energy requirement (RER) = 600 kcal/day |
| Day 1 goal | 50% RER = 0.5(600) = 300 kcal |
| Day 2 goal | 100% RER = 600 kcal |
| % Protein calories | 25% |
| Nonprotein calories | 50% from lipid, 50% from dextrose |
There are 4 kcal/g protein; therefore, the animal needs 18.75 grams of protein: (75 kcal ÷ 4 kcal/g = 18.75 g). | |
50% dextrose to provide 50% nonprotein calories = 112.5 kcal. 50% dextrose solution = 1.7 kcal/mL; therefore, the animal needs 66 mL 50% dextrose solution: (66 mL = 112.5 kcal ÷ 1.7 kcal/mL). | |
Dosed at 8 mM/1000 kcal delivered; therefore, the animal needs 2.4 mM potassium phosphate: (2.4 mM = [8 mM × 300 kcal] ÷ 1000 kcal). | |
POSTPROCEDURE
• Delivery:
○ Administer 50% of the goal infusion, and monitor for metabolic complications the first day, advancing to the goal rate the next day if no problems occur that cannot be addressed.
• Catheter care:
○ Examine the catheter site at least daily for signs of phlebitis or infection, changing the dressing as needed.
• Monitoring:
○ In addition to the routine monitoring appropriate for any animal on IV fluid therapy, animals receiving PN should be monitored for metabolic complications.
○ Animals should have blood glucose monitored at least every 12 hours and serum electrolytes at least once daily depending on circumstances and whether any PN supplementation is required.
○ When a packed cell volume (PCV) is performed, the serum should be examined for evidence of lipemia.
Chan DL. Parenteral nutritional support. In: Ettinger SJ, Feldman EC, editors. Textbook of veterinary internal medicine. ed 7. St Louis: Elsevier; 2010:701-707.
Chan DL, Freeman LM, Labato MA, et al. Retrospective evaluation of partial parenteral nutrition in dogs and cats. J Vet Intern Med. 2002;16(4):440-445.
Pyle SC, Marks SL, Kass PH. Evaluation of complications and prognostic factors associated with administration of total parenteral nutrition in cats: 75 cases (1994-2001). J Am Vet Med Assoc. 2004;225(2):242-250.
Pericardiocentesis
CONTRAINDICATIONS
Small volume of pericardial effusion (difficult to safely enter the pericardial space)
EQUIPMENT, ANESTHESIA
• 0.5-12 mL 2% lidocaine for local anesthesia; volume based on body weight 1-70 kg. Note: Discomfort of lidocaine infusion can be reduced by adding 0.05-0.2 mL 8.4% sodium bicarbonate and warming to body temperature (armpit method).
PREPARATION: IMPORTANT CHECKPOINTS
• Ultrasound guidance: not usually used during the procedure. Immediately prior to the procedure, however, ultrasound-based localization of an area with maximal pericardial effusion and minimal lung is extremely useful.
• Electrocardiogram (ECG) may be used during the procedure; appearance of premature ventricular complexes (PVCs) suggests catheter/stylet is touching heart and should not be advanced farther. However, such contact with the heart is generally palpable (heartbeat hitting catheter tip), and PVCs may occur independently of the catheter in animals with pericardial effusion.
• Clinicians should tell clients that the procedure is often palliative, and effusion can recur, with the likelihood and speed of recurrence depending on the underlying cause. Rapid re-effusion or even hemodynamic collapse postcentesis is possible with atrial tear and with malignancies such as hemangiosarcomas.
POSSIBLE COMPLICATIONS AND COMMON ERRORS TO AVOID
• Effusion that clots in the syringe, either due to excessive advancement (catheter tip is in the heart) or to a rapidly bleeding pericardial effusion. Differentiation: disconnect syringe from catheter/stylet. If catheter is intracardiac, flow should be pulsatile ± vigorous.
PROCEDURE
• The animal is restrained in left lateral recumbency for the whole procedure. The approach will be via the right side of the thorax to minimize the risk of catheter-induced coronary artery laceration.
• Ultrasound is used for confirming that pericardial effusion is present in sufficient volume to allow safe centesis (width of effusion on echocardiogram > 1 cm). In the absence of ultrasound guidance, the site of entry is at the level of the costochondral junction, right fifth intercostal space.
• The area is widely clipped of hair and aseptically prepared. The wide clipping of hair should allow the procedure to be done without a sterile drape.
• Lidocaine is infused subcutaneously, intramuscularly (intercostal muscle), and subpleurally. Doing so requires that the needle be repeatedly withdrawn until almost out of the skin and then redirected and reinserted until a region at least 2 cm square and spanning the entire thickness of the chest wall has been infiltrated.
• The clinician puts on sterile gloves and makes a 3-mm skin incision in the center of the site using the scalpel blade.
• The catheter, which covers a metal stylet, is attached to the 6-mL syringe, and the tip of the catheter-stylet combination is inserted into the skin incision.
• The clinician advances the syringe-catheter-stylet combination (which is grasped in the palm as a handle to advance the catheter and stylet) while drawing back on the syringe plunger to create negative pressure. The orientation is perpendicular to the chest wall and is neither cranially nor caudally directed.
• When a flashback of effusion is seen to enter the syringe, the syringe-styletcatheter apparatus is advanced only 2-3 mm farther into the chest.
• With continuing negative pressure on the syringe (effusion flowing into syringe), the syringe and stylet are kept in the same position, while the catheter is advanced several centimeters farther. In this way, the catheter is seated well within the pericardial space.
• Using a large-volume syringe (35 or 60 mL), the effusion is withdrawn. Ideally, an assistant helps by emptying one syringe while the clinician is filling the other with pericardial effusion; thus, two syringes can aseptically be traded back and forth until the pericardium is empty.
• When the effusion has partially or completely been removed, the catheter is withdrawn, and the skin incision is closed with tissue glue.
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