Introduction

Open-heart surgery with cardiopulmonary bypass (CPB) in dogs has typically been reported in medium- to large-breed dogs weighing > 10 kg using normothermia or moderate hypothermia (28°C) achieved by blood cooling with a heart exchanger.

In small and toy dogs, a conventional CPB cannot be used because of the small surgical field and low bodyweight (BW). Formerly, we reported open-heart surgery using low-flow CPB combined with surface-induced deep hypothermia (sHT) in toy-breed dogs below 3.5 kg BW, in which the lowest BW was 1.62 kg.

Currently, we made improvements in CPB technique including cardiac anesthesia, pump method, and cardioplegia, and successes in several complicated mitral valvuloplasty cases required a long aortic cross clamp (ACC) over 90 minutes in small dogs.

Open-Heart Assist Methods

Formerly, open-heart assist methods were surface-cooling deep hypothermia (sHT) (20°C) under isoflurane anesthesia and low-flow cardiopulmonary bypass (CPB) with a small-volume prime circuit. Recently, we used blood-cooling with a heart-exchanger, and low-moderate hypothermia (esophageal temperature; ET 20–25°C) with moderate-flow CPB using a small-volume prime circuit.

Cardiac Anesthesia

Formerly, anesthesia was induced with thiamylal sodium under heavy premedication of hydroxyzine, acepromazine, and atropine, and maintained with moderate isoflurane (2-1.5-1-0.5%) and oxygen.

Currently, we use light premedication of ketamine 2.5 mg/kg IV, midazolam 0.2 mg/kg IV, fentanyl 5 μg/kg IV, and atropine 25 μg/kg IV, maintained with light isoflurane (1.5-1-0.5%) and oxygen, and fentanyl 10 μg/kg/h CRI. In addition, dopamine and dobutamine were infused with a small volume of CRI before and after ending CPB. As a result, arterial blood pressure was moderately elevated, and urine volume increased more than those of previous anesthesia before and during the CPB pump.

Formerly, deep hypothermia (TH) was induced by surface cooling using ice bags followed with blood cooling using heat exchanger of CPB into 20°C of target ET.

Recently, we used only blood cooling using heat exchanger of CPB into 20–25°C (low-moderate TH) of target ET. As a result, CPB pump time and the operative duration was shorter and recovery time minimal.

Cardiopulmonary Bypass Pump Technique

Formerly, blood was drained by gravity via jugular vein in which the tip of a drain tube was inserted into the right atrium, and through the soft reservoir, roller pump, heat exchanger and artificial lung, oxygenated blood was sent into the carotid artery toward the heart.

As before, a drain tube and sending tube were inserted into the jugular vein and the carotid artery, respectively. Recently, we used 8-10-12 Fr venous drain tube (Flexmate, Toyobo) with many side holes on the tip which were homemade. As a result, blood drain volume was increased and high-flow CPB became possible. However, we used middle-flow CPB (60–80 ml/min) under low-moderate TH during aortic cross clamp and high-flow (100 ml/kg/min) during rewarming (up to 37°C). We also used Terumo-Baby RX-05 (Terumo) as an artificial lung and a small circuit priming volume of 170 ml to prevent blood excessive hemodilution. Also, after cardioplegia, the same volume of water was filtrated using a hemoconcentrator (BC20 Plus, Maquet) to prevent over-hemodilution.

The priming solution was a combination of Ringer acetate (circuit priming volume – [added blood volume + drug]), 20% mannitol (5 ml/kg), Hespander 9 ml/kg, cefazolin (200 mg/100 ml), haptoglobin (1 ml/100 ml), and cross-matched donor blood added to maintain hematocrit at ~ 20%.

Cardioplegia

Formerly, cold (4°C) cardioplegia solution (St. Thomas II solution: Miotector®, Mochida Pharmaceutical Co. Ltd., Tokyo) was immediately injected antegrade with 10 ml/kg through an aortic root cannula by a syringe and repeated with 5 ml/kg every 20 minutes after aortic cross clamp (ACC).

Currently, first cold (4°C) cardioplegia solution was added with KCl up to K⁺ 20 mEq/L, and increased from 10 ml/kg to 20 ml/kg. After that, the solution without KCl was repeated every 20 minutes with 10 ml/kg. The infusion pressure was held in mean 80–100 mm Hg, not exceeding > 150 mm Hg.

Results

During 2008, we successfully performed all open-heart surgery in all 6 toy dogs weighing < 3.5 kg, in which the lowest bodyweight was 1.62 kg in a Chihuahua.

This combined method of deep sHT and low-flow CPB may be safely used for open-heart surgery in toy-breed dogs.

Currently, recovery from anesthesia was smooth and uneventful in all 7 dogs, in which ACC requires a mean 105.1 minutes (range 90–117) at a mean lowest ET 22°C (range 20.3–25.2°C). The improvements of CPB technique including cardiac anesthesia, pump method, and cardioplegia made possible a long ACC, moderate pump flow under low-moderate TH.

Future Problems

1.  Open-heart surgery in even lower bodyweight small dogs will require development of a new smaller CPB circuit and a smaller-size venous tube with many side holes on the tip.

2.  Prolongation of long ACC time over 2 hours will require improvement by a new cardioplegia method.

References

1.  Kanemoto I, Taguchi D, Yokoyama S, Mizuno M, Suzuki H, Kanamoto T. Open heart surgery with deep hypothermia and cardiopulmonary bypass in small and toy dogs. Vet Surg. 2010;39:674–679.

2.  Klement P, del Nido PJ, Mickleborough L, MacKey C, Klement G, Wilson GJ. Technique and postoperative management for successful cardiopulmonary bypass and open-heart surgery in dogs. J Am Vet Med Assoc. 1987;190:869–874.

3.  Lew LJ, Fowler JD, Egger CM, Thomson DJ, Rosin MW, Pharr JW. Deep hypothermia low flow cardiopulmonary bypass in small dogs. Vet Surg. 1997;26:281–289.

4.  Orton EC. Heart surgery strategies. In: Monnet E, ed. Small Animal Soft Tissue Surgery. Ames, IA, USA: Wiley-Blackwell; 2013: 785–791.