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A SIMPLE TECHNIQUE OF pH-STAT STRATEGY FOR INFANTS UNDERGOING DEEP HYPOTHERMIC CIRCULATORY ARREST. |
To enhance cerebral protection for neonates undergoing Deep Hypothermic Circulatory Arrest (DHCA) for repair of complex congenital heart defects, the authors currently utilize the following pH-stat blood gas strategy. Based on extensive outcome data from the Boston Circulatory Arrest Study and studies by Kurth, et al., pH-stat management may provide improved cerebral physiologic recovery following periods of DHCA.
Purpose:
To provide a simplified method of ph-stat blood gas management for patients undergoing DHCA for repair of complex congenital heart defects. This case report involves a modified Norwood I procedure for hypoplastic left heart syndrome.
Materials:
The cardiopulmonary bypass (CPB) circuit consists of a Lilliput I oxygenator with an integral reservoir (COBE Cardiovascular; Arvada, CO), ¼" rollerpump arterial raceway with Pulsatile Flow Controller (Stockert S-III; Germany), CDI 500 in-line blood gas analyzer (Terumo Cardiovascular Systems; Ann Arbor, MI), LG-6 Leukoguard arterial filter (Pall Corp; East Hills, NY), and a ¼ x ¼" AV-Loop.
The CO2 source is 100% CO2, connected to the ¼" side of a ¼ " x male luer lock DLP perfusion adaptor (Medtronic; Grand Rapids, MI). The male luer end is then connected to the female port of a ¼ x ¼ " luer lock connector (COBE Cardiovascular; Arvada, CO).
Methods:
CO2 priming of the circuit is key to the effectiveness of this method of blood gas management. After sterile assembly of the CPB circuit, the authors CO2 prime through the LG-6 arterial filter at 5 LPM for a minimum of three minutes. The LG-6 leukocyte reducing filter is a large prime (220cc) adult filter. We have incorporated this into our circuit since 1994 for all single ventricle patients, as the authors believe the benefit of leukoreduction outweighs the additional hemodilution. Immediately after CO2 priming is completed, assure that all vents are closed. Gas flow to the oxygenator remains off until CPB is commenced. This "traps" the CO2 in the system, essentially using CO2 as a prime additive. Only minimal CO2 then needs to be added for management on bypass.
The prime consists of Plasmalyte A (Baxter Healthcare; Deerfield, IL), 2000 units/L beef lung heparin, sodium bicarbonate 2 meq/kg/L prime, 12.5 gm 25% albumin, mannitol 0.5mg/kg, calcium chloride 20mg/kg plus 200mg/unit packed red blood cells. All blood is washed and leukoreduced, and added to keep the dilute hematocrit greater than 18%.
The patients are anesthetized with Fentanyl and Propofol infusions. Additional Fentanyl (50mcgs/100cc prime) and Pavulon (0.3mg/kg) are added to the CPB circuit. One hypothermia blanket is placed under the child and one hypothermia blanket is placed over the child's lower extremities for adjunctive surface cooling. The blankets are connected to a blanket warmer (Cincinnati Sub-Zero; Cincinnati, OH) to maintain normothermia until the patient is heparinized. After heparinization, the hypothermia blanket underneath the child is attached to the Heater/Cooler (CSZ; Cincinnati, OH) of the CPB circuit. The water bath of the blanket warmer and the heater/cooler are set to 13oC. The water lines to the oxygenator are turned off at this time.
Adequate heparinization is assured prior to cannulation, using the Hepcon/HMS (Medtronic Hemotec; Parker, CO). CPB is initiated with a minimum activated clotting time (ACT) of 300 seconds. Circulating heparin levels and ACT's are measured three minutes after initiation of bypass and every 20-30 minutes thereafter. Adequate circulating heparin levels are maintained on bypass, despite ACT results which may measure >999 seconds because of hemodilution or hepatic immaturity.
Global cooling is the goal. Rectal, nasopharyngeal and tympanic temperatures are measured. A temperature probe (Mon-a-therm; Mallinckrodt Medical, St. Louis, MO) is also placed on the sole of the patients' foot. The skin temperature helps reflect peripheral perfusion. This temperature is very effective in helping us dose the appropriate vasodilator and in determining the length of cooling time prior to circulatory arrest. Ice packs are placed around the patient's head with the initiation of bypass. Core cooling with the water bath set at 130C is immediately begun after the start of bypass. The patient is cooled to 180C rectal and/or nasopharangeal. The authors keep the tympanic temperature >150C and the arterial blood infusate temperature >130C.
The proximal main pulmonary artery is cannulated with an 8 French DLP arterial cannula (Medtronic; Grand Rapids, MI). An 18 French DLP single venous cannula is placed in the right atrium.
CPB is initiated with a FiO2 0.60 and a gas: blood flow of 0.5:1. After assuring adequate gas transfer across the membrane oxygenator, the FiO2 is immediately adjusted down to as low as 0.25-0.30, keeping the paO2 > 100mmHg. The gas flow is immediately decreased to 0.05-0.1 LPM, keeping the pCO2 40mmHg at the actual measured temperature. This would reflect a pCO2 >80 mmHg measured at 37oC. The FiO2 and gas flow are finely adjusted to keep the pCO2 40 mmHg at the actual temperatures during the entire cooling phase. By keeping the gas flow so low at the initiation of bypass, CO2 usually only needs to be added to the circuit briefly as the temperatures reach 20-220C. Often, using this method of pH-stat management, we do not need to add any additional CO2 to the circuit. When CO2 does need to be added, using the aforementioned connection to the CPB circuit, the CO2 is titrated at less than 0.5 LPM flow, and is only added for a few seconds. As soon as a change in the paCO2 is seen, shut off the CO2 flow and clamp the CO2 line, but leave it in place in case additional CO2 would be needed. Rarely is more than a brief, one-time "dose" of CO2 required. If you overshoot on the addition of CO2, minimal regulation is needed with the gas flow to the circuit. With careful titration, it is not often necessary to blow off CO2 to maintain a balanced pCO2. The CDI 500 allows for real-time data. Blood gases are sent to the lab after approximately ten minutes of CPB to calibrate the CDI and for confirmation of electrolytes.
Cooling is continued for a minimum of twenty minutes, or longer if needed, to achieve global cooling. Temperatures are monitored carefully not only for their absolute values, but for the gradients between all the temperatures monitored. Blood flows are maintained at 170-200cc/kg during the entire cooling phase. Acidosis is treated with sodium bicarbonate. Low blood pressures are not treated at any time with vasoconstrictive agents. Adequate flows and good perfusion of all vascular beds are achieved with adequate vasodilation. Phentolamine (0.2mg/kg) or Phenoxybenzamine (0.25mg/kg in prime and 0.25-0.50mg/kg infusion over 24 hours) is used on all cases for alpha blockade. Once temperatures have been reached and two minutes prior to termination of CPB, Sodium Thiopental (8 mg/kg) is given into the CPB circuit. Glucose levels are stabilized at 150mg/dl prior to DHCA on all neonates. Two minutes later, bypass is terminated and the patient is exsanguinated out the venous line. Simultaneously, 15cc/kg crystalloid cardioplegia is administered retrograde via the ductus and the cardioplegia solution is also drained into the circuit out the venous line. Once the patient is adequately drained the cannulae are removed and the Norwood I repair is completed in 45-60 minutes of DHCA.
Following the repair, CPB is reestablished with the arterial cannula in the neoaorta and the venous cannula in the right atrium. The venous line remains clamped until the patient is adequately filled with volume from the circuit. Rewarming is begun immediately and the ice is removed from the head. A warming gradient of 80C between the venous blood temperature and the water bath temperature is diligently maintained, keeping the warming time to less than or equal to 10C every three minutes. Additional vasodilators are added as needed to ensure global warming of all vascular beds. Nitroglycerine, or additional Phentolamine or Phenoxybenzamine, is used. Additional mannitol (0.25-0.50 mg/kg) and sodium bicarbonate will be added as warming progresses. Calcium chloride (20mg/kg) is added when temperatures reach 280C, keeping the ionized levels at 1.3 mmol/dl.
Conventional ultrafiltration (CUF) with an HPH-Mini (Minntech; Minneapolis, MN) is utilized on all cases. Ultrafiltration begins with rewarming and continues until CPB terminates. The authors prefer to wean from bypass with a hematocrit >30%. If ultrafiltration cannot achieve this, additional washed packed red blood cells are added. A unit of reconstituted whole blood is administered as the patient is being weaned from bypass. The blood in the pump is salvaged, washed and returned to the patient. This blood protocol is very patient specific. Any combination of packed cells, whole blood, platelets, and cell salvage blood is used, attempting to keep donor blood minimal and hematocrit optimal. CUF also allows us to treat high glucose levels by dilutional washout with 0.9% sodium chloride.
This method of pH-stat management continues until the patient warms to 280C. Blood gases are then treated by conventional alpha-stat strategy. Bypass is terminated with rectal and nasopharyngeal temperatures no greater than 350C, and a skin temperature greater than 300C.
Conclusion: This method of pH-stat blood gas management is simple and easily achieved. No carbogen tanks are needed and it is easily duplicated case to case. This pH-stat strategy is used for all pediatric patients undergoing DHCA, patients with aortopulmonary collateral arteries, and any infants cooled to <280C. This guideline is offered in an attempt to simplify the management of pH-stat blood gas strategy for infants undergoing DHCA. By using CO2 as a prime additive, minimal intervention is needed to maintain a pH of 7.40 and paCO2 of 40mmHg at actual temperatures.
REFERENCES
Priestley MA, Golden JA, O'Hara IB, McCann J, Kurth CD: Comparison of Neurologic Outcome After Deep Hypothermic Circulatory Arrest with Alpha-stat and pH-stat Cardiopulmonary Bypass in Newborn Pigs. J Thorac Cardiovasc Surg 2001;121:336-43
Kurth, CD, O'Rourke, MM, O'Hara, IB: Comparison of pH-stat and Alpha-stat Cardiopulmonary Bypass on Cerebral Oxygenation and Blood Flow in Relation to Hypothermic Circulatory Arrest in Piglets. Anesthesiology 1998;89:110-8
du Plessis, et al.: Perioperative Effects of Alpha-stat versus pH-stat Strategies for Deep Hypothermic Cardiopulmonary Bypass in Infants. J Thorac Cardiovasc Surg 1997;114(6):991-9