New circuit developed for cardiopulmonary support

Patrick L. VanderWal, MS, CCP, perfusionist, Children's Hospital of Wisconsin.

The postoperative care of children with congenital heart disease occasionally requires the use of extracorporeal support devices. At Children's Hospital of Wisconsin, the preferred device has been the traditional ECMO (extracorporeal membrane oxygenation) circuit, utilizing a roller head pump and a true silicone membrane oxygenator. This technology has been very useful and has saved hundreds of children in acute cardiopulmonary crisis. However, due to its size, it requires blood in the prime and its somewhat complex circuitry and the design of the oxygenator takes a minimum of 45 minutes to an hour to set up and prime the system.

To reduce the perfusionist's response time, a traditional ECMO circuit is kept preassembled and primed with crystalloid at all times. This significantly has reduced the response time, however, due to the need for blood in the prime, blood availability often is the limiting factor for a quick response. In an effort to eliminate this factor, a miniaturized circuit has been developed allowing for support of even our small neonates with an asanguinous prime. The circuit also can be set up and primed in less than five minutes.

This new rapid response cardiopulmonary support circuit (CPS) utilizes centrifugal pump technology combined with a hollow fiber oxygenator. The use of these components allows for the rapid prime and de-bubbling of the system. Specifically, the system is composed of a Biomedicus centrifugal pump, Medtronic Mini-Max oxygenator with plasma resistant fibers and an integrated heat exchanger. A Terumo CDI 500 is used for in-line blood gas analysis of arterial gas parameters and venous saturation and HCT tracking. The circuit was designed to fit on a fully mobile Biomedicus VAD cart and has a battery capable of powering the pump for up to two hours. The cart has the capacity to carry both oxygen and compressed air, so the Secrest blender can be utilized throughout transport from anywhere in the hospital, for example, from the catheterization laboratory, intensive care unit or operating room.

The new system was designed for short-term support. Hollow fiber oxygenators are suboptimal for long-term support. The fibers have an increased susceptibility to "wet out" reducing their capability for gas transfer compared to the true silicone membrane used with the traditional ECMO circuits. Centrifugal pumps may cause considerably more hemolysis at low flow rates compared to roller pumps. Therefore, if long-term support is indicated, transition to the tradition ECMO circuit may be warranted.

The new CPS system went live May 1 and currently is offered to all cardiac patients in crisis less than 15 kg. Circuits designed to support children greater than 15 kg are in development.

Mechanical ventilation

Sara M. Eckes, RRT-NPS; Khris E. O'Brien, RRT, respiratory care practitioners, Children's Hospital of Wisconsin.

The many advances in respiratory care in recent years have become an integral part in the care of patients at Children's Hospital of Wisconsin. Using the bronchial hygiene protocol, respiratory care practitioners determine the best modality to optimize the bronchial hygiene of each post-surgical cardiac patient. We also provide a valuable service helping to determine the optimal ventilation modalities for intubated patients.

The invasive ventilators available are Infant Star with or without Star Sync, Drager Babylog, Servo 300/300A and Servo I. Specialty ventilators also are available for invasive and non-invasive ventilation. They are the Pulmonetic's LTV portable ventilator, Vision BiPAP and Sensormedics 3100A/3100B oscillatory ventilators.

Having a variety of ventilators to choose from provides a way for us to customize the ventilatory needs of each patient. Many of the ventilators have similar modes, but the characteristics of each ventilator differ.

Components of ventilation
Most of our ventilators have six primary components to each mode:

1. PIP (positive inspiratory pressure): The amount of pressure used to deliver a breath.
2. TV (tidal volume): The amount of volume delivered with each breath.
3. RR (respiratory rate): The number of breaths per minute.
4. PEEP (positive end expiratory pressure) or CPAP (continuous positive airway pressure): The amount of pressure maintained in the lungs at all times.
5. IT (inspiratory time): The amount of time it takes to deliver the breath.
6. FiO2 (fraction of inspired oxygen).

Other optional components are available with certain ventilators. These include:

1. PS (pressure support): Only used on spontaneous breaths, adds extra pressure to assist patient in overcoming the resistance of the endotracheal tube.
2. Trigger sensitivity: Pressure or flow sensitivity. Determines how the ventilator recognizes when a patient is breathing spontaneously and determines how much work of breathing is involved for the patient to trigger a breath.

How each component is used depends on the ventilator and mode being used. The goal is to enhance ventilation while decreasing work of breathing and reducing barotrauma.

Modes of ventilation
CPAP with or without PS: CPAP is a mode that allows a patient to breathe spontaneously at an elevated positive pressure baseline. PS is an option for spontaneous breaths only. The primary use of PS is to overcome the resistance of the tubing and decrease work of breathing. The preset pressure support will be delivered when the patient triggers inspiration.

IMV (intermittent mandatory ventilation): This mode delivers a set PIP, a set PEEP and a set RR with a set I-time at pre-determined intervals. The patient is allowed to breathe at CPAP/PEEP levels in between mechanical breaths, but the mode makes no attempt to synchronize the breathing efforts with the machine.

SIMV (synchronized intermittent mandatory ventilation) plus pressure support.

SIMV/VC (volume control): In this mode a specific tidal volume and RR are set.

SIMV/PC (pressure control): With this mode, instead of setting a volume, a specific peak inspiratory pressure is set as well as a RR.

In both modes, the RR determines the maximum number of "ventilator breaths" the patient will receive in one minute. The patient may breathe at will and will receive the set pressure support on non-mechanical breaths. However, if it is "time" for a mechanical breath, the ventilator will synchronize the mechanical breath (either volume or pressure) with the spontaneous breath instead of the pressure-supported breath.

Pressure control: This mode usually is used for patients without breathing capacity. Here, the PIP is constant and the VT will vary with compliance/resistance. The patient always will receive the set RR, but if the patient initiates a breath he or she will receive a ventilator breath.

Volume control: This mode also is usually used for patients without breathing capacity. Volume control delivers a certain preset volume during a preset time with a constant flow. The PIP may vary with changes in compliance/resistance. If the patient initiates a breath, the patient will receive a ventilator breath.

VG (volume guarantee) or PRVC (pressure regulated volume control): Similar modes used on two different ventilators. VG is available on the Infant Drager and PRVC is available on all Servo ventilators. Both modes aim to deliver a desired volume using the lowest possible pressure. Rapid changes in compliance or resistance are dealt with slowly over several breaths. This protects the patient's lungs from sudden increases in pressure.

Volume support: This is a spontaneous breathing mode. Peak inspiratory pressures are determined by the patient's ability to maintain ventilation. The patient is given pressure to deliver a minimum preset tidal volume. If the patient is able to maintain adequate spontaneous tidal volume, no pressure is added. If not, pressure will be added to help the patient reach the minimum tidal volume set. If the apnea alarm is activated, the ventilator will switch to PRVC.

Servo (auto mode): The auto mode is suitable for patients who have a respiratory drive, are able to trigger breaths but require a backup rate, have changing ventilatory needs and still require additional monitoring. Auto-mode options are available with Servo brand ventilators in the following modes:

- PRVC partners with volume support.
- Volume control partners with volume support.
- Pressure control partners with pressure support.

Auto-mode switching: Control to spontaneous mode will switch when the patient triggers two spontaneous breaths in a row. Spontaneous to control mode will switch when apnea intervals are achieved.

SIMV/PRVC + PS: This is a new mode currently available on the Servo I only. This mode provides the benefit of PRVC in a spontaneous breathing mode, but still allows for traditional weaning options. RR and volume both can be weaned allowing for spontaneous breathing with the PS option.

Ventilator adjustments
Children need to be ventilated differently than adults. With our smaller children and premature infants we primarily use pressure ventilation instead of volume ventilation, which is used in larger children and adults.

Whenever mechanical ventilation is necessary, ask a few of these questions first.

• Is the patient breathing spontaneously?
• Should the patient be breathing spontaneously?
• Does the patient have good chest wall excursion?
• Does the patient have good bilateral air entry?
• How much does the patient weigh?
• Does the patient have adequate gas exchange?
• Are blood gases acceptable?
• What impact will the disease process or anatomy of the patient have in determining the patientÕs ability to ventilate adequately?
• Which ventilator and what available options best match your patients needs?

Summary
While many ventilator manufacturers call different modes by different names, the basics of operation are similar. Respiratory care practitioners must be experts on the modes, options and limitations of each ventilator.

Collaboration between care providers is vital as mechanical ventilation does not just involve the lungs but encompasses the whole body. Making ventilator adjustments may only correct a temporary underlying problem. One must look at the whole picture when making decisions in mechanical ventilation. It all starts with choosing the right ventilator and mode. Inevitably, the underlying cause or problem must be corrected to facilitate extubation.

Respiratory care practitioners work closely with nurses and physicians involved in the patient's care, enabling the best possible delivery of care for each patient. As respiratory care advances, the need for critical thinking abilities will increase. The role that respiratory care practitioners now play in weaning and maintaining the ventilator also will change with time. Respiratory care practitioners are vital to the management of ventilators with the proper knowledge and understanding of equipment and disease processes.

Continuous milrinone home infusion therapy

Gen M. Owens, RN, MSN, CPNP, cardiology nurse practitioner, Herma Heart Center, Children's Hospital of Wisconsin.

Milrinone (Primacor) is classified as an inotropic agent with a mechanism of action that essentially relaxes the vascular muscle contributing to vasodilation by inhibiting the enzyme phosphodiesterase (the major enzyme in cardiac and vascular tissue). In the past few years, Children's Hospital of Wisconsin has seen an increased use of Milrinone therapy on many of the postoperative cardiovascular inpatients, such as patients who are at an increased risk of developing low cardiac output states. Milrinone also is indicated for use in patients with congestive heart failure.

Recently, we have begun discharging patients on home infusion Milrinone therapy. The types of patients most likely to require home infusion include patients with a diagnosis of protein losing enteropathy (a condition that develops as a result of increased lymphatic pressure in the gut), patients with dilated cardiomyopathy and patients in end-stage cardiac failure awaiting heart transplantation.

There are several necessary steps required in preparing the discharge of a patient on Milrinone home infusion therapy. Central venous access is a must. The central access team should place a new central venous line (CVL) and educate the parent or caregiver to care for the central line. The caregiver also must be taught how to perform dressing changes, understand the side effects and medication interactions of Milrinone and the necessary procedure in the event the patient's CVL becomes dislodged.

At Children's Hospital, the inpatient case manager is notified of the plan of care prior to discharge. The case manager helps ensure insurance coverage of home Milrinone therapy, as well as facilitates the delivery of the infusion pump, medication and central venus line dressing supplies to the home.

The usual dose of Milrinone infusion home therapy ranges from 0.5mcg/kg/min up to 0.75mcg/kg/min. The patient is followed closely in the Herma Heart Center outpatient clinic with appointments on a weekly or bi-weekly basis depending on patient stability and therapy effectiveness.

So far, there have been 17 patients sent home on infusions of Milrinone. The time requirement for continuous home infusion depends on the need for therapy. Patients requiring Milrinone for dilated cardiomyopathy or end-stage cardiac failure most likely will require the therapy until the time of transplant.

D-Transposition of the great vessel

Ndidiamaka Musa, MD, pediatric critical care specialist, Children's Hospital of Wisconsin, assistant professor, Pediatrics, Medical College of Wisconsin.

The following was taken from a presentation Musa gave in June to Children's Hospital critical care staff.

First described in 1797, transposition of the great vessel (D-TGA) is a congenital heart defect in which the great arteries are transposed. There is atrio-ventricular concordance, but ventricular-arterial discordance. (The aorta arises from the right ventricle and the pumonary artery arises from the left ventricle). (Figure 1)

This gives rise to a circulation in parallel, which is the dominant physiology of this defect. Thus arterial oxygenation requires mixing between parallel circulations. (Figure 2)

Effective blood flow - anatomic right to left and left to right shunts participate in functional gas exchange at a capillary level and is important for survival. Blood flow also is dependent on the number, size and position of the shunts. (Figure 3).

Diagnosis and preoperative assessment
The diagnosis should be suspected in any neonate with severe cyanosis. Along with echo(cardiography) confirmation, prostaglandens (PGE1) is initiated. In patients with D-TGA with intact septum balloon atrial septostomy by the bedside with echo guidance can ensure adequate intercirculatory mixing.

Management goals are to:

• Assist in "mixing" to improve effective pulmonary blood flow (PBF) and systemic blood flow (SBF) by performing a balloon atrial septostomy (BAS) and beginning infusion of PGE1.
• Optimize systemic output (increase mixed venous saturation Ð SVC sat) by preload/inotropic support and optimizing hemoglobin.
• Minimize pulmonary edema (increase pulmonary venous saturation - PV sat) by minimizing atelectasis and encouraging diuresis.

Repair
In 1975, the first successful arterial switch operation was reported. Since then, surgical techniques and perfusion methods have evolved, improving outcomes. There is a predicted 10-year survival of 93 percent for uncomplicated D-TGA with IVS or VSD as reported by Wernovsky et al in the Journal of Thoracic Cardiac Surgery Vol 109(2) 289-302. (Figure 4).

Postoperative management
After separation from cardiopulomnary bypass (CPB) and initial stabilization, the patient is transferred to the Pediatric Intensive Care Unit. After review of the anesthesia records, physical exam, initial labs and chest x-ray, postoperative management should include the following:

• Sedation and neuromuscular blockage (NMB).
• Monitor cardiac output and SVC sat and signs of end-organ perfusion.
• Optimize preload:

• Watch for signs of low cardiac output syndrome (LCOS). LCOS was thought to be related to a number of factors including an inflammatory response post cardiopulmonary bypass, effects of myocardial ischemia from aortic cross clamp, hypothermia and reperfusion injury.

Management of LCOS

Minimize oxygen consumption and demand by:

• Using sedation/NMB.
• Treating fevers aggressively.

Maximize oxygen delivery SVO2 by:

• Optimizing preload (blood and 5 percent albumin).
• Using inotropy (Epinephrine).
• Afterload reduction (Milrinone/Nipride).

Diagnose and treat residual lesions.

Postoperative complications
Postoperative complications include residual lesions (supravalvar pulmonary stenosis, VSD and ASD) or myocardial ischemia from a kinked or clotted coronary artery.

Click here for updated information about Project ADAM.

ADAM Act becomes national law

U.S. Sen. Russ Feingold held a news conference at Children's Hospital of Wisconsin Monday, July 21 to announce that the Senate approved the Automatic Defibrillators in Adam's Memory (ADAM) Act. Adam was a 17-year-old Whitefish Bay High School student who collapsed and died during a basketball game in 1999. Project ADAM was created as a result of Adam's death and the deaths of several other young athletes in the Milwaukee area, in an effort to help schools install and use automatic external defibrillators (AEDs). The ADAM Act is based on the successes Project ADAM has had in Wisconsin. The act was initiated and written by Herma Heart Center staff members at Children's Hospital of Wisconsin and backed by Feingold.

Project ADAM helps Wisconsin schools implement public access defibrillation programs, establish training in cardiopulmonary resuscitation and AED use and maintain the equipment. The ADAM Act will allow programs like Project ADAM to apply for grant funds to establish national information centers. These centers will serve as information clearinghouses to increase public access to defibrillation in schools.

Project ADAM staff and the Lemel family are committed to applying for these funds in order to establish the first national clearinghouse at Children's Hospital.

For more information about Project ADAM or the ADAM Act, call (414) 266-3889.