Cyanotic Heart Disease

Cyanosis occurs in patients with inadequate Q _p or those in whom the pulmonary and systemic circulations occur in parallel with inadequate mixing of oxygenated and deoxygenated blood. Neonates whose Q _p is dependent on a patent ductus arteriosus (PDA) may present hours to days after birth, with severe hypoxemia and acidosis as the ductus closes. Decreased Q _p may be due to obstructed flow from the inlet of the pulmonary ventricle (e.g., tricuspid atresia) or from its outlet (e.g., severe pulmonary stenosis, pulmonary atresia [PA] with intact ventricular septum, or tetralogy of Fallot). The PDA allows blood to bypass the level of obstruction. An atrial or ventricular septal defect is also required to “decompress” the systemic venous return to the systemic side of the circulation and allow mixing of oxygenated and deoxygenated blood. The blood in the systemic ventricle therefore consists of desaturated systemic venous blood (via the septal defect) and a smaller volume of saturated pulmonary blood depending on the degree of pulmonary obstruction (Q _p :Q _s <1). Although Q _s may initially be normal, with ductal closure systemic DO ₂ falls rapidly due to both decreasing Q _p :Q _s and greater O ₂ extraction. This results in anaerobic metabolism, acidosis, and myocardial dysfunction.

Neonates may also present with significant cyanosis in the presence of inadequate mixing of parallel pulmonary and systemic blood flow as is seen with d-transposition of the great arteries (d-TGA) or with obstructed pulmonary vein outflow as seen with total anomalous pulmonary venous return (TAPVR). Patients with d-TGA with intact ventricular septum will present at birth with severe cyanosis but with preserved Q _s . Acidosis and myocardial dysfunction will develop over time as O ₂ saturations fall, though the rapidity will depend on the presence and size of an atrial shunt to provide mixing. Patients with obstructive TAPVR also present with cyanosis but may demonstrate significant respiratory insufficiency due to pulmonary hypertension with increased transmural pressure across the pulmonary vasculature and pulmonary edema formation.

The differential diagnosis of a neonate who presents with severe cyanosis must include the possibility of a ductal dependent pulmonary circulation with a closing ductus arteriosus. For neonates with suspected CHD the initial evaluation should include the following:

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  Four-extremity blood pressure measurements (e.g., upper and lower limb systolic blood pressure difference >10 mm Hg usually indicates aortic arch abnormalities).

  
  
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  Preductal and postductal oxygen saturation measurements (oxygen saturation as measured by pulse oximetry [SpO ₂ ]) to identify differential cyanosis (i.e., preductal SpO ₂ > postductal SpO ₂ ) and reverse differential cyanosis (i.e., postductal SpO ₂ > preductal SpO ₂ ). A preductal-postductal SpO ₂ difference of more than 10% usually indicates ductal dependent CHD. Reverse differential cyanosis is usually observed in TGA but was also described in supracardiac total anomalous pulmonary venous connection.

  
  
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  Hyperoxia test to differentiate between CHD and non-CHD (e.g., SpO ₂ <85% on both room air and 100% oxygen usually indicates CHD).

Other causes of central cyanosis in a neonate should be evaluated concurrently with initiating prostaglandin E ₁ (PGE ₁ ) while establishing a definitive diagnosis. These causes include pulmonary causes such as persistent pulmonary hypertension, congenital diaphragmatic hernia, pneumothorax, or pleural effusion, as well as central nervous system depression due to perinatal asphyxia, neurologic disorder, or maternal sedation. In addition to four-extremity blood pressure measurements, preductal and postductal saturation levels, and hyperoxia testing, the diagnosis can be further elucidated by physical examination; chest x-ray examination to evaluate cardiac size, position, and shape and vascular markings; electrocardiogram to assess ventricular hypertrophy, conduction defects, or arrhythmia; and arterial blood gas measurement to assess gas exchange and acidosis. Ultimately, an echocardiogram will be required but will not always be available on presentation.

Acyanotic Heart Disease

Children with acyanotic heart disease will present emergently when Q _s is inadequate. This may represent (1) ductal dependent systemic circulation with obstructed flow into or out of the LV (e.g., mitral stenosis, aortic valve disease, aortic coarctation), (2) myocardial dysfunction (e.g., cardiomyopathy, anomalous coronaries), or (3) maldistribution of pulmonary blood flow with left-to-right shunts (e.g., ventricular septal defect [VSD]). These lesions result in inadequate systemic DO ₂ either by actual obstruction to flow through the left heart or by excessive Q _p with a functional decrease in blood flow to the systemic circulation. As the pathophysiology for each of these defects worsens, the ability of the lungs to oxygenate blood is impaired due to increased intravascular and extravascular lung water and elevated pulmonary vascular resistance (PVR).

Ensure Adequate Pulmonary and Systemic Output

Intravenously administered PGE ₁ dilates the ductus arteriosus to provide Q _p or Q _s to neonates with ductal dependent lesions. An infusion of PGE ₁ should be initiated for any clinically unstable neonate with severe cyanosis or cardiovascular failure to reestablish flow. This should be done before confirming the presence of a ductal dependent lesion. Patients who may benefit from early PGE ₁ initiation include (1) those with right-sided cardiac lesions who need to maintain Q _p (tricuspid atresia, PA, tetralogy of Fallot with PA, Ebstein anomaly), (2) those with left-sided cardiac lesions that need to maintain Q _s (hypoplastic left heart syndrome, critical aortic coarctation, interrupted aortic arch), and (3) those with parallel circulations (d-TGA) to augment effective mixing of circulations, though this is likely to be insufficient without an atrial septal defect (ASD) of adequate size. However, PGE ₁ administration may be not helpful in d-TGA with intact ventricular septum with a small ASD because there is limited mixing and critical aortic stenosis because the ductus is distal to the area of obstruction and the LV is still subjected to outflow; PGE ₁ administration may be relatively contraindicated for patients with obstruction of the blood flow into or out of the left atrium (e.g., TAPVR with obstruction). In those conditions the benefits of PGE ₁ outweigh the risks, and PGE ₁ should be started pending definitive diagnosis. Consideration must also include the serious side effects of PGE ₁ infusion, which may be dose related. Up to 38% of patients may develop one or more adverse reactions, the most common being apnea (18%), hypotension (13%), and fever (11%). The starting dose for the PGE ₁ infusion is 0.01 to 0.03 mcg/kg/min, though this should be initiated at a higher dose (0.05 to 0.1 mcg/kg/min) if the ductus has closed and the patient is in extremis. To decrease the risk of side effects, once patency is established, the dose should be minimized if possible. In addition to the presence of a PDA, lesions with obstructed Q _p or Q _s also require a site of mixing of Q _p and Q _s , typically an ASD. As described earlier in the case of a minimal ASD, reestablishing patency of the ductus is unlikely to provide adequate mixing of the circulations, and an atrial septostomy may be required. This should be considered before transport so that the accepting team can have the necessary personnel and equipment present on the infant’s arrival.

Once ductal patency has been restored, cardiac output must be optimized and balanced between pulmonary and systemic circulations. In the absence of a fixed obstruction the distribution of total cardiac output to the pulmonary and systemic circulation is determined by the relative resistance of each vascular bed ( Table 42.1 ). PVR is affected by pH, pCO ₂ , pO ₂ , lung volume, noxious stimuli, hematocrit, and a number of medications. Patients with excessive Q _p and consequent low DO ₂ to the systemic circulation can be managed by increasing PVR to decrease Q _p and increase Q _s . Conversely, decreasing PVR will increase Q _p and decrease the relative flow to the systemic circulation. Therefore significant care should be taken with overventilation or oxygenation of patients with ductal dependent systemic circulation. In discussing this with the transport team, it is essential that the communication be concise and goal directed.

Similarly, alterations in PVR can help improve hemodynamics in other cardiac lesions. Minimizing PVR can decrease RV afterload to encourage more forward flow out of the RV to increase LV preload and possibly cardiac output as long as the LV is compliant. Manipulation of PVR also provides some control over intracardiac shunting and effective cardiac output. Shunting through large intracardiac shunts is affected by the relative resistances of the downstream circuits. In the case of an ASD the compliance of the ventricle will determine its filling pressure and therefore the pressure gradient across the ASD. Shunt flow across an ASD is typically left to right because the right ventricle (RV) is typically more compliant than the LV. However, in newborn infants with an ASD the RV may be relatively stiff or noncompliant, and the direction of shunt flow may be right to left, resulting in some arterial oxygen desaturation. For VSDs and extracardiac shunts the downstream resistance is provided by the systemic and pulmonary vasculature. Decreased PVR will increase Q _p :Q _s and reduce effective systemic perfusion. In most cases the PVR is substantially less than the systemic vascular resistance (SVR), and shunt flow will be toward the pulmonary circuit.

Stabilize Hemodynamics

Myocardial function is determined by both the contractile function and the loading conditions of the heart. The general approach to improving cardiac output is to augment contractility with inotropic support, to optimize preload with careful titration of fluid resuscitation or diuretic therapy, and/or to minimize afterload of the RV and/or LV with vasodilators or by altering PVR as described earlier. Additional support of LV function can be achieved with positive pressure ventilation by decreasing wall stress with resultant decrease in the afterload on the LV. The adequacy of DO ₂ can also be improved by minimizing O ₂ consumption with temperature control, sedation, and mechanical ventilation.

Myocardial contractility is controlled by the presence of intracellular calcium and therefore can be improved by increasing ionized calcium or infusing inotropic medications, such as catecholamines or phosphodiesterase inhibitors. Calcium supplementation plays an essential role in augmenting LV function in children. The underdeveloped sarcoplasmic reticular system in the neonatal myocyte causes the myocardium to be more dependent on extracellular calcium concentration than the adult myocardium. Because intracellular calcium plays a central role in myocardial contractility in neonates, blood levels of ionized calcium should be normalized to augment stroke volume. Infants with chromosome 22q11 deletions (velocardiofacial syndrome, DiGeorge syndrome) are particularly susceptible to low calcium levels.

Catecholamines, or beta-adrenergic agents, represent a class of agents that provide varying effects on contractility, heart rate, and vascular resistance depending on the specific receptors stimulated. The more frequently used agents include epinephrine, dopamine, dobutamine, and isoproterenol. Epinephrine is typically used at a dose of 0.03 to 0.3 mcg/kg/min, with predominant beta-1 effects and increased contractility at lower doses. At higher doses, alpha-1 effects predominate with vasoconstriction. Dopamine is used at doses of 3 to 10 mcg/kg/min, though its use may be limited because it is associated with a greater incidence of arrhythmias, as well as increased PVR at higher dosing. Dobutamine is an effective positive inotropic agent but frequently produces unacceptable degrees of tachycardia. Isoproterenol is used predominantly for its chronotropic effect. Inotropic drugs increase contractility by increasing cytosolic Ca ²⁺ concentration, which may also impair relaxation of the heart, decrease ventricular compliance, and limit preload. In addition, augmenting inotropy will increase myocardial energy utilization. Milrinone is an inotropic agent that works through an alternative mode of action. As a phosphodiesterase-3 inhibitor, it potentiates the action of cyclic adenosine monophosphate (cAMP) to provide inotropic effects, as well as vasodilation. Data suggest that milrinone minimizes low cardiac output syndrome in some postoperative patients. Milrinone may serve an important role in patients with decompensated heart failure and preserved blood pressure by reducing afterload, particularly given that beta-adrenergic receptors are desensitized in these patients, resulting in a blunted response to catecholamines. It should be remembered, however, that the half-life of milrinone is relatively long at 2 to 3 hours, and its use can cause persistent hypotension and should be used in caution with patients with renal insufficiency. Vasopressor agents such as norepinephrine and vasopressin that increase afterload should be used with great caution in patients with poor myocardial function. These agents do, however, serve an important role in increasing diastolic blood pressure to maintain myocardial perfusion, as well as in the treatment of severe vasoplegia with hypotension due to sepsis or systemic inflammation.

Preload, or the amount of blood that distends the ventricle before contraction, determines the effectiveness of contraction as described by the Frank-Starling law. The impact of preload on output will depend on the compliance of the ventricle. In a compliant ventricle, enhancing preload will increase stroke volume and cardiac output. However, in a poorly compliant ventricle, augmenting preload will negatively impact the strength of contraction, elevate venous pressure, and generate pulmonary edema. The neonatal myocardium is stiffer than that of older children; therefore it both requires a greater than normal preload to maintain output and is less tolerant of excess preload. An infant presenting with congestive heart failure, pulmonary edema, and a stable systemic blood pressure may respond to diuretics to reduce ventricular wall stress and relieve pulmonary edema without compromising ventricular output. However, an infant with a myopathic ventricle presenting with hypoperfusion, hypotension, and acidosis is more likely to require carefully titrated fluid administration to optimize preload and augment cardiac output. Chest x-ray examination and liver size can aid in determining fluid and diuresis requirements. When titrating fluid, careful monitoring of central venous pressure (CVP) is crucial because small changes in volume can cause overdistention in a noncompliant ventricle. In the absence of a CVP, which is typical on transport, we recommend administering smaller-volume boluses (5 mL/kg to 10 mL/kg) with frequent reassessment of the effectiveness of the fluid bolus(s). If there appears to be a positive response with a reduction in heart rate and/or improved blood pressure and perfusion, volume augmentation of preload should continue. However, if there is a worsening of vital signs and/or signs of fluid overload, one should consider stopping volume augmentation and administer diuretics instead.

Reducing afterload may improve myocardial function by decreasing ventricular wall tension and myocardial oxygen consumption and improving stroke volume. Afterload can be optimized by using agents that vasodilate (nicardipine, nitroprusside, milrinone, dobutamine), as well as by avoiding medications (high-dose dopamine, epinephrine, norepinephrine) or situations (pain, agitation) that raise SVR. Patients with left-to-right shunts and LV volume overload have shown improved function with cautious reduction of systemic afterload. Children with LV outflow tract obstruction and pressure overload, such as severe aortic stenosis, may have massively increased, fixed afterload. Administering vasodilators in these patients will not increase Q _s but rather may cause shock, myocardial ischemia, or life-threatening arrhythmias. In this situation, afterload reduction is accomplished by relief of the fixed obstruction using surgical or cardiac catheterization techniques.

Mechanical ventilation may be needed to support respiratory failure and decrease O ₂ utilization in the case of poor DO ₂ . In addition, positive pressure ventilation impacts preload, contractility, and afterload of the right and left ventricles to directly alter cardiac output. During spontaneous ventilation, inspiration generates negative intrathoracic pressure, which enhances venous return to the heart. The increase in preload to the RV augments stroke volume. Positive pressure, on the other hand, increases intrathoracic pressure and reduces preload to the RV. This will be further accentuated by other factors that impact preload, such as hypovolemia or sepsis. Mechanical ventilation augments LV output primarily by altering its transmural pressure. The change in pleural pressure from negative to positive decreases the transmural pressure and lowers the afterload against which the LV must pump. Although this may have minimal impact on a normally functioning myocardium, it can provide significant support to a dysfunctional LV.

Changes in lung volumes also can impact RV afterload by altering PVR. Pulmonary vasculature resistance is determined by the resistance of the alveolar and extraalveolar vessels. When the lung is hyperinflated beyond functional residual capacity (FRC), the small alveolar vessels become compressed, and PVR is increased. Conversely, hypoventilation (with resultant atelectasis) distorts and collapses the larger extraalveolar vessels and increases PVR. Further, atelectasis may also decrease alveolar pO ₂ and cause hypoxic pulmonary vasoconstriction. Therefore maintaining lung volume at FRC without overdistention or atelectasis results in optimal PVR. Because minor changes in lung volume and ventilation can have significant effects on hemodynamic function, transitioning from negative pressure to positive pressure ventilation during resuscitation can significantly destabilize a patient, particularly in the presence of hypovolemia or vasodilation from sedative agents or sepsis. Preload should be optimized in such patients to compensate for increased intrathoracic pressure with initiation of positive pressure ventilation. These effects can be exacerbated in patients with significantly decreased cardiac function. In such cases it is prudent to be prepared for cardiovascular collapse, including the ability to rapidly mobilize extracorporeal membrane oxygenation (ECMO) support if readily available.

Transport Hazards

By virtue of its mobile and underresourced nature, transport is a hostile environment with a multitude of potential hazards that can affect both the patient and the crew: noise, vibration, and poor lighting; exposure to gravitational forces; extreme heat/cold with alterations in thermoregulation; changes in barometric pressure (leading to hypoxia and gas expansion with altitude); limited resources (e.g., personnel, equipment, supplies, therapies); difficulties monitoring patient status; and communication and interpersonal challenges. The safe completion of every transport is contingent upon thorough assessment of these possible risks.

Overview of Critical Care Transport

Critical care transport (CCT) is a unique discipline whose goal is to take critical care services to the patient and rapidly begin advanced care. CCT borrows the use of ambulances, stretchers, and monitors from the prehospital environment and adds critical care–trained personnel and specialized equipment (e.g., isolette, ventilator, intraaortic balloon pump, ECMO pump).

Each CCT has its own variables related to the patient (e.g., age, weight, comorbidities); disease process (e.g., severity of illness, complications, treatments); referring and receiving institutions (e.g., resources, interfacility distance); time of day, geographic and environment conditions (e.g., inaccessible area, traffic, weather); and the transport team’s resources (e.g., personnel, vehicles, equipment, medications, supplies) and communication skills (e.g., interactions with referring staff, patient, family).

Transport Modes

Mode of transport (ground versus air) is determined based on resource availability, transport specifics, and the characteristics of each transport vehicle: ground, rotor-wing, and fixed-wing ambulance ( Table 42.2 ).

TABLE 42.2

Characteristics of Transport Vehicles Used for Pediatric Interfacility Transport

Transport Vehicle	Advantages	Disadvantages	Comments
Ground ambulance	• Larger work space for crew • No equipment restrictions • No altitude-related hazards/physiologic risks • Able to board large medical teams • Able to transport very large patients • Able to carry heavy equipment • Able to stop/pull over • Able to divert to another facility • Readily available • Low cost	• Subject to traffic and road conditions that may: • Delay arrival time • Increase out-of-hospital time • Increase risk for tubes/lines dislodgment • Limited gas availability over longer distances	• Should be considered for: • Stable patients • Short interfacility distance (e.g., urban transports) • Patients likely to require assessments/interventions difficult to perform in flight (e.g., intubation, CPR) • Transports with anticipated need for large medical teams (>3-4 crew members), and/or heavy equipment • Situations when air transport is unavailable (e.g., weather restrictions, lack of resources, cost-prohibitive) • Situations when air transport may be hazardous (e.g., PPHN, tension pneumothorax)
Rotor-wing ambulance	• Reduces transport time to the referring facility • Reduces total out-of-hospital time • Reduces time to diagnostic/therapeutic procedure • Able to land at: • Referring facility’s helipad • Remote landing sites (e.g., road, field, parking lot) • Able to land immediately if the aircraft malfunctions • Able to quickly divert to another facility	• Very small work space • Staff/patient weight and habitus restrictions • Environmental exposure (e.g., extreme heat/cold) • Unpressurized cabin leads to altitude-related hazards/physiologic risks (e.g., hypobaric hypoxia, expanding gases) • Equipment requires FAA approval • Not all programs have IFR capability (most fly under VFR conditions) • Weather restrictions • Not readily available • High cost	• Should be considered for: • Unstable patients • Patients requiring emergent transport for time-sensitive diagnostic/therapeutic procedures • Long interfacility distance (e.g., rural transports)
Fixed-wing ambulance	• High speed over long distances • IFR capability • Pressurized cabin • Able to land at alternate airport • Compared with rotor-wing: • Less-restrictive weight requirements • Usually smoother flight • Lower cost	• Requires: • Flight plan • Airport-to-airport destinations • Ambulance rides to and from the airport • Insurance preauthorization • Involves multiple transfers: • Bedside to stretcher • Stretcher to ambulance • Ambulance onto tarmac • May need transition to/from aircraft-specific stretcher • Tarmac into airplane • Airplane to tarmac • Tarmac into ambulance • Stretcher off-loaded from ambulance • Stretcher to bedside • Equipment requires FAA approval • Similar weather restrictions as rotor-wing • May not be able to pressurize cabin at sea level, hence risking altitude-related hazards • Diversion takes longer: • May not be able to land at closest airport (depending on the type of aircraft and required runway)	• Should be considered for: • Very long interfacility distance (including international transports) • Stable patients • Patients susceptible to altitude-related risks who are unlikely to tolerate flight in unpressurized cabin (i.e., rotor-wing)

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