**4. Pre and post-operative care of HLHS**

Adequate pre-operative management of HLHS requires knowledge of the fetal circulation in this disorder. In patients with aortic atresia and very severe aortic stenosis—in general, there is a lower pO2 to the fetal brain with probably decreased blood flow to the brain as well. There is retrograde flow into coronaries in these instances and the ascending aorta is quite small. Normally the lungs will receive only 11% of the combined ventricular output [24], but the blood flow that the lungs see in HLHS fetus is usually higher [25]. In HLHS, it is conceivable that the relatively higher oxygen saturation could impact the normal microvascular development in the pulmonary artery and veins [26].

Postnatally, pulmonary venous return (normally fully saturated blood) must be routed to the right atrium. This mixes with desaturated blood in the right atrium and is ejected by the right ventricle. By default, the right ventricle supplies both systemic (through the PDA) and pulmonic circulations. Blood may need to flow retrograde to supply head and neck vessels and coronaries arteries. The delicate balance between systemic vascular resistance and pulmonic vascular resistance plays a major role. As suggested in the above section, ischemia of the brain and coronaries may occur.

In the postnatal period, we need an adequate interatrial communication, widely patent ductus arteriosus and a balanced pulmonary vascular resistance (PVR) to systemic vascular resistance (SVR). There is an inherent tendency for high pulmonary blood flow—in otherwise uncomplicated cases, with concurrent systemic steal. The right ventricle is volume overloaded at baseline. So, the immediate goals of pre-op care rest on preserving ductal patency and balancing PVR and SVR. One needs to be able to diagnose a high pulmonary to systemic flow ratio (Qp:Qs) state. Some of the clinical manifestations include hypotension, decreased urine output, delayed capillary refill, and lactic acidosis. Patients may also present with tachypnea, increased work of breathing, and respiratory distress.

## **4.1 Keeping the duct patent**

The ductus arteriosus can be kept patent via the use of prostaglandin E1 (PGE1). The previously described starting dose range of 0.05–0.10 mcg/kg/min [27] has fallen out of favor to a lower starting dose of 0.01 mcg/kg/min [28]. It is most probable that these previously higher doses were the starting doses for patients who had not been prenatally diagnosed; and who came to medical attention later in the neonatal period. The risk of precipitating apnea is higher at higher starting doses. The recommended maintenance dose of PGE is 0.01–0.04 mcg/kg/min. At our institution, we will use doses as low as 0.003 mcg/kg/min–0.005 mcg/kg/min while patients await surgical repair. There are still institutions that will maintain their patients on PGE1 for prolonged periods of time.

The other means by which the ductus arteriosus can be maintained patent is through stenting of the ductus arteriosus. For patients with HLHS, this is generally done as part of the Hybrid procedure.

## **4.2 Limiting pulmonary blood flow**

Pulmonary blood flow may be limited by manipulating pulmonary vascular resistance. This may be done by the use of sub-ambient oxygen otherwise called hypoxemic mixture [29]. The literature on this is sparse; particularly there are no randomized controlled trials. There has been hesitation to use this widely due to concerns about the cellular effect of hypoxemic mixture on tissue oxygenation. The other concern is that a non-intubated patient receiving a hypoxic mixture may hypoventilate or become apneic due to prostaglandin administration; therefore, the PaO2 levels may fall drastically [30]. The use of a hypoxemic mixture requires vigilance in terms of frequent checking of blood gases—to look at PaO2 and base deficit. Attempts to maintain Qp:Qs of approximately 1:1, with an ideal goal for PaO2 of 35–45 mmHg should be made. A useful thing to be aware of is that a central line placed in the right atrium of a patient with HLHS and aortic atresia is generally representative of an arterial gas in such patients. Hypoxemic mixture can be administered via high flow nasal cannula or via oxy-hood or a combination of both or through the endotracheal tube of an intubated patient.

Adjusting the ventilator settings of an intubated patient to allow for higher PaCO2 will also restrict pulmonary blood flow, as well as blending CO2 into the circuit [31, 32]. The latter is also not commonly practiced.

#### **4.3 Optimization of systemic circulation**

One can increase systemic circulation by use of afterload reduction agents such as milrinone—which can be titrated to effect as we monitor saturations; clinical cardiac output, base deficit, lactate levels, and PaO2 [2]. Optimization of oxygen-carrying capacity by keeping hematocrit >40% is also ideal. Ultimately, if all these maneuvers fail and one still needs to wait for intervention, then muscle relaxation of the patient may need to be considered in order to decrease metabolic demand, fully take over respiratory support and manipulated PaCO2.

Some of the maneuvers to limit pulmonary blood flow and increase systemic circulation are listed in the table below.

