Glenn physiology


•The essence of Glenn physiology is that PBF is non-pulsatile and is driven by venous pressure from the SVC (or from both SVCs when bilateral). The cavopulmonary anastamosis raises cerebral venous pressure by an amount equal to the transpulmonary gradient, which may vary between 2-5 mmHg in favorable circumstances and reach 10-15 mmHg or more in patients with elevated PVR.

•The arterial oxyhemoglobin saturation is determined by the amount of pulmonary blood flow as well as the pulmonary vein saturation (assuming complete atrial mixing). Because PVR represents a second, serial downstream resistor in the cerebral circulation, elevations in PVR may cause decreased cerebral blood flow – and therefore pulmonary blood flow – decreasing the volume of oxygenated blood returning to the atrium and causing arterial hypoxemia. Alternatively, elevations in cerebral vascular resistance (e.g. respiratory alkalosis) may diminish PBF by increasing proximal resistance and exacerbate hypoxemia. Further, low Qp may be due to low cardiac output or to left atrial hypertension (e.g. restrictive atrial septum or LV non-compliance).

•PVR may be elevated in a number of circumstances. (1) Native PVR may be elevated due to lung disease, as in patients born with HLHS/ intact atrial septum or those with chronic aspiration or prolonged mechanical ventilation. (2) Branch pulmonary arteries may be hypoplastic due to asymmetric flow preoperatively. For example, in some patients the LPA experiences diminished flow following S1P. Relatedly, some patients with asymmetric elevated resistance may experience thrombus formation, particularly following PAplasty. (3) Similarly, some patients with diminuitive arterial flow may experience pulmonary vein stenosis, raising total pulmonary resistance. (4) Postoperative bleeding or effusion may cause compressive atelectasis, which may increase PVR and/or cause pulmonary venous desaturation. Treatment considerations

•When possible, early extubation allows for negative intrathoracic pressure, which increases the driving gradient for cerebral venous drainage into the thorax and promotes PBF.

• If CXR demonstrates significant pulmonary edema, this may be due to an elevated resistance to pulmonary lymphatic drainage (thoracic duct drains into the innominate vein). Early diuresis and low fat feedings may be effective.

• If significant hypoxemia is present (e.g. SaO2<75%):

•Consider and treat causes of elevated PVR.

•Ensure ETT patency and position.

•The use of increased FiO2 may diminish pulmonary vein desaturation and decrease PVR. •Examine for and treat pleural effusions, hemothorax, pneumothorax. •The use of inhaled nitric oxide is rarely necessary except in patients at risk for (or documented) elevated PVR (e.g. PVRi >2-3 iWU). •Persistent hypoxemia should prompt evaluation for venovenous (SVC to atrium or IVC) collaterals.

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