harmful to other organs, leading to secondary organ dysfunction or failure, most typically the lungs, kidneys, and gastrointestinal tract Hypotension is a late sign of acute circulatory failure, especially in neonates and infants, due to their higher systemic vascular resistance and vasoactive capacity compared with older children Skin and muscles are affected early during acute circulatory failure as a result of blood being shunted away in order to perfuse other organs This leads to ischemia of these vascular beds Providers should recognize a prolonged capillary refill as a surrogate marker of decreased superior vena cava oxygen saturation, and if associated with hypotension, infers an increased mortality risk.6 In addition, the presence of neurologic dysfunction may suggest reduction in cerebral oxygen delivery beyond the point of cerebral autoregulation and is an ominous clinical sign in patients with acute circulatory failure (Table 64.1) Table 64.1 Signs and Laboratory Findings of Acute Circulatory Failure Organ System Respiratory Circulatory Instability Signs Tachypnea, increased WOB, grunting Acute Circulatory Failure Signs Laboratory Derangements Respiratory failure with hypoxia PaO2/FiO2 65 or 20 mm Hg over baseline PaCO2 Cardiovascular Tachycardia, Tachycardia, bradycardia, capillary SvO2 25 weak distal pulses weak central pulses BNP >400 pg/mL Renal Oliguria Anuria, tubular necrosis Creatinine elevation >95th percentile for age or doubling of creatinine from admission Neurologic Agitation, anxiety, Lethargy, somnolence, GCS 80 µg/dL, GCS >6 hypoglycemia Gastrointestinal Ileus, feeding GI bleeding, distended abdomen with intolerance signs of peritonitis Hepatic Right upper Jaundice AST >200 IU/L quadrant tenderness, ALT >200 IU/L hepatomegaly INR >1.5 in the absence of systemic anticoagulation Hematologic Endothelial and DIC Platelets 2 mmol/L ALT, Alanine aminotransferase; aPTT, activated partial thromboplastin; AST, aspartate aminotransferase; BNP, brain-type natriuretic peptide; DIC, disseminated intravascular coagulation; GCS, Glasgow Coma Scale; INR, international normalized ratio; NH4, ammonium; O2ER, oxygen extraction ratio; PaCO2, partial pressure of carbon dioxide; PaO2/FiO2, partial pressure of oxygen/inspired fraction of oxygen ratio; PT, prothrombin time; SvO2, mixed venous oxygen saturation; WOB, work of breathing Diagnosis of Acute Circulatory Failure In extreme presentations, the clinical diagnosis of acute circulatory failure is usually straightforward However, the majority of cases of acute circulatory failure present in a more subtle or insidious manner, sometimes presenting with respiratory (tachypnea, wheeze) or circulatory signs (tachycardia) that may lead frontline providers to initiate therapies that are inappropriate or even harmful Frontline providers in the emergency department or the wards should be vigilant for clues during history taking or examination Furthermore, the mode of presentation may vary by age; for instance, infants at risk of acute circulatory failure will develop poor feeding or irritability with feeds, whereas an older child may complain of excessive fatigue or sleep difficulties In the ICU, the diagnosis of acute circulatory failure is established by noninvasive and invasive methods The noninvasive methods include assessment of vital signs, physical examination, pulse oximetry, near-infrared spectroscopy (NIRS) monitoring, and echocardiography The invasive methods include central venous pressure monitoring, co-oximetry, and assessment of cardiac output via transpulmonary thermodilution and pulse contour analysis Laboratory Studies The presence of anion gap metabolic acidosis is indicative of acute circulatory failure Inadequate oxygen delivery will lead to lactate and lactic acid formation due to anaerobic metabolism via the Cori cycle.7 It is generally accepted that in the normal circulation, lactate levels less than 2 mmol/L correlate with superior vena cava O2 saturation of 70% or greater.8 In addition, lactate levels greater than 6 mmol/L are associated with increased rate of adverse outcomes including mortality in neonates following cardiac surgery.9 Due to these associations, lactate levels are widely used in the ICU Monitoring Pulmonary Artery Catheterization The pulmonary artery catheter (PA catheter), also referred to as the Swan-Ganz catheter, is a balloon-tipped catheter that is used to assess mixed venous oxygen