Shock and Hypotension in the Newborn Workup

Attempt to determine the type of shock (eg, hypovolemic, cardiogenic, maldistributive) to guide the therapeutic approach. In neonates who are hypotensively compromised, the authors encourage the early use of a bladder catheter. Hourly urine output is one of the few objective methods of evaluating hypoperfusion that leads to specific organ failure, and its accurate objective measurement can augment clinical decision making.

Obtain the hematocrit, electrolyte levels, blood cultures, blood gases (for acid/base status), and glucose level as soon as vascular access is attained. Among laboratory investigations, data supporting the diagnosis of shock include metabolic acidosis on an arterial blood specimen in the face of reasonable oxygenation.

An elevated plasma lactate level with a normal pyruvate result (infrequently measured) suggests anaerobic metabolism triggered by tissue hypoxia-ischemia.

Other pertinent tests include the following:

• Automated Doppler: Provides blood pressure readings through a noninvasive method

• Manual oscillometric techniques: Used for noninvasive blood pressure testing

• Infant blood pressure testing: Invasive methods include direct manometry using an arterial catheter and the use of an in-line pressure transducer and continuous monitor

Specific studies must be performed to determine the cause (eg, sepsis, cardiac lesions, anemia) and sequelae (eg, renal, hepatic, endocrine) of shock.

Echocardiography can provide useful insight into the pathophysiology involved in the hemodynamic instability as well as aid in therapeutic strategies. Newer modalities such as functional magnetic resonance imaging (MRI) are being evaluated to assess cardiac function. This technology is promising, but it is limited to research findings and is subject to the availability of an MRI scanner. It also has the limitation of an inability to perform repeated longitudinal measurements at the point of care, whereas this can be done with bedside echocardiography.

Noninvasive hemodynamic monitoring

With advances in technology, newer noninvasive hemodynamic monitoring devices such as Near-InfraRed Spectroscopy (NIRS) and electrical cardiometry are being used in many centers across the world. Both devices have the limitations of availability, cost and accuracy, and being noninvasive.

NIRS works on a principle similar to that of pulse oximetry, but it uses the NIR light of frequency ranging between 730 nm and 810 nm. It can be applied on the forehead for cerebral saturations on either side and also on the flanks for renal saturation or over the abdomen for splanchnic saturations. In the presence of shock, NIRS can help in early diagnosis and management. Cerebral NIRS measures cerebral regional oxgen saturations (CrSO2) in the watershed areas (anterior cerebral artery [ACA] and middle cerebral artery [MCA]) with a normal CrSO2 of 60-80%. In shock, these levels drop far below 50% and show improvement with fluid management, ionotropic support, and blood transfusion depending on the underlying pathology. Mesenteric NIRS measures splanchnic regional oxygen saturations (SrSO2) , where there is variable flow and lower extraction, thus splanchnic saturations are generally in the range of 5-20% more than CrSO2. Similarly, renal  circulation (RrSO2) is also of variable flow and higher extraction, ranging 5-20% more than CrSO2. In the presence of shock, regional saturations drop significantly and, especially in hypovolemic shock, these organs may be significantly affected due to the brain-sparing redistribution of blood.

Electrical cardiometry (EC) or noninvasive cardiac output monitoring devices work on the principle of measuring changing electrical impedance over that area of the thorax in relationship with blood flow during various phases of the cardiac cycle. It is a novel technique that noninvasively provides continuous bedside information on various parameters, such as cardiac output, systemic vascular resistance, cardiac contractility, and fluid status in the body. EC can help to differentiate various types of shock as well as monitor response to therapy.

Research is ongoing for other markers of septic shock, such as mannose-binding lectin, ^[10] vasopressin, ^[11] and the use of electrical cardiometry. ^[12]

Mixed venous blood gases may be more helpful than arterial measurements, because mixed venous blood gases reflect oxygen extraction and waste products at the tissue level. Conversely, arterial blood reflects lung function and the gas composition of blood before it is delivered to the tissues.

Comparison of simultaneous arterial and mixed venous blood gas determinations may be more useful in assessing cardiac output, tissue oxygenation, and acid-base balance.

The value of capillary blood gas determinations is limited, because they may only reflect diminished perfusion to the periphery and not reflect central perfusion.

Echocardiography and Doppler flow velocimetry may provide semi-quantitative and semi-qualitative noninvasive analysis of preload, myocardial function, and afterload. This may help in understanding the underlying pathophysiology for hemodynamic instability. In conjunction with other clinical parameters and monitoring tools, echocardiographic assessment can be used in selecting fluid resuscitation therapy or appropriate inotropic or vasopressor/vasodilator therapy.

Assessment of left ventricular output (LVO)

In the setting of a low LVO and an underfilled left ventricle (LV), volume expansion is the first-line management. With a normal LVO but impaired LV contractility, dobutamine or epinephrine may be the initial choice. A low LVO with paradoxical movement of the interventricular septum would benefit from dobutamine administration. If blood pressure is low despite normal or high LVO and patent ductus arteriosus (PDA) is not evident, a vasopressor (eg, dopamine or epinephrine) can initially be instituted. In case of a hemodynamically significant PDA, additional treatment for pharmacologic PDA closure maybe considered.

Assessment of superior vena cava (SVC) flow

SVC flow in newborn infants is reported to be a novel marker of systemic blood flow, as it is not affected by the presence of any persistent fetal shunts like PDA and patent foramen ovale (PFO). ^[13] Low SVC flow (< 41 mL/kg/min) has been used to diagnose hypotension and to predict poor long-term outcomes. ^[14] SVC Doppler is also used as a surrogate for LVO in the presence of a PDA. ^{[15, 16]}

Assessment of the preload inferior vena cava (IVC) size and collapsibility index are useful parameters for assessing right heart filling pressure in spontaneously breathing infants, and they have the potential to inform fluid responsiveness in neonates with septic shock. ^{[15, 16]}

Cardiac function can be further assessed using various functional echocardiography parameters that are used for measuring systolic and diastolic function. ^{[16, 17]}  A detailed description of these is beyond the scope of this article.

Patent ductus arteriosus

PDA is a significant cause of hypotension in preterm infants. Although the increase in LVO and other compensatory mechanisms may initially offset the effects of ductal shunt on systemic circulation, effective LVO is reduced over time. This can lead to organ hypoperfusion, and treatment of shock in such situations should be directed toward closing the PDA.