Shock in Pediatrics Workup

Although the overall clinical appearance is critically important in determining the presence or absence of shock, certain objective signs may help to solidify or better define the diagnosis. These include the following:

• Acid-base status
• Complete blood count (CBC) with differential
• Complete metabolic panel
• B-type natriuretic peptide (BNP)
• Arterial oxygen tension and mixed venous oxygen saturation
• Cardiac index (CI)
• Central venous pressure (CVP) and/or pulmonary capillary wedge pressure (PCWP)
• Arterial and venous blood gases

Stabilization of the airway and breathing and an aggressive response to improving the circulation of any patient who presents clinically in shock takes precedence over any workup that might delay resuscitation. Nevertheless, additional studies may help to identify an etiology and may help to guide ultimate therapy of the patient in shock.

A patient in shock produces lactic acid that results in metabolic acidosis, which can be detected by a decrease in serum bicarbonate or measured directly by obtaining a serum lactate value. Diarrhea also leads to direct bicarbonate loss, which may exacerbate metabolic acidosis in a patient with shock due to dehydration from diarrhea. Measurement of serum lactate levels may help to distinguish bicarbonate loss from lactic acidosis due to shock.

Basic laboratory tests for a child with metabolic acidosis should include measurements of electrolytes, blood urea nitrogen (BUN), creatinine, and serum glucose levels, as well as an arterial and/or venous blood gas determination and a urinalysis.

In assessing the CBC, the hemoglobin concentration is particularly important because it determines the blood's oxygen-carrying capacity. In patients with anemia who present in severe shock, consider transfusing as soon as possible.

A significantly elevated or depressed white blood cell (WBC) count, along with a WBC differential suggestive of infection, could support the diagnosis of septic shock. Similarly, thrombocytopenia may herald a bleeding disorder that could result in internal hemorrhage or disseminated intravascular coagulation (DIC), which may accompany septic shock.

A complete metabolic panel may provide a wealth of information about the patient in shock. Hypernatremia suggests intravascular volume contraction consistent with hypovolemic shock. A decreased serum carbon dioxide suggests a metabolic acidosis that may reflect a significant lactic acidosis from anaerobic metabolism associated with shock. Hypovolemia may result in elevated BUN and creatinine levels.

Other abnormalities may reflect hypoxic-ischemic damage to other organ systems in the body. For example, liver injury may result in elevated liver function enzymes such as aspartate transaminase (AST) and alanine transaminase (ALT).

B-type natriuretic peptide (BNP) is a hormone produced by ventricular myocytes that is released in response to myocardial wall stress. Plasma BNP levels have been shown in adult and pediatric studies to be elevated in sepsis and in congestive heart failure with cardiogenic shock.

Elevated levels of BNP reflect myocardial stress, and improvement in cardiac function is associated with normalization of BNP levels.^{[7, 8]}

Classically, the cardiac index (CI) is determined by pulmonary artery catheter measurement of the cardiac output (CO). CO can also be determined by various invasive or noninvasive methods, including Doppler echocardiography, the pulse contour cardiac output (PiCCO) catheter, and the femoral artery thermodilution catheter (FATD). The CI is calculated by dividing the CO by the body surface area (BSA).

Normal CI is 3.5-5.5L/min/m², and values between 3.3 and 6L/min/m² have been associated with optimal outcomes from pediatric septic shock.^{[9, 10]} Monitoring changes in CI during intravascular volume or cardiotropic infusions may help to guide and optimize administration of these therapies.

A blood sample taken from the right atrium through a central venous catheter or blood sampled from the port through a pulmonary oxygen catheter (Swan-Ganz catheter) placed in the right atrium consists of mixed venous blood returning to the heart. Mixed venous blood gas can be determined with the venous hemoglobin (Hb) oxygen saturation or directly measured by co-oximetry.

By comparing the mixed venous oxygen saturation (SvO₂) with the arterial oxygen saturation (SaO₂), the arteriovenous oxygen saturation difference can be determined. In a patient with a relatively normal SaO₂ (90-100%), the normal SvO₂ is 70-80%, as the tissues typically extract 28-33% of oxygen delivered to them. If the oxygen extraction difference is greater than 33%, perfusion to the tissue capillary beds may be inadequate, reflecting a state of shock.

Alternatively, if the oxygen extraction difference is less than 25%, oxygenated blood may be shunting past tissue capillary beds as a result of inappropriate distribution of blood flow (ie, distributive shock with arteriovenous shunts resulting from vasodilation).^[9] Sepsis can also inhibit the cellular metabolic machinery, decreasing oxygen extraction and leading to an increase in venous saturation.

A central venous catheter wedged in a pulmonary vein may transduce the pressure generated by the blood in that vessel. If central venous pressure (CVP) or pulmonary capillary wedge pressure (PCWP) is low, this may reflect inadequate intravascular volume.

Care must be taken in relying entirely on such measurements. The cardiac filling pressure measured by these catheters reflects ventricular function and compliance, not necessarily intravascular volume alone. Volume expansion by as much as 30% has been shown not to change measurements of right atrial pressure and CVP.^[11] Alternatively, changes in ventricular afterload or compliance lead to changes in PCWP or CVP without altering preload.

Nevertheless, such values, taken in context with the clinical examination findings, may help to determine clinical status. A normal CVP in a normal, compliant heart is typically 1-3cm H₂ O. Pressures much higher than 10cm H₂ O may reflect volume overload or poor right-sided heart compliance or function.

The same may be said for the relationship between PCWP and left atrial compliance. Volume administration is generally thought to be maximal at PCWP measurements of 12-18cm H₂ O in patients with adequate left-sided heart function.

An arterial blood gas (ABG) test helps to determine the arterial oxygen tension (PaO₂) of the blood, assisting in titration of supplemental oxygen delivery to the patient in shock. In addition, ABG findings help to determine the patient's acid-base status, which reflects the degree of systemic shock and the patient's response to therapy.

A central venous blood gas (VBG) test allows for evaluation of the central venous oxygen saturation (ScVO₂) of the venous blood returning to the heart. An ScVO₂ of more than 70% suggests that oxygen delivery to the tissues of the body has been optimized and that more than 70% of the hemoglobin returning from the tissues is still bound to oxygen.

American College of Critical Care Medicine guidelines recommend resuscitating children with septic shock to an ScVO₂ of more than 70%.^[12] A prospective, randomized trial from Brazil that used an ScVO₂ of more than 70% as a resuscitation target in conjunction with traditional signs of perfusion reported a significant reduction in patient mortality.^[13] This suggests that ScVO₂ monitoring and targeted, goal-directed resuscitation rescued a certain number of children from cryptic shock not evident by simply monitoring vital signs, CVP, and urine output alone.

Never delay resuscitation of the patient in shock in order to perform chest radiography or other radiography. However, evaluation of the cardiac silhouette on a chest radiograph may help to delineate cardiogenic shock, which may feature cardiomegaly (see the image below), from hypovolemic shock, in which the heart size appears small.

The chest radiograph may also reveal signs of pneumonia or other pulmonary disorders. Respiratory distress in a patient in shock may result from acute respiratory distress syndrome (ARDS), which may develop in any patient in shock or from pneumonia and sepsis.

A new technology currently under investigation in select pediatric intensive care units is near-infrared spectroscopy (NIRS).^{[14, 15, 16]} A probe placed over a patient's skin, such as the forehead over the brain, the flank over the kidneys, or the abdomen, sends an infrared signal through the skin and reports pooled-tissue oxygen saturation. Because most of the blood in any given region is predominantly venous, the oxygen saturation is close to that of the tissue venous oxygen saturation in that region.

Arterial blood makes a certain contribution to the value reported by the NIRS unit; thus, the value reported is slightly higher than that of venous oxygen saturation. However, the reported values have been shown to correlate with venous oxygen saturations, allowing for a noninvasive measurement and an observation of the trend of increased or decreased tissue oxygen saturation to be followed in critical tissue beds such as the brain, kidneys, or mesenteric region.

Such information may help to identify adequate or inadequate oxygen delivery (DO₂). It is analogous to determining venous oxygen saturation and may help to guide evaluation and response to therapy.