Shock:The Final Common Pathway

The final common pathway of most severe disease states is that of shock. Simply defined, shock is the failure of the circulation to provide adequate tissue perfusion. Although shock may not be present in all patients requiring emergent resuscitation, if untreated or treated inadequately, most will eventually deteriorate into a shock state. Once an illness progresses to a shock state, further deterioration involves a complex interaction between the underlying disease, host factors and the psychophysiology of the shock state itself.

Because of its central role in severe decompensated disease, a working knowledge of the classification and approach to shock is essential. When the diagnosis is known

Classification and causes of shock

Cardiogenic (inadequate pump function)

Cardiac rupture

Congestive heart failure

Dysrhythmia

Intracardiac shunt (e.g., septal defect)

Ischemia/infarction

Myocardial contusion

Myocarditis

Valvular dysfunction

Distributive (misdistrubution of the circulating volume)

Adrenal crisis

Anaphylaxis

Capillary leak syndromes

Neurogenic

Sepsis

Toxicologic

Obstructive (extracardiac obstruction to circulation)

Air embolism

Cardiac tamponade

Massive pulmonary embolus

Tension pneumothorax

Hypovolemic (Inadequate circulating volume)

Adrenal crisis

Hemorrhage

Severe dehydration

treatment is directed at both the underlying cause as well as the shock state itself. For those patients in whom the diagnosis is unknown, general resuscitative measures and treatment of shock proceeds alongside the diagnostic evaluation. Table 1.1 outlines the major classes of shock and gives examples of individual etiologies of each class. Many patients have compound presentations when more than one root cause is present.

The Recognition of Occult Shock

Many of the traditional clinical indicators of shock, such as blood pressure (BP) and heart rate (HR), lack the sensitivity to identify all patients in shock. In fact, more sophisticated indices, such as pooled venous oxygen saturation measured through a central catheter, can demonstrate a mismatch between the delivery of oxygen to the tissues and its consumption in some patients with normal or elevated BPs. Moreover, evidence suggests that using such indices to guide therapy in septic shock (not simply the BP) results in better outcomes. Thus, the early identification of shock before the traditional vital signs are grossly deranged (in its so-called "occult" form) is essential to management and disposition.

In the ED, shock is still most often recognized by the presence of persistent hypotension (e.g., systolic BP of < 90 mm Hg in an adult) Nonetheless, there are many other clinical indicators that when considered together can alert the clinician to the presence of early shock, leading to appropriately vigorous resuscitation. Table 1.2 gives a list of clinical parameters that can assist in making the diagnosis of early or "occult" shock.

Table Clinical parameters in the diagnosis of shock

Parameter	Comment
Heart rate	Tachycardia (HR >100 in non-pregnant adults) is present in most patients with shock; however, its presence may be masked by multiple factors including spinal cord injury, medications, intra-abdominal catastrophe, older age and cardiac conduction abnormalities.
Blood pressure	Hypotension (arbitrarily systolic BP < 90) is a late finding in shock. In early shock, it may actually be transiently elevated. Measurements, in particular with standard BP cuff, become less accurate in shock states. A narrow pulse pressure may be present in hypovolemic shock. A wide pulse pressure may be seen in distributive shock.
Shock index	Heart rate/systolic blood pressure. An index of >0.9 is a more sensitive indicator of shock than either blood pressure or heart rate alone.
Pulsus paradoxus	A wide variation of blood pressure with respiration (>10 mm Hg) may indicate obstructive shock (e.g., cardiac tamponade)
Respirations	Either high (>24/min) or low (< 12/min) rates may suggest a shock state, as may very shallow or deep breathing
Skin signs	Cool and clammy skin is often an indicator of a shock state although certain distributive shock states may have warm and dry skin (neurogenic and early septic shock). Delayed capillary refill (>2 seconds) is another sign of shock.
Urine output	Most often reduced (< 30 ml/h) in shock states.

Diagnosis and Treatment in the Critically Ill Patient

In the classical medical model, the physician performs a history and physical examination before proceeding to diagnostic tests and then treatment. But the ED patient often requires treatment emergently and often in the absence of a diagnosis. This paradigm is taken to its extreme in the setting of resuscitation. There is clearly no time for history-taking or detailed physical examination in a patient who is pulseless and apneic. Treatment of this patient, regardless of the underlying diagnosis, must be immediate and maximal at the onset of the patient encounter (in this case by securing an airway, providing rescue breathing and performing chest compressions).

Because there are final common pathways for most disease processes, (e.g., the loss of spontaneous circulation and profound coma), the approach to any resuscitation overview always begins with general supportive measures that may not be specific to the underlying disease process. As more data is gathered, both by assessing the patient�s response to therapy and obtaining incremental data from the ongoing history, examination and bedside laboratory testing, the resuscitation becomes more specific, focusing therapy to the most likely pathologies. Such upward reversal of disease momentum mirrors its downward spiral�powerful, broad therapies are used to reverse the intense downward momentum of end-stage disease, followed by more specific and focused therapy as the curve of disease momentum becomes less steep.