Emergency Medical Services. Группа авторов

      Any condition that lowers cardiac output or peripheral vascular resistance may decrease blood pressure. Alterations of heart rate (very low or very high) can lower cardiac output, and hence can reduce blood pressure secondary to decreased cardiac filling. Decreasing stroke volume may lower cardiac output with a possible reduction in perfusion as well. Cardiac output may be reduced by lower circulating blood volume (e.g., hemorrhage or dehydration), by damage to the heart (e.g., myocardial infarction or myocarditis), or by conditions obstructing blood flow through the thorax (e.g., tension pneumothorax, cardiac tamponade, or extensive pulmonary embolism).

To aid in the evaluation and treatment of shock, it is often useful for the physician and emergency medical services (EMS) personnel to categorize the etiology of the shock condition [4]. Most EMS clinicians are familiar with the “pump‐pipes‐fluid” model of the cardiovascular system, with the pump representing the heart, pipes representing the vascular system, and fluid representing the blood [5]. Categorizing shock into four categories may help to organize assessments and management approaches. Accurate physical assessment is vital for the EMS clinician to determine the categories of the shock state (Table 7.1).

Evaluation

The diagnosis of shock depends on a combination of key historical features and physical findings in the proper clinical setting. For example, tachycardia and hypotension in an elderly patient with fever, cough, and dyspnea may represent pneumonia with septic shock. Hemorrhagic shock should be suspected in a middle‐aged man with epigastric pain, hematemesis, melena, and hypotension. Hypotension, tachycardia, and an urticarial rash in a victim of a recent bee sting strongly suggest distributive shock secondary to anaphylaxis. Obstructive shock precipitated by a tension pneumothorax should be suspected in a patient with hypotensive trauma who has unilateral decreased breath sounds and tracheal deviation to the opposite side.

Table 7.1 Categories of shock

Type of shock	Disorder	Examples	Comments
Hypovolemic	Decreased intravascular fluid volume	A. External fluid loss Hemorrhage Gastrointestinal losses Renal losses Cutaneous loss B. Internal fluid loss Fractures Intestinal obstruction Hemothorax Hemoperitoneum Third spacing	Hypovolemic shock states, especially hemorrhagic shock, produce flat neck veins, tachycardia, and pallor
Distributive	Increased “pipe” size: peripheral vasodilation	A. Drug or toxin induced B. Spinal cord injury C. Sepsis D. Anaphylaxis E. Hypoxia/anoxia	Distributive shock states usually show flat neck veins, tachycardia, and pallor. Neurogenic shock due to a cervical spinal cord injury tends to show flat neck veins, normal or low pulse rate, and pink skin
Obstruction	Pipe obstruction	A. Pulmonary embolism B. Tension pneumothorax C. Cardiac tamponade D. Severe aortic stenosis E. Venocaval obstruction	Obstructive shock states tend to produce jugular venous distension, tachycardia, and cyanosis
Cardiogenic	“Pump” problems	A. Myocardial infarction B. Arrhythmias C. Cardiomyopathy D. Acute valvular incompetence E. Myocardial contusion F. Myocardial infarction G. Cardiotoxic drugs/poisons	Cardiogenic shock states tend to produce jugular venous distension, tachycardia, and cyanosis

      An important problem in the prehospital diagnosis of shock is the frequent inaccuracy of field assessment. For example, in one analysis, emergency medical technicians (EMTs) made vital sign errors more than 20% of the time [6]. Subsequently, when critical medical decisions are based on the data gathered in the field, multiple assessments should be performed.

EMS clinicians should look for the signs and symptoms of system‐wide reduction in tissue perfusion, such as tachycardia, tachypnea, mental status changes, and cool, clammy skin (Box 7.1). When available, adjunctive technologies can provide improved recognition and assessment of shock by revealing reductions in expired CO₂, hypovolemia, obstruction or poor cardiac contractility, and elevated serum lactate levels.

Vital signs that fall outside of expected ranges must be correlated with the overall clinical presentation. Vital signs have a broad range of normal values. They must be interpreted in the context of the individual patient. A petite 45‐kg, 16‐year‐old girl with lower abdominal pain with a reported blood pressure of 88 mmHg systolic by palpation may have a ruptured ectopic pregnancy, or may just be at her baseline blood pressure. An elderly patient with significant epistaxis may be hypertensive due to catecholamine release and vasoconstriction despite being relatively volume depleted. Consideration should be given to patient age, comorbid conditions, and medications that may affect the interpretation of vital signs.

Box 7.1 Signs and symptoms of shock

      Cardiovascular

       Tachycardia, arrhythmias, hypotension

       Jugular venous distension in obstructive and cardiogenic shock states

       Tracheal deviation away from the affected side in tension pneumothorax

      Central nervous system

       Agitation, confusion

       Alterations in level of consciousness

       Coma

      Respiratory

       Tachypnea, dyspnea

      Skin

       Pallor, diaphoresis

       Cyanosis (in obstructive and cardiogenic shock cases), mottling

      In the noisy field environment, EMS clinicians often measure blood pressure by palpation rather than auscultation. Blood pressure by palpation provides only an estimate of sBP [7]. Without an auscultated diastolic pressure, the pulse pressure (the difference between systolic and diastolic pressure) cannot be calculated. A pulse pressure less than 30 mmHg or 25% of the sBP may provide an early clue to the presence of hypovolemic or obstructive shock [3]. Conversely, a wide pulse pressure may be indicative of distributive shock. Dividing the pulse rate by the sBP typically produces a ratio of approximately 0.5 to 0.8, which is called the “shock index.” When that ratio exceeds 0.9, then a shock state may be present [8].

      Previously healthy patients with acute hypovolemic shock may maintain relatively normal vital signs with up to 25% blood volume loss [3]. Sympathetic nervous system stimulation with vasoconstriction and increased cardiac contractility may result in normal blood pressure in the face of decreasing intravascular volume, especially in the pediatric population. In some patients with intra‐abdominal bleeding (e.g., ruptured abdominal aneurysm, ectopic pregnancy), the pulse may be relatively bradycardic despite significant blood loss [9].

      EMS personnel may equate “normal” vital signs with normal cardiovascular status [5]. The field team may be lulled into a false sense of security initially if the early signs of shock are overlooked, and then they are caught off guard when the patient’s condition dramatically worsens during transport. Following