Common causes of hypovolemic shock are hemorrhage from trauma and gastrointestnal bleeding (i.e., ulcers). A less common cause is dehydration from vomiting, diarrhea or low fluid intake. A 15% blood/fluid loss causes tachycardia. A 15-30% loss causes tachycardia, tachypnea, decreased pulse pressure and prolonged capillary refill. Only when the loss is about 30-40% does the systolic pressure begin to drop. The skin is cool and clammy, accompanied by restlessness and anxiety.
1. External hemorrhage is controlled by pressure;
2. Two large bore IV’s are placed and 2 liters (peds: 20ml/kg x 3) of normal saline or lactated Ringer’s solution is infused wide open to maintain a urine output of 30 cc/h (peds: 1 cc/kg/h);
3. A loss of 30% of blood volume requires the administration of blood (type-specific packed cells or O-negative in an emergency);
4. Possible traumatic abdominal hemorrhage requires a diagnostic peritoneal lavage and
5. A surgical consultation is required.
A normal heart-rate or bradycardia rather than tachycardia is sometimes seen in hemorrhagic shock (5-50%) of cases). It is called paradoxical or relative bradycardia, although the heart rate is normal (60-100) in most cases (the median rate is 80, although a few cases are <60). A more precise term would be hemorrhagic non-tachycardia. Originally thought to be a vagal response to blood in the peritoneal cavity (i.e., abdominal trauma, splenic rupture, bleeding ovarian cyst, ruptured ectopic pregnancy), it was later discovered in thoracic and extremity trauma as well.
The reason for the response is the following: at a loss of about 15% body fluid/blood, sympathetic activity is increased and vasoconstriction and tachycardia occur. As blood loss approaches 30% and the systolic pressure decreases the left ventricle is now contracting around a reduced volume. This triggers stimualtion of unmyelinated afferent vagal fibers in the left ventricle and bradycardia occurs. This reflex (vago-vagal reflex) prevents further sympathetic stimulation and reduction of left ventricular volume, preserving organ perfusion. Evidence: the efferent response is abolished by atropine. As blood volume and pressure decline further, the baroreceptor response overrides the reflex and tachycardia resumes, continuing until terminal bradycardia and cardiac arrest occur.
The reflex does not seem to be rate-dependent ,and it is not consistently seen. Current thinking is that the reflex is often overridden by the sympathetic/ baroreceptor response. It is not present in hypovolemia from dehydration. In summary, an increase in heart-rate is a useful parameter for the assessment of bleeding, but its absence does not rule out severe hemorrhage. Profound shock may occur with a normal heart rate or bradycardia. It is thus an unreliable assessment tool. Hypotension and an alteration in behavior are more reliable signs.
Treatment: fluid/blood resuscitation at all stages, as noted previously.
Orthostatic Vital Signs
Confusion exists with this topic, sometimes for conceptual reasons but also because of ambiguous terminology. Orthostatic vital signs are heart-rate and blood-pressure. The word “orthostatic” means assuming an erect position. When a person stands upright the heart-rate increases slightly (about 10 beats per minute), the systolic pressure decreases slightly and the diastolic increases slightly (compensatory baroreceptor activity). As the body loses about a liter of fluid, gravity begins to have an effect. The first sign of hypovolemia is an increase in heart-rate, followed by a slight decrease in pulse pressure.
is the correct but rarely used term for “positively orthostatic”. In a patient with blood loss or dehydration, an increase in pulse-rate of 30 beats per minute on standing represents a blood/fluid loss of about a liter. When about 35% of the blood volume is lost (about 2 liters in the average adult), the systolic pressure begins to drop (orthostatic tachycardia and hypotension).
Being “positively orthostatic” may also mean having orthostatic (postural) hypotension, or a drop in blood-pressure (>20/10 mmHg) on standing without an increase in heart-rate. This is usually not a sign of hypovolemia but occurs in patients on beta-blockers, alpha-blockers, calcium-channel blockers, nitrates, phenothiazines, with alcohol ingestion (impairment of vasoconstriction) and in the rare person with autonomic dysfunction. It may be seen with hypovolemia in the occasional elderly patient with a weak baroreceptor response, and as mentioned in the previous section, with blood loss
resulting in paradoxical bradycardia. Paradoxical bradycardia is not seen in hypovolemia from dehydration.
Orthostatic vitals are recorded as follows:
the patient lies for three minutes and the blood-pressure and heart-rate are recorded. He then stands (or sits up) for one minute and they are retaken. The patient has orthostatic
tachycardia (positively orthostatic) if the heart-rate increases by 30 beats per minute, or if he becomes dizzy or light-headed with a lesser increase. Orthostatic hypotension is present if only the blood pressure decreases >20/10 mmHg. Both conditions are considered “positively orthostatic”.
An older term, the tilt test, is occasionally encountered in the literature. The test was originally designed with the patient lying, then the patient was tilted to a sitting position and the heart-rate and blood-pressure retested. The test was positive if the pulse rate increased 15 to 20 beats, or the systolic blood pressure decreased 15 to 20 mmHg. The test is no longer considered valid (orthostatics from lying to sitting will not detect a 1000 cc blood/fluid loss). However, if the heart-rate increases 30 or more beats per minute or the person becomes dizzy or light-headed from lying to sitting, this is a positive test. Results were written as “tilt positive” or “tilt negative”. When practitioners today speak of a tilt test, they mean (hopefully) current orthostatics.
The elderly and and occasionally others do not always follow the rules. Cardiac syncope and poor autonomic functioning may result in false positives or negatives. An entity in some elderly, postural orthostatic tachycardia syndrome, is a fall in blood pressure, tachycardia, near-syncope and symptoms of a transient ischemic attack (TIA), thought to be caused by autonomic dysfunctioning and not hypovolemia.
The accuracy of orthostatic vital signs is frequently challenged. A recent study showed that some normal subjects had a heart-rate increase from 5 to 39 beats per minute (with a mean of 17) from supine to standing. Both systolic and diastolic pressures rose. The cause was related to baroreceptor stimulation with both a-adrenergic and b-adrenergic effects. The conclusion was that a wider than previously thought variability exists in the physiologic response to standing.
To conclude, orthostatics are of value if the procedure is accurately performed and accompanied by a careful history. Excluding other factors, lightheadedness or dizziness on standing or sitting upright is a positive test regardless of the heart-rate. Patients are located within a bell-curve. At the edges are rare false positives and negatives.
Treatment: Fluid/blood resuscitation as previously described.
The capillary refill is the time it takes for blanching of the skin to return to normal when the nailbed or hypothenar emminence is quickly squeezed. It is a simple measurement of blood/fluid loss. The upper limit of normal in males is 2 seconds; in females 2.9 seconds. A delay greater than this implies a fluid deficit of about 100 cc/kg. The test has been challenged. Recent studies indicate that its value as an isolated test for mild-to-moderate hypovolemia is minimal and even misleading. However, if orthostatics are abnormal, the validity increases. The test is more sensitive in children.
Treatment: Fluid/blood resuscitation as previously described.
Abdominal Aortic Aneurysm (AAA)
A rare but potentially catastrophic cause of hypovolemic shock is a leaking abdominal aortic aneurysm. Because of a weakened tunica media from atherosclerosis, the abdominal aorta may slowly balloon out. A common presentation is an older male with sudden onset of severe lower abdominal and back pain, sometimes radiating to the groin and accompanied by hypotension. This represents a leaking aneurysm. Lower extremity pulses may or may not be weak compared to upper ones. The physical exam reveals a tender abdominal pulsatile mass.
Treatment: a call is made to the surgeon for immediate surgery, while 2 IV’s are started and blood is typed and crossed for 10 units.