Mechanisms of Circulatory Shock. The essentials of shock are easier to understand if the circulatory system is thought of as a four-part mechanical device made up of a pump (the heart), a complex system of flexible tubes (the blood vessels), a circulating fluid (the blood), and a fine regulating system or “computer” (the nervous system) designed to control fluid flow and pressure. The diameter of the blood vessels is controlled by impulses from the nervous system which cause the muscular walls to contract. The nervous system also affects the rapidity and strength of the heartbeat, and thereby the blood pressure as well.

Shock, which is associated with a dangerously low blood pressure, can be produced by factors that attack the strength of the heart as a pump, decrease the volume of the blood in the system, or permit the blood vessels to increase in diameter.

Types of Circulatory Shock. There are five main types:

 Hypovolemic (low-volume) shock occurs whenever there is insufficient blood to fill the circulatory system. Neurogenic shock is due to disorders of the nervous system. Anaphylactic (allergic) shock and septic shock are both due to reactions that impair the muscular functioning of the blood vessels. And cardiogenic shock is caused by impaired function of the heart.

Hypovolemic (Low-Volume) Shock. This is a common type that happens when blood or plasma is lost in such quantities that the remaining blood cannot fill the circulatory system despite constriction of the blood vessels. The blood loss may be external, as when a vessel is severed by an injury, or the blood may be “lost” into spaces inside the body where it is no longer accessible to the circulatory system, as in severe gastrointestinal bleeding from ulcers, fractures of large bones with hemorrhage into surrounding tissues, or major burns that attract large quantities of blood fluids to the burn site outside blood vessels and capillaries. The treatment of hypovolemic shock requires replacement of the lost volume.

Neurogenic Shock. This type, often accompanied by fainting, may be brought on by severe pain, fright, unpleasant sights, or other strong stimuli that overwhelm the usual regulatory capacity of the nervous system. The diameter of the blood vessels increases, the heart slows, and the blood pressure falls to the point where the supply of oxygen carried by the blood to the brain is insufficient, which can bring on fainting. Placing the head lower than the body is usually sufficient to relieve this form of shock.

Anaphylactic (Allergic) Shock. This type (see also anaphylaxis) is a rare phenomenon that occurs when a person receives an injection of a foreign protein but is highly sensitive to it. The blood vessels and other tissues are affected directly by the allergic reaction. Within a few minutes, the blood pressure falls and severe dyspnea develops. The sudden deaths that in rare cases follow bee stings or injection of certain medicines are due to anaphylactic reactions.

Septic Shock. This type, resulting from bacterial infection, is being recognized with increasing frequency. Certain organisms contain a toxin that seems to act on the blood vessels when it is released into the bloodstream. The blood eventually pools within parts of the circulatory system that expand easily, causing the blood pressure to drop sharply. Gram-negative shock is a form of septic shock due to infection with gram-negative bacteria.

Cardiogenic Shock. This type may be caused by conditions that interfere with the function of the heart as a pump, such as severe myocardial infarction, severe heart failure, and certain disorders of rate and rhythm.

ICP / ICT Monitoring

Definition:      The primary goals of intracranial pressure (ICP) monitoring are identification of intracranial pressure trends and evaluation of therapeutic interventions in order to minimize ischemic injury in the brain-injured patient. Intracranial hypertension (sustained ICP equal to or greater than 15 mm Hg) results when the brain's protective mechanisms to shunt cerebrospinal fluid (CSF) to the subarachnoid space or to vasoconstrict cerebral arterioles fail to maintain the ICP below 15 mm Hg. Intracranial hypertension compromises the relationship between systemic blood pressure and the resistance that must be overcome to accomplish cerebral perfusion. When cerebral perfusion pressure (CPP), calculated by subtracting the ICP from the mean arterial pressure, falls below 50 mm Hg, secondary brain ischemia, herniation, and, ultimately, brain death occur. ICP monitoring allows for early detection of intracranial hypertension and subsequent aggressive management.

            ICP monitoring is accomplished by the use of a fluid-filled monitoring system attached to an intraventricular catheter. The ICP waveform resembles a dampened arterial blood pressure waveform and is considered normal when the pressure is between 0 and 15 mm Hg. In addition, a stopcock within the system allows for therapeutic drainage of CSF and for sampling for infection surveillance.

Indications

1- Severe traumatic brain injury

2- Intracranial hemorrhage

3- Cerebral edema

4- Post-craniotomy

5- Space-occupying lesions such as epidural and subdural hematomas, tumors, abscesses, or aneurysms which occlude the CSF pathway

6- Reye syndrome patients who develop coma, posturing, and abnormal responses to obnoxious stimuli

7- Encephalopathy from lead ingestion, hypertensive crisis, or hepatic failure

8- Meningitis/encephalitis resulting in malabsorption of CSF

Contraindications

1.                     1-Central nervous system infection

2- Coagulation defects

3- Anticoagulant therapy

4- Scalp infection

5- Severe midline shift resulting in ventricular displacement

6- Cerebral edema resulting in ventricular collapse

Complications

`1- Intracranial infection

2- Intracerebral hemorrhage

3- Air leakage into the ventricle or subarachnoid space

4- CSF leakage

5- Overdrainage of CSF leading to ventricular collapse and herniation

6- Loss of monitoring or drainage capabilities due to the occlusion of the catheter with brain tissue or blood

                7- Inappropriate therapy because of erroneous ICP readings due to dampened waveforms,      electromechanical failure, or operator error (i.e. inappropriate leveling)