11- Circulatory shock, fetal and pulmonary
circulation. Circulatory shock
It means too little blood flow, too little delivery of oxygen and other
nutrients tothe tissue cells. Physiological types of shock are:
1- Cardiogenic shock:
It results from inadequate cardiac output. These include myocardial
infarction (MI), toxic stases of heart, sever heart valve dysfunction, and
2heart arrhythmias.
Hypovolemic shock:
It means
diminished blood volume. Hemorrhage is the most common cause of
hyovolemic shock. It decreases the filling pressure of circulation and
decreases venous return. Loss of plasma from the circulatory system
even without loss of red blood cells, can sometimes be sever enough to
reduce the total blood volume.
Sever plasma loss occurs in the following conditions:
1- Intestinal obstruction; the distention of the intestine partly blocks
venous blood flow in the intestinal circulation , which increases intestinal
capillary pressure. This causes fluid leak from capillaries into intestinal
walls and into the intestinal lumen. 2- Sever burn.
Dehydration is the other cause of hpovolemic shock.
The main causes of dehydration are:
1- Excessive sweating.
2- Fluid loss in sever diarrhea or vomiting.
3-Excess loss of fluid by kidneys.
4- Inadequate intake of fluid and electrolytes.
5- Destruction of the adrenal cortex with consequent failure of the
kidneys to reabsorb sodium, chloride, and water, which occur in the
absence
of
the
adrenocortical
hormone
(aldosterone).
3- Neurogenic shock:
Shock results without any loss of blood volume. Vascular
capacity increases, the major causes of this is sudden loss of vasomotor
tone. Factors that can cause loss of vasomotor tone include the
following:
1- Deep general anesthesia.
2- Spinal anesthesia.
3- Brain damage.
4- Anaphylactic and histamine shock:
It is an allergic condition. It results from an antigen-antibody reaction.
Basophiles and mast cells release histamine or histamine-like substance.
Histamine increased capillary permeability with rapid loss of fluid and
protein into tissue space. Also Intravenous injection of large amount of
histamine causes histamine shock.
5- Septic shock:
It means bacterial infection (blood poisoning). It is important to the
clinician because this type of shock is more frequency than other kind of
shock. It causes death in the modern hospital. Causes of septic shock
1- Peritonitis caused by spread of
are:
infection from uterus and fallopian tubes.
2-Peritonitis resulting
3from rupture of the gut.
Generalized infection from skin.
4Generalized
infection
from
gas
gangrene
bacilli.
5- Infection from the kidney or urinary tract caused by colon
`
bacilli.
The features of septic shock are high fever,
vasodilatation, high cardiac output, and development of micro blood
Trauma to the body
clots.
causes either hemorrhage or damage the capillaries sufficient to allow
excessive loss of plasma into tissue. Also trauma causes neurogenic
shock
by
pain.
Compensated shock (non progressive shock):
The sympathetic reflexes prevent further deterioration of the circulation,
and attempt to return arterial pressure back to normal level. The
sympathetic reflexes provide immediate recovery within 30 seconds after
hemorrhage..
Other factors include the
1-Central following:
nervous system ischemic response.
2Formation of angiotensin by the kidneys.
3- Formation of vasopressin (antidiuretic hormone).
4- Other compensatory mechanisms such as absorption of large quantities
of fluid from the intestine, from the interstitial spaces into blood
capillaries, conservation of water and salt by the kidneys, and increased
thirst and increased appetite for salt.
Irreversible shock
(progressive shock):
Hemorrhage beyond certain critical level causes shock to become
progressive. Many destructive enzymes have been released into the body
fluid from damaged tissue , much acidosis has developed, high energy
phosphates in tissues of the body are diminished in sever degrees of
shock.
Physiological treatment in shock:
1- Replacement therapy:
If a person is in shock caused by hemorrhage, the
best therapy is transfusion of whole blood. If the shock is caused by
plasma loss, the best therapy is administration of plasma. When
dehydration is the cause, administration of electrolyte solution can
correct the shock. Whole blood is not always available, plasma can
substitute. Sometimes plasma also is unavailable, dextran solution can
substitute.
2- Sympathomimetic drugs:
Smpathomimetic drug is a drug
that mimics sympathetic stimulation. These drugs include norepiphrine,
epinephrine, and a large number of long-acting drugs that have the same
effect of them. They are beneficial in neurogenic and anaphylactic shock.
They have not proved to be very valuable in hemorrhagic shock because
the sympathetic nervous system has become maximally activated by
circulatory reflexes.
3- Head-Down position:
When the pressure falls too low especially in hemorrhagic and
neurogenic shock, placing the patient with the head at least 12 inches
lower than the feet helps in promoting venous return and increasing COP.
4-
Oxygen
therapy:
It is less value because the problem is not inadequate oxygenation of
blood in the lungs but inadequate transport of blood after it is
5Treatment
with
oxygenated.
glucocorticoids:
Glucocorticoids are frequently given to patients in sever shock for several
reasons: 1- they increase the strength of the heart in late stages of shock;
2- they prevent release of lysosomal enzymes into the cytoplasm of the
cells; and 3- they may aid in the metabolism of glucose by the severely
damaged cells.
Circulatory arrest:
In which all blood flow
stops. This occurs frequently on the surgical operating table as a result of
cardiac arrest or ventricular fibrillation. Ventricular fibrillation can be
stopped by strong electroshock of the heart. Cardiac arrest often results
from little oxygen in the anesthetic gaseous mixture or from a depressant
effect of the anesthesia itself. A normal cardiac rhythm can be restored by
removing the anesthetic and immediately applying cardiopulmonary
resuscitation procedures with supplying the patient's lungs with adequate
quantities of oxygen. In general, more than 5 minutes of total circulatory
arrest can cause at least some degree of permanent brain damage in more
than one half of patients. Circulatory arrest for as long as 10 minutes
permanently destroys mental power.
Placental and fetal circulation
Placental circulation:
Placenta is maternal portion, is in effect a large blood sinus. Into this
lake project the villi of the fetal portion containing the small branches of
the fetal umbilical arteries and vein. The functions of placenta are:
1- The placenta is fetal lung where O2 and CO2 are exchanged, but the
cellular covering the villi are thicker and less permeable than alveolar
membrane in the lungs, exchange is much less efficient.
2- The placenta is also the rout by which all nutrient materials enter the
fetus and by which fetal wastes are discharged to the maternal blood.
Figure (47).
Fetal circulation:
The blood in the umbilical vein is about 80 % saturated with O 2 compared
with 98 % saturation in the arterial circulation of the adult. The ductus
venosus diverts some of blood directly to the inferior vena cava, and the
remainder mixes with the portal blood of the fetus. The portal and
systemic venous blood of fetus is only 26% saturated. The saturated of
mixed blood in the inferior vena cava is approximately 67%. Most of
blood entering the heart through the inferior vena cava is diverted
directly to the left atrium via the patent foramen oval. Most of the blood
from the superior vena cava enters the right ventricle and expelled into
the pulmonary artery. The resistant of the collapsed lungs is high, and the
pressure in the pulmonary artery is several mmHg higher than it is in the
aorta, so that most of the blood in the pulmonary artery passes through
the ductus arteriosus to the aorta. The relatively unsaturated blood from
the right ventricle is diverted to the trunk and lower body of the fetus,
while the head of the fetus receives the better oxygenated blood from the
left ventricle. From the aorta, some of the blood is pumped into the
umbilical arteries and back to the placenta. The O2 saturation of the blood
in the lower aorta and umbilical arteries of the fetus is approximately
60%. About 55% of fetal cardiac output goes through placenta. Figure
(48).
Changes in fetal circulation at birth:
1-The placental circulation is cut off and the peripheral resistance
suddenly rises. The pressure in the aorta rises until it exceeds that in the
pulmonary artery. The ductus arteriosus constricts within a few hours
after birth, producing functional closure, and permanent anatomical
closure follows in the next 24-48 hrs.
2- The infant gasps several times and the lungs expand. Pulmonary
blood flow increases markedly. Blood returning from the lungs raises the
pressure in left atrium closing the foramen oval by pushing the valve
against the inter-atrial septum. Closure of foramen oval prevents the leftto-right flow of arterial blood and improves the oxygenation of systemic
arterial blood.
3- Closure of two umbilical arteries and umbilical vein.
Figure (47): Placental circulation (Ganong's review of medical physiology
2010).
Figure (48): Fetal circulation(Ganong's review of medical physiology
2010). .
Pulmonary circulation
Systolic pulmonary arterial pressure is 25 mmHg, and diastolic is 8
mmHg. The pulmonary vascular resistance is about 1/8 that of systemic
circulation, and this why pulmonary arterial pressure is much lower than
aortic pressure. The wall of pulmonary artery and its large branches are
about 30% as thick as the wall of the aorta. The main velocity of the
blood in the root of the pulmonary artery is the same as that in the aorta.
Pulmonary veins are an important blood reservoir when a normal
individual lies down the pulmonary blood volume increase by up to 400
ml and when the person stands up is discharged into the general
circulation. This shift is the cause of decrease in vital capacity in the
supine position and is responsible for the occurrence of orthopnea in
heart failure.
The blood flow through the lung:
The blood distribution in lung is affected by many factors:
1- Alveolar oxygen concentration: When concentration of oxygen in
alveoli decreases below normal (below 70 mmHg) the pulmonary
capillaries will constrict. In pulmonary circulation hypoxia causes
vasoconstriction which shunt blood away from poorly ventilated area
toward well-ventilated area where gas exchange can occur.
2- Autonomic nervous control: Stimulation of vagus causes
vasodilatation and sympathetic cause vasoconstriction.
3- Effect of hydrostatic pressure (effect of gravity): gravity has marked
effect on the pulmonary circulation. In the upright position, the upper
portions of the lungs are above the level of the heart, and the bases are at
or below it. Consequently, in the upper part of the lungs, blood flow is
less and ventilation is less than at base. So in normal adult, we can
divide the lung into three zones:
A- Zone I: No blood flow during any part of cardiac cycle.
B- Zone II: Intermittent blood flow only when systolic pressure is
greater than the alveolar pressure but diastolic pressure is less than
alveolar pressure.
C- Zone III: Continuous bloods flow because the alveolar capillary
pressure remains greater than alveolar pressure during the entire
cardiac cycle.
4- Effect of exercise: During exercise, increase in pulmonary blood
flow. The whole lung is considered as zone III.