L17. Shock-Sh.pdf

CIRCULATORY SHOCK Prepared and Presented by Dr. Shimaa Mohammad Yousof Associate Prof. of Physiology SCU 2024-2025 OBJECTIVES At the end of this lecture the student should be able to: 1- Define shock 2- Describe the types and physiologic causes of shock. 3...

CIRCULATORY SHOCK Prepared and Presented by Dr. Shimaa Mohammad Yousof Associate Prof. of Physiology SCU 2024-2025 OBJECTIVES At the end of this lecture the student should be able to: 1- Define shock 2- Describe the types and physiologic causes of shock. 3- Describe the physiological response to shock according severity. 4. Enumerate the general symptoms of shock. 5- Explain briefly the main lines in treatment of shock. 25 September 2024 CVS-lecture 3-dentistry faculty  WHAT IS “SHOCK”?  Theterm SHOCK denotes “any situation in which a decrease in blood flow to the organs and tissues damages them”.  This damage occurs due to too little blood flow and too little flow of oxygen and nutrients.  Arterial pressure is usually, but not always, low in shock.  PHYSIOLOGIC CAUSES OF SHOCK Decreased Cardiac Output  Cardiacoutput: “is the amount of blood pumped by each ventricle per minute”.  Shockusually results from inadequate cardiac output  any condition that reduces the cardiac output far below normal will likely lead to circulatory shock.  PHYSIOLOGIC CAUSES OF SHOCK  Cardiac abnormalities: decrease the ability of the heart to pump blood, e.g. myocardial infarction.  Factors that decrease venous return  decrease CO  The causes of decreased venous return are: *diminished blood volume (most common), *decreased vascular tone, especially of the venous blood reservoirs, *or obstruction to blood flow at some point in the circulation.  CLASSIFICATION OF SHOCK  (1) Hypovolemic shock: - Cause: decrease in blood volume. - e.g. secondary to hemorrhage or of plasma in burns.  (2) low-resistance shock: - Cause: a decrease in total peripheral resistance. - e.g. excessive release of vasodilators (as in allergy and infection).  (3) cardiogenic shock: Cause: a marked decrease in CO. e.g. during a heart attack.  CLASSIFICATION OF SHOCK Hypovolemic shock Low-resistance shock Cardiogenic shock (vasogenic) Decreased blood volume *Increase in the vascular Decreased cardiac output capacity. *Occurs in: anaphylactic septic and neurogenic shock For shock to occur , at least, one element should be affected in the circulatory system:  The heart  Blood vessels  The blood.  STAGES OF SHOCK  Shock is divided into the following three major stages: 1. A nonprogressive stage (sometimes called the compensated stage) the normal circulatory compensatory mechanisms eventually cause full recovery without help from outside therapy.  STAGES OF SHOCK 2. A progressive stage  without therapy, the shock becomes steadily worse until death. 3. An irreversible stage  the shock has progressed to such an extent that all forms of known therapy are inadequate to save the person’s life, even though, for the moment, the person is still alive.  BODY RESPONSES TO SHOCK Sympathetic Reflex Compensations in Shock  The decrease in arterial pressure after hemorrhage + decreases in pressures in the pulmonary arteries and veins in the thorax  causes powerful sympathetic reflexes (initiated mainly by the arterial baroreceptors and other vascular stretch receptors).  BODY RESPONSES TO SHOCK  These reflexes stimulate the sympathetic vasoconstrictor system throughout the body, resulting in three important effects: 1. The arterioles constrict in most parts of the systemic circulation, thereby increasing the total peripheral resistance. 2. The veins constrict, thereby helping to maintain adequate venous return despite diminished blood volume. 3. Heart activity increases markedly, sometimes increasing the heart rate from the normal value of 72 beats/min to as high as 160 to 180 beats/min.  1- Constriction of blood vessels going to the heart.(veins) 2- Increase the pump (heart) force of contraction due to 3- Constriction of increased venous return and peripheral.blood sympathetic stimulation. vessels (arterioles) Result: increased blood pressure to the survival range.  BODY RESPONSES TO SHOCK What is the effect of the sympathetic stimulation on the coronary and cerebral blood flow?? Does it lead to decreased blood flow to the heart and the brain? NO, WHY?  BODY RESPONSES TO SHOCK  BECAUSE: 1- The sympathetic stimulation does not cause significant constriction of either the cerebral or the cardiac vessels. 2- In both these vascular beds, local blood flow autoregulation is excellent  which prevents moderate decreases in arterial pressure from significantly decreasing their blood flows.  BODY RESPONSES TO SHOCK  In Nonprogressive Shock (Compensated Shock):  Nonprogressive shock: If shock is not severe enough to cause its own progression  the person eventually recovers.  The sympathetic reflexes and other factors compensate enough to prevent further deterioration of the circulation.  The factors that cause a person to recover from moderate degrees of shock are all the negative feedback control mechanisms of the circulation that attempt to return cardiac output and arterial pressure back to normal levels.  BODY RESPONSES TO SHOCK  1. Baroreceptor reflexes: which elicit powerful sympathetic stimulation of the circulation.  BODY RESPONSES TO SHOCK  2. Reverse stress-relaxation of the circulatory system: which causes the blood vessels to contract around the diminished blood volume, so that the blood volume that is available more adequately fills the circulation. Stress Relaxation Reverse Stress relaxation  BODY RESPONSES TO SHOCK  3. Formation of angiotensin by the kidneys  - constricts the peripheral arteries - output of water and salt by the kidneys,  both of which help prevent progression of shock.  BODY RESPONSES TO SHOCK .Compensatory mechanisms that return the blood volume back toward normal:  including absorption of large quantities of fluid from the intestinal tract,  absorption of fluid into the blood capillaries from the interstitial spaces of the body  increased thirst and increased appetite for salt  which make the person drink water and eat salty foods if able,.  BODY RESPONSES TO SHOCK  “ProgressiveShock” Is Caused by a Vicious Circle of Cardiovascular Deterioration  Thepositive feedbacks  further depress cardiac output in shock thus causing the shock to become progressive.  Some of the more important feedbacks are the following:  BODY RESPONSES TO SHOCK  Cardiac Depression: When the arterial pressure falls low enough  coronary blood flow decreases below that required for adequate nutrition of the myocardium  This weakens the heart muscle and thereby decreases the CO more  a positive feedback cycle has developed  the shock becomes more and more severe.  BODY RESPONSES TO SHOCK  Positive Feedback Deterioration of Tissues in Shock and the Vicious Circle of Progressive Shock:  Positive feedback does not necessarily lead to a vicious circle.  Whether a vicious circle develops depends on the intensity of the positive feedback.  In mild degrees of shock, the negative feedback mechanisms of the circulation such as sympathetic reflexes, can easily overcome the positive feedback influences  cause recovery.  BODY RESPONSES TO SHOCK  Irreversible Shock:  Aftershock has progressed to a certain stage, transfusion or any other type of therapy becomes incapable of saving the person’s life. Different types of positive feedback  that can lead to the progression of shock. SYMPTOMS OF SHOCK PLEASE STUDY THE SYMPTOMS IN THE PICTURE PHYSIOLOGY OF TREATMENT IN SHOCK  Treatment by the Head-Down Position. Placing the patient with the head at least 12 inches lower than the feet  helps tremendously in promoting venous return  thereby also increasing cardiac output. This head-down position is the first essential step in the treatment of many types of shock.  Replacement Therapy:  Blood and Plasma Transfusion.  PHYSIOLOGY OF TREATMENT IN SHOCK  Treatment of Shock with Sympathomimetic Drugs— Sometimes Useful, Sometimes Not. - A sympathomimetic drug is a drug that mimics sympathetic stimulation. - These drugs include norepinephrine, epinephrine.  Oxygen Therapy. - Because the major deleterious effect of most types of shock is too little delivery of oxygen the tissues, giving the patient oxygen to breathe can be of benefit in many instances. References Guyton and Hall Textbook of Medical Physiology, 2021, Chapter 24. THANK YOU