By V. Kayor. Keiser University.
Tricuspid regurgitation Denition Management Retrograde blood ow from the right ventricle to the Functional tricuspid regurgitation usually resolves with rightatrium during systole purchase lioresal 10 mg online muscle relaxant topical. Cardiac arrhythmias A cardiac arrhythmia is a disturbance of the nor- Aetiology mal rhythm of the heart buy lioresal 25mg without a prescription muscle relaxer zoloft. Tachycardias are also subdivided according to their Clinical features origin: Most patients are asymptomatic but occasionally post- r Sinustachycardia discount lioresal 25mg online spasms calf muscles. If bradycardia is episodic and severe cheap lioresal 10 mg otc muscle relaxant 2631, syncope r Ventricular tachyarrhythmias such as ventricular may occur. However, in patients with bundle branch block Most cases do not require treatment other than with- and in cases where the rapid rate of supraventricu- drawal of drugs or treatment of any underlying cause. Carotid sinus massage typically leads to a Denition sudden and sometimes prolonged sinus pause. Aetiology/pathophysiology Sinustachycardia is a physiological response to main- tain tissue perfusion and oxygenation. Clinical features Investigations Palpitations with an associated rapid, regular pulse rate. In addition anti-arrhythmic drugs may be required to Management controlanytachycardia. Atrial arrhythmias Sinus node disease Atrial ectopic beats Denition Sinusnode disease or sick sinus syndrome is a tachy- Denition cardia/bradycardia resulting from damage to the sinus Atrial ectopic beats include extrasystoles and premature node. Aetiology/pathophysiology Aetiology Sinusnode disease is relatively common in the elderly Atrial ectopics are common in normal individuals. All dueto ischaemia, infarction or degeneration of the sinus cardiac cells have intrinsic pacemaker ability. The condition is characterised by prolonged in- ually depolarise until a threshold is reached at which tervals between consecutive P waves (sinus arrest) and point rapid depolarisation occurs and a cardiac action periods of sinus bradycardia. This is most rapid in the sinoatrial may allow tachycardias (typically atrial brillation) from node, the normal pacemaker of the heart. This combination of fast and slow or group of cells the gradual depolarisation is more rapid supraventricular rhythms is known as tachy-brady syn- than usual, or if the voltage threshold for rapid depolar- drome. Clinical features Atrial utter presents with palpitations, dizziness, syn- Investigations cope or cardiac failure. Massage of the Management carotid sinus causes a transient increase in block with Atrial ectopic beats do not require treatment, although consequent slowing of the ventricular rate. If atrial ectopic beats are fre- Investigations quent they may progress to other atrial arrhythmias. Atrial utter produces a characteristic regular sawtooth utter waves at a rate of 300 bpm seen best in lead V1. Atrial utter is a rapid atrial rate between 280 and 350 bpm, most commonly 300 bpm. It may be caused by thyro- logical assessment, recurrence may be prevented by ra- toxicosis. Normally once a cardiac cell has been depolarised it is refractory to re-stimulation for a short period. This pre- vents waves of cardiac depolarisation owing in a retro- Atrial brillation grade direction. If, however, the conduction through the myocardiumisslow(usuallyduetomyocardialdamage), Denition adjacent cells may have recovered from their refractory Atrial brillation is a quivering of atrial myocardium period allowing restimulation and hence the formation resulting from disordered electrical and muscle activity. Incidence rate,inthe elderly who depend on atrial function to Common achieve sufcient ventricular lling, or if there is associ- ated signicant cardiac damage. Patients may Sex present with palpitations, acute cardiac failure or the M > F gradual onset of increasing shortness of breath. On ex- amination there is an irregularly irregular pulse with Aetiology varying pulse volume. There is also loss of the a wave of Causes may be divided into cardiac and systemic. Inacuteatrialbrillation,underlyingischaemia ease, mitral valve disease, cardiomyopathies and pul- such as a recent myocardial infarction or unstable monary disease. Thelonger the atrial brillation has been present, merous circuits have different cycle times, the result is a the less the likelihood of restoring sinus rhythm. Digoxin does not missions, but an irregularly irregular pulse of between prevent recurrence. Atrial brilla- r Control of the ventricular rate is achieved with drugs tion may be paroxysmal with attacks lasting minutes to such as digoxin, calcium channel blockers and/or - hours. Aetiology/pathophysiology The majority of junctional tachycardias are due to re- Investigations/management entry circuits. If Usually there is a slow anterograde pathway from atria the retrograde pathway is slow with delayed atrial con- to ventricles and a fast retrograde pathway back to the traction, inverted P waves appear between complexes. The re- may produce an immediate cessation of the arrhyth- entrant circuit is concealed as it slow, close to the mia. Complications Aetiology Sudden cardiac death may rarely occur if atrial brilla- Abnormalconnectionbetweenatriumandventricle(e. Pathophysiology Management r Re-entrant tachycardias are treated with drugs that NormallythefastconductionthroughthebundleofKent allows the adjacent area of ventricle to be rapidly depo- block retrograde conduction through the accessory larised (preexcitation), whilst the remainder of the ven- pathway, e. Verapamil and digoxin are contraindicated as two pathways may form a re-entry circuit with the fast they accelerate anterograde conduction through the accessory pathway causing a retrograde stimulation of accessory pathway. Clinical features Prognosis In sinus rhythm WolffParkinsonWhite syndrome is With age the pathway may brose and so some patients asymptomatic. Denition Aventricular ectopic/extrasystole/premature beat is an extramyocardial depolarisation triggered by a focus in Prognosis the ventricle. Ventricular ectopics worsen the prognosis in patients with underlying ischaemic heart disease but there is no evidence that anti-arrhythmic drugs improve this. Aetiology/pathophysiology Ventricular ectopics are not uncommon in normal indi- viduals and increase in incidence with advancing age. Common causes include ischaemic heart disease and Ventricular tachycardia hypertension. Ectopic beats may arise due to any of Denition the mechanisms of arrhythmias, such as a re-entry cir- Tachycardia of ventricular origin at a rate of 120220 cuit or due to enhanced automaticity (which may occur bpm. When ventricular ectopic beats occur regularly Ventricular tachycardia is normally associated with un- after each sinus beat, it is termed bigeminy, which is fre- derlying coronary, ischaemic or hypertensive heart dis- quently due to digoxin. Clinical features Patients are usually asymptomatic but may feel uncom- Pathophysiology fortable or beaware of an irregular heart or missed beats. The underlying mechanism is thought to be enhanced On examination the pulse may be irregular if ectopics automaticity,leadingtore-entrycircuitasinothertachy- are frequent. In ventricular tachycardia there is a small (or sometimes large) group of ischaemic or electrically non- homogeneouscells,typicallyresultingfromanacutemy- Investigations r ocardial infarction. Clinical features r Echocardiography and exercise testing may be used The condition is episodic with attacks usually lasting to look for underlying structural or ischaemic heart minutes. The presenting pic- Denition ture is dependent on the rapidity of the tachycardia and Torsades de pointes or twisting of the points is a con- the function of the left ventricle, as well as general con- dition in which there is episodic tachycardia and a pro- dition of the patient (e. Low serum potas- It is thought that the long QT interval allows adjacent sium or magnesium may predispose to arrhythmias, so cells, which are repolarising at slightly different rates, levels should be checked. The QT interval is prolonged by biochemical abnormalities and Complications drugs, and is also prolonged in bradycardic states. Cardiac arrest due to pulseless ventricular tachycardia or ventricular brillation. Clinical features It typically recurs in frequent short attacks, causing pre- syncope, syncope or heart failure. Management r Any underlying electrolyte disturbance should be identied and managed. It is now customary to use these in patients Denition known to have a high risk of sudden cardiac death. Chaoticelectromechanicalactivityoftheventriclescaus- ing a loss of cardiac output.
Approach to Diagnosis To complete the assessment of an abdominal mass discount lioresal 10 mg without prescription muscle relaxant causing jaundice, one may choose among several different investigational tools order 10 mg lioresal fast delivery back spasms 32 weeks pregnant. This noninvasive buy lioresal 10 mg on line muscle relaxant rx, safe discount 10mg lioresal with amex spasms meaning in urdu, cheap and widely available method identifies the mass and provides information on its origin and nature. Hollow organs may be demonstrated radiographically through the use of contrast media (e. Description Proctalgia fugax is a sudden severe pain in the anus lasting several seconds or minutes and then disappearing completely. Although some observations suggest a rectal motility disorder, the symptom appears more likely to result from spasm of the skeletal muscle of the pelvic floor (specifically, the puborectalis). History and Physical Examination Proctalgia fugax occurs in about 14% of adults and is somewhat more common in females than males. The pain may be excruciating, but since it is so short-lived patients seldom report it to their physician. In 90% of instances it lasts less than five minutes and in many cases less than a minute. Differential Diagnosis Perianal disease may cause pain but it usually accompanies, rather than follows, defecation. One should be particularly careful to exclude the presence of an anal fissure, which may be difficult to see on anal inspection. Pain originating from the coccyx may be accompanied by coccygeal tenderness both externally and from within the rectum. An acute attack of anal pain lasting several hours may indicate a thrombosed hemorrhoid. Saloojee Examination of the abdomen is an important component of the clinical assessment of anyone presenting with suspected disease of the gastrointestinal tract. As in all other parts of the examination, care must be taken to show respect and concern for the patient while ensuring an appropriate and thorough examination. While performing the examination it is useful to keep in mind the concepts of sensitivity and specificity. How confident can we be that a suspected physical finding is in fact present and has clinical significance? For example, how sensitive and specific is our bedside examination for hepatomegaly? What is the clinical significance of an epigastric bruit heard in a thin 20-year-old female versus a 55-year-old hypertensive, obese male? In the following sections we will describe an appropriate sequential examination of the abdomen and highlight some of the potential pitfalls of this process. When describing the location of an abnormality it is useful to divide the abdomen into four quadrants. Imagine a perpendicular line through the umbilicus from the xiphoid process to the symphysis pubis. The overall appearance of the abdomen can be described as scaphoid (markedly concave), protruberant, or obese. One should examine the skin for cutaneous lesions, vascular markings, dilated veins and striae. Division of the abdomen into nine quadrants: the left upper quadrant, right regions. Auscultation It is useful to auscultate the abdomen for bowel sounds and bruits prior to palpation or percussion. Bruits are vascular sounds created by turbulent flow and may indicate partial arterial occlusion. Arterial bruits are usually heard only during systole and best heard with the diaphragm of the stethoscope, as they are high pitched. Renal bruits may be heard midway between the xiphoid process and the umbilicus, 2 cm away from the midline. About 20% of normal persons will have a vascular bruit, so that the auscultation of an abdominal bruit has to be placed within the clinical context. This is found an area approximated by an ellipse between the umbilicus and the midclavicular line where it crosses the right subcostal margin. There are, however, no studies to suggest this is a helpful finding in routine examination. Friction rubs are a rare sound indicating inflammation of the peritoneal surface of an organ. However, even with careful auscultation of patients with known liver tumours, fewer than 10% are found to have a rub. Bowel Sounds Bowel sounds should be listened for prior to palpation or percussion, but the yield of this examination is low. Listening in one spot, such as the right lower quadrant, is generally sufficient since bowel sounds are transmitted widely through the abdomen. Rushes of very high pitched bowel sounds First Principles of Gastroenterology and Hepatology A. Shaffer 32 coinciding with crampy pain may indicate hyperperistalsis and acute small bowel obstruction. Palpation Palpation of the abdomen should be done in an orderly sequence with the patient in the supine position. Light palpation should be done in all four quadrants, assessing for areas of potential tenderness. With one hand, using the pads of the fingertips, palpate in a gentle, circular motion. It is thought that using one hand for deep palpation may increase the risk of missing a mass. Involuntary guarding and rebound tenderness are signs of peritoneal inflammation (peritonitis). Guarding refers to contraction of abdominal wall muscles when the abdomen is palpated. Involuntary guarding occurs as a protective mechanism when peritonitis is present. It is useful in defining organomegaly and the presence of free intra-abdominal fluid (ascites), as discussed below. The patient is asked to breathe deeply and slowly, in order to bring the liver edge down to the examining fingertips of the right hand. The examiner moves the right hand in a cephalad direction about 2 cm with each expiration. When the liver edge is palpable, trace the edge First Principles of Gastroenterology and Hepatology A. Percuss in a cephalad direction in the right midclavicular line until an area of dullness is encountered. Percuss for the upper border starting in the right midclavicular line in the third intercostal space. Move down one interspace at a time until the percussion note changes from resonant to dull. To confirm the change of percussion note strike the third and fourth fingers laid in adjacent interspaces. Measure the distance between the upper and lower percussion edges in the mid-clavicular line. The diaphragm of the stethoscope is placed at the right costal margin in the midclavicular line. A finger moves up the abdomen in the mid-clavicular line, scratching gently and with consistent pressure. When the liver edge is reached, there is a sudden increase in the scratching sound heard through the stethoscope. In one comparative study the scratch test was not felt to offer any advantage over the techniques of palpation and percussion.
As the blood ows through the circulatory system discount 10 mg lioresal amex muscle relaxant in pediatrics, its initial energy cheap 25 mg lioresal overnight delivery muscle relaxant during pregnancy, pro- vided by the pumping action of the heart generic lioresal 10 mg with amex spasms gallbladder, is dissipated by two loss mecha- nisms: losses associated with the expansion and contraction of the arterial walls and viscous friction associated with the blood ow lioresal 10mg online spasms just before sleep. Due to these energy losses, the initial pressure uctuations are smoothed out as the blood ows away from the heart, and the average pressure drops. By the time the blood reaches the capillaries, the ow is smooth and the blood pressure is only about 30 torr. The pressure drops still lower in the veins and is close to zero just before returning to the heart. In this nal stage of the ow, the movement of blood through the veins is aided by the contraction of muscles that squeeze the blood toward the heart. The rate of blood ow Q through the body depends on the level of physical activity. Of course, as the aorta branches, the size of the arteries decreases, result- ing in an increased resistance to ow. Although the blood ow in the nar- rower arteries is also reduced, the pressure drop is no longer negligible (see Exercise 8-2). The ow through the arterioles is accompanied by a much larger pressure drop, about 60 torr. Since the pressure drop in the main arteries is small, when the body is horizontal, the average arterial pressure is approximately constant throughout the body. The arterial blood pressure, which is on the average 100 torr, can support a column of blood 129 cm high (see Eq. This means that if a small tube were introduced into the artery, the blood in it would rise to a height of 129 cm (see Fig. If a person is standing erect, the blood pressure in the arteries is not uni- form in the various parts of the body. The weight of the blood must be taken into account in calculating the pressure at various locations. For example, the average pressure in the artery located in the head, 50 cm above the heart (see Exercise 8-4a) is Phead Pheart gh 61 torr. In the feet, 130 cm below the heart, the arterial pressure is 200 torr (see Exercise 8-4b). Thus, a person may feel momentarily dizzy as he/she jumps up from a prone position. This is due to the sudden decrease in the blood pressure of the brain arteries, which results in a temporary decrease of blood ow to the brain. The same hydrostatic factors operate also in the veins, and here their eect may be more severe than in the arteries. When a person stands motionless, the blood pressure is barely adequate to force the blood from the feet back to the heart. Thus when a person sits or stands without muscular movement, blood gathers in the veins of the legs. This increases the pressure in the capillaries and may cause temporary swelling of the legs. Hormones are molecules, often proteins, that are produced by organs and tissues in dierent parts of the body. They are secreted into the blood stream and carry messages from one part of the body to another. Hormones aecting the heart are produced in response to stimuli such as need for more oxygen, changes in body tempera- ture, and various types of emotional stress. These small vessels that receive blood from the arteries have an average diameter of about 0. The walls of the arterioles contain smooth muscle bers that contract when stimulated by nerve impulses and hormones. The con- traction of the arterioles in one part of the body reduces the blood ow to that region and diverts it to another. Since the radius of the arterioles is small, con- striction is an eective method for controlling blood ow. Poiseuilles equation shows that if the pressure drop remains constant, a 20% decrease in the radius reduces the blood ow by more than a factor of 2 (see Exercise 8-5). A stress-induced heart condition called stress cardiomyopathy (broken heart syndrome) has only recently been clearly identied by Western medicine. The syndrome occurs most frequently after a sudden intense emotional trauma such as death in the family, an experience of violence, or extreme anger. The symptoms are similar to an acute heart attack, but the coronary arteries are found to be normal and the heart tissue is not damaged. It has suggested that the condition is triggered by an excessive release of stress-related hormones called chatecholamines. During the period of ow, the velocity of the blood is about three times as high as the overall average value calculated in Exercise 8-6. For example, when the total ow rate is 5 liter/min, the blood velocity in the capillaries is only about 0. The kinetic energy of the blood becomes more signicant as the rate of blood ow increases. This energy is no longer neg- ligible compared to the blood pressure measured at rest. In healthy arteries, the increased velocity of blood ow during physical activity does not present a problem. During intense activity, the blood pressure rises to compensate for the pressure drop. Assuming a Reynolds number of 2000, the critical velocity for the onset of turbulence in the 2-cm-diameter aorta is, from Eq. But as the level of physical activity increases, the ow in the aorta may exceed the critical rate and become turbulent. In the other parts of the body, however, the ow remains laminar unless the passages are abnormally constricted. Laminar ow is quiet, but turbulent ow produces noises due to vibrations of the various surrounding tissues, which indicate abnormalities in the circu- latory system. These noises, called bruit, can be detected by a stethoscope and can help in the diagnosis of circulatory disorders. In the United States, an estimated 200,000 people die annually as a consequence of this disease. In arteriosclerosis, the arterial wall becomes thickened, and the artery is narrowed by deposits called plaque. Sixty to seventy percent is considered severe, and a narrowing above 80% is deemed critical. The increased kinetic energy is at the expense of the blood pressure; that is, in order to maintain the ow rate at the higher velocity, the potential energy due to pressure is converted to kinetic energy. Because of the low pressure inside the artery, the external pressure may actually close o the artery and block the ow of blood. When such a blockage occurs in the coronary artery, which supplies blood to the heart muscle, the heart stops functioning. Stenosis above 80% is considered critical because at this point the blood ow usually becomes turbulent with inherently larger energy dissipation than is associated with laminar ow. As a result, the pressure drop in the situa- tion presented earlier is even larger than calculated using Bernoullis equation. The blood impinging on the arterial wall may dislodge some of the plaque deposit which downstream may clog a narrower part of the artery. If such clogging occurs in a cervical artery, blood ow to some part of the brain is interrupted causing an ischemic stroke. The artery has a specic elasticity; therefore, it exhibits certain springlike prop- erties. Specically, in analogy with a spring, the artery has a natural fre- quency at which it can be readily set into vibrational motion.
The most common cause is thrombosis in association with an atheromatous plaque that has cracked or Symptoms ruptured buy discount lioresal 25mg online muscle relaxant machine. There may be a previous history of angina leftatriumorventricle 25mg lioresal with amex muscle relaxant food,ormitraloraorticvalvelesions or myocardial infarction generic lioresal 10mg with visa spasms sleep. The size and location of the infarct depend on which Examination artery is involved (Fig buy 10 mg lioresal amex muscle relaxant high. Occlusion of: Once any distress has been alleviated by pain control there may be no signs. T pericardial friction rub Posterior infarction is rare and does not produce Q T mitral regurgitation (papillary muscle dysfunc- waves, but gives a tall R wave in V1. The Twaves may eventually become upright, but in full thickness untreated myocardial infarction Q waves persist indenitely. Ventricular hypertrophy Large R waves occur over the appropriate ventricle in the chest leads (V12 for right ventricular hypertrophy and V56 for left ventricular hypertrophy). Causesinclude ischaemic heart disease, myocardial infarction, cardiomyopathy, hypertension and aortic stenosis. Fascicular block There are three fascicles to the bundle of His: right, left anterior and left posterior. Sinoatrial disease (sick sinus syndrome) This is a chronic disorder often associated with ischaemic heart disease in which sinus bradycardia and/or episodic sinus arrest can alternate with episodes of rapid supraventricular arrhythmia. Earlymortality(within4weeks)ischieywithintherst Several studies in the late 1980s showed that in- 2handusuallyfromventricularbrillation. Anypatient travenous streptokinase reduced mortality in patients suspected of having a myocardial infarction requires: reachinghospitalwithmyocardialinfarctionfromjust. It is cheaper than alternatives pressure and treat heart failure but can cause allergic reactions. Shock: the patient is hypotensive, pale, cold, sweaty aneurysm may be demonstrated by echocardiogra- andcyanosed. There is a pansystolic or late sysytolic ous) or nitrates (venous) if blood pressure allows mitral regurgitant murmur. Echocardiography con- T inotropes dopamine and dobutamine increase rms the diagnosis. Supraventricular extrasystoles: common, but rarely ditis, and the presence of antibodies to heart muscle. Supraventricular tachycardia: arise from the atria or Invasive and non-invasive atrioventricular junction. If the rate is Patients with ongoing angina (or other evidence of < 50beats/min and the patient is hypotensive, give ischaemia) at rest or on minimal exertion or left atropine 0. Patients in whom angiography is not serious if they complicate anterior rather than infe- planned should undergo exercise testing towards the rior infarcts. Echocardiography should be Many physicians would consider cardiac pacing performed to assess left ventricular function. Ventricular brillation: this is frequently within 6h must be stressed and strategies to help smokers used. V en tricul ar asystol e D directcurren t el ectrocardiogram 84 Cardiovascular disease be considered. The cholesterol did not result in a signicant reduction in intensivelipid-loweringstatinregimenprovidedgreater the primary outcome of major coronary events, but protectionagainstdeathormajorcardiovascularevents did reduce the risk of other composite secondary than the standard regimen. Duringnearly12million randomly assigned to receive either 10 mg or 80mg person years at risk between the ages of 40 and of atorvastatin per day. There was an absolute with about a half, a third and a sixth lower ischaemic reduction in the rate of major cardiovascular events heart disease mortality in both sexes at ages 4049, of2. Inpatientswithonlyoneoftheseriskfactorslong-termantithrombotictherapywitheither warfarinoraspirinatadoseof75325mg/dayisrecommended,andinpatientswithnoneoftheseriskfactors long-term aspirin therapy at a dose of 75325mg/day is recommended. Recommendations for patients with atrial utter are similar, although the evidence base is less strong. Management controlling the ventricular rate, either alone or in combination with b-blockers. Check serum potassium, echocardiogram and thyroid The incidence of ischaemic stroke (embolic or function. Long-term amiodarone reduces the frequency of relapse, although side effects can limit its use. The rate is basically regular but is neal microdeposits, photosensitivity, skin discoloura- affected by 2:1, 3:1 and variable block. Management Medical therapy Drugs such as sotalol, amiodarone, propafenone and Quinine, ecainide and amiodarone have all been ecainide can be effective in restoring sinus rhythm. In tachycardias involv- for 3 weeks before elective cardioversion and for at ing accessory connections, agents that affect fast least 4 weeks after sinus rhythm has been maintained channel dependent tissue (propafenone, ecainide, is recommended. Potassium current blockers, such as sotalol or Atrial tachycardia amiodarone, represent an alternative therapy. The main types are WolffParkinsonWhite tachycardia and LownGanongLevine syndromes. First try unilateral carotid sinus massage or Valsalva accessory pathway (bundle of Kent) that bypasses the manoeuvre. Its short half-life (10s) means that complex is positive in lead V1; in type B, it is negative. Verapamiliscontraindicatedinpatients form a circuit through which impulses repeatedly taking b-blockers. In atrial brillation most ventricular complexes sia is used when rapid results are required and other are broad because of the presence of d waves on procedures have failed. Amiodarone may changes so that the complexes appear to twist con- beusedtoslowconductionintheaccessorypathway. It is often self-limiting, but if sustained may cause hypo- Ventricular brillation tension and shock. Patients with cardiac disease but without asystole) resulting limitation of physical activity. Patients with cardiac disease resulting in Management slight limitation of physical activity. Patients with cardiac disease resulting in mationabout basicandadvancedlifesupport, includ- marked limitation of physical activity. Patients with cardiac disease resulting in inability to carry on any physical activity without discomfort. If any physical Cardiovascular disorders activity is undertaken, discomfort is increased. In response to increased volume load, ventricular volume increases (the Group 1 entitlement heart dilates). Angina: driving must cease when symptoms occur strength of contraction increases as the cardiac at rest, with emotion or at the wheel. Acute coronary syndromes: if successfully treated traction declines as stretch becomes extreme. Cardiacoutputisdiminishedbydenition,resulting after 1 week provided no other urgent revascularisa- in reduced perfusion to vital organs. Sympathetic nervous activity and plasma noradren- at least 40% prior to hospital discharge and there is aline (norepinephrine) levels increase, leading to no other disqualifying condition. If not successfully increased heart rate, myocardial contractility and treated by coronary angioplasty, driving may arterial and venous tone. Renal blood ow is reduced, leading to activation of activity, and salt and water retention. These me- reduced peripheral blood ow and circulatory chanisms increase both pre- and afterload. Preload is the extent to which cardiac muscle is stretched prior to contraction; it is reected by the Aetiology ventricular volume at the end of diastole the end- diastolic volume. Pulmonary congestion Ejection fraction is reduced and there may be dilata- causes dyspnoea, orthopnoea and paroxysmal noc- tion of the heart.
Diusion is the main mechanism for the delivery of oxygen and nutrients into cells and for the elimination of waste products from cells discount lioresal 25 mg without a prescription spasms spanish. On a large scale generic 25mg lioresal otc spasms 2, diusive motion is relatively slow (it may take hours for the colored solution in our example to diuse over a distance of a few centimeters) buy 10mg lioresal mastercard spasms right side under rib cage, but on the small scale of tissue cells 25 mg lioresal for sale spasms right arm, diusive motion is fast enough to provide for the life function of cells. Although a detailed treatment of diusion is beyond our scope, some of the features of diusive motion can be deduced from simple kinetic theory. Consider a molecule in a liquid or a gas which is moving away from the starting point 0. The molecule has a thermal velocity v and travels on the average a distance L before colliding with another molecule (see Fig. As a result of the collision, the direction of motion of the molecule is changed randomly. On the average, however, after a certain number of collisions the molecule will be found a distance S from the starting point. A statistical anal- ysis of this type of motion shows that after N collisions the distance of the molecule from the starting point is, on the average, S L N (9. A frequently used illustration of the random walk examines the position of a drunkard walking away from a lamppost. If the length of each step is 1 m, after taking 100 steps he will be only 10 m away from the lamppost although he has walked a total of 100 m. After 10,000 steps, having walked 10 km, he will be still only 100 m (on the average) from his starting point. Let us now calculate the length of time required for a molecule to diuse a distance S from the starting point. Therefore, the mean free path of a diusing molecule is short, about 108 cm (this is approximately the distance between atoms in a liquid). Gases are less densely packed than liquids; consequently, in gases the mean free path is longer and the diusion time shorter. In a gas at 1 atm pressure, the mean free path is on the order of 105the exact value depends on the specic gas. Consider a cylinder containing a nonuniform distribu- tion of diusing molecules or other small particles (see Fig. Although this solution for the diusion problem is not exact, it does illustrate the nature of the diusion process. The net ux from one region to another depends on the dierence in the density of the diusing particles in the two regions. The ux increases with thermal velocity v and decreases with the distance between the two regions. In our previous illustration of diusion through a uid, where L 108 cm and v 104 cm/sec, the diusion coecient calculated from Eq. By comparison, the measured diusion coecient of salt (NaCl) in water, for example, is 1. Thus, our simple calculation gives a reasonable esti- mate for the diusion coecient. The diusion coecients for biologically important molecules are in the range from 107 to 106 cm2/sec. Oxygen, nutrients, and waste products must pass through these membranes to maintain the life functions. In the simplest model, the biologi- cal membrane can be regarded as porous, with the size and the density of the pores governing the diusion through the membrane. If the diusing molecule is smaller than the size of the pores, the only eect of the membrane is to reduce the eective diusion area and thus decrease the diusion rate. If the diusing molecule is larger than the size of the pores, the ow of molecules through the membranemaybebarred. The permeability depends, of course, on the type of membrane as well as on the diusing molecule. Permeability may be nearly zero (if the molecules cannot pass through the membrane) or as high as 104 cm/sec. Many membranes, for example, are permeable to water but do not pass molecules dissolved in water. As a result water can enter the cell, but the components of the cell cannot pass out of the cell. In the type of diusive motion we have discussed so far, the movement of the molecules is due to their thermal kinetic energy. Some materials, however, are transported through membranes with the aid of electric elds that are gen- erated by charge dierences across the membrane. We have shown that over distances larger than a few millimeters diusion is a slow process. Therefore, large living organisms must use circulating sys- tems to transport oxygen nutrients and waste products to and from the cells. The evolution of the respiratory system in animals is a direct consequence of the inadequacy of diusive transportation over long distances. At rest, an average 70-kg adult requires about 70 Cal of energy per hour, which implies a consumption of 14. It has been determined that in a person only about 2% of oxygen consumed at rest is obtained by diusion through the skin. The lungs can be thought of as an elastic bag suspended in the chest cav- ity (see Fig. When the diaphragm descends, the volume of the lungs increases, causing a reduction in gas pressure inside the lungs. The trachea branches into smaller and smaller tubes, which nally terminate at tiny cavities called alveoli. Itis here that gas is exchanged by diusion between the blood and the air in the lungs. The lungs of an adult contain about 300 million alveoli with diameters ranging between 0. The total alveolar area of the lungs is about 100 m2, which is about 50 times larger than the total surface area of the skin. In fact, the full 1 volume of the lungs is about 6 liter, and at rest only about liter is exchanged 2 during each breath. The oxygen requirement, of course, rises with increased physical activity, which results in both faster and deeper breathing. While diusion through the skin can supply only a small fraction of the oxygen required by large animals, the oxygen needs of small animals may be completely satised through this channel. The energy consumption and, hence, the oxygen requirement of an animal is approximately proportional to its mass. The amount of oxygen diusing through the skin is proportional to the surface area of the skin. Now, if R is a characteristic linear dimension of the animal, the volume is proportional to R3, and the skin surface area is proportional to R2. It is possible to obtain an estimate for the maximum size of the animal that can get its oxygen entirely by skin diusion. A highly simplied calcula- tion outlined in Exercise 9-7 shows that the maximum linear size of such an animal is about 0. Therefore, only small animals, such as insects, can rely entirely on the diusion transfer to provide them with oxygen. However, during hibernation when the oxygen requirements of the animal are reduced to a very low value, larger animals such as frogs can obtain all the necessary oxygen through their skin. In fact some species of frog hibernate through the winter at the bottom of lakes where the temperature is constant at 4C.