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Release of acetylcholine from the motor neuron would be impaired compared to normal buy keppra with american express symptoms after conception. The concentration of acetylcholine in the myoneural junction is the net effect of acetylcholine being released into the junction from the motor neuron and decomposition of the acetylcholine by acetylcholinesterase in the junction buy generic keppra canada medicine song 2015. Anticholinesterases would block acetylcholine breakdown and quality 250 mg keppra medicine 7253, thus, increase the concentration of acetylcholine in the junction. The endplate potential is proportional to the number of nicotinic receptors occupied by acetylcholine and would likely be increased in the presence of an anticholinesterase. In the presence of anticholinesterases, less motor neuron stimulation (and, thus, less acetylcholine release) would be needed to maintain normal concentrations of acetylcholine in the vicinity of the motor endplate. Finally, anticholinesterases have no effect on the release of acetylcholine from the motor neuron terminal; these enzymes affect acetylcholine after it is released. At L, a muscle group produces a maximum isometric twitch force of 10 g, whereas it produces ao maximum isometric force of 7 g at 0. Starting at an optimum preload, the muscle will shorten if it moves a load less than the maximum isometric force possible at its optimum preload until it reaches a length whose maximum isometric force-generating capacity is the same as the load being moved. The muscle cannot move beyond this point because, at shorter lengths, there will not be enough crossbridges to support the load being moved. Prevent contraction of the muscle following activation of ligand-gated calcium channels B. Not affect tonic contraction of the muscle stimulated by activation of ligand-gated membrane channels D. Opening of ligand-gated channels will contract the muscle tonically, first due to the influx of calcium through the channel and second by activating the latch state mechanism in the cell. While cleaning out his garage of garden fertilizers, malathion insecticide, and herbicides, he becomes easily fatigued, especially while reaching up and outward to clean shelving. During his history and physical examination 2 weeks later, the patient reveals he’s had no sudden falls or injuries at any time prior to appearance of his symptoms nor did he experience symptoms solely while cleaning his garage. He does complain that he is becoming more short of breath while doing physical activity. His physical revealed that he had normal reflexes but had trouble abducting or holding out his arms for any length of time. When he tries to keep his eyelids open for extended time, his left lid starts to droop. When the patient tries to smile, his smile resembles a grimace though the patient is not in any pain. The myogram shows marked diminution of compound muscle action potentials within 12 minutes of stimulated electroshocks to the muscle of four stimuli per second (normal equals about 30 minutes). He instead immediately prescribes therapy that includes intermittent use of pyridostigmine for the patient’s symptoms. Based on the patient’s history and physical examination, postulate what type of potential causes were eliminated by the patient’s physician and why. Postulate what connection might exist between the patient’s physical and clinical test results and his symptoms. Similarly, this is not indicated by intact muscle reflexes as well as the fact that his symptoms are bilateral and involve more central muscles (eyelids, shoulders, neck). Thus, it is unlikely that the patient was exposed to toxic concentrations of malathion while cleaning his garage. This would also account for the patient’s fatigue, inability to sustain muscle contractions (arm abduction, eye lid lift), and the fact that his muscles weaken when stimulated repetitively. However, in a normal patient, stimuli of two to four cycles per second produce a 10% to 15% reduction in muscle activation after about 30 minutes of stimulation. The patient’s history, physical, and test results are consistent with the onset of myasthenia gravis, which generally appears suddenly without notable cause and affects muscle of the face and upper extremities and bulbar muscles. Explain how the blood lipid profile can be applied to assess cardiovascular health. Discuss the clinical usefulness of blood tests including the basic metabolic panel, complete metabolic panel, hematocrit, and complete blood cell count. Distinguish the different types of anemia and understand how these differences dictate the type of treatment protocols that would be used. Explain the function and clinical significance of erythropoietin as well as its involvement in blood doping. Understand how white blood cells form an integrated response to prevent and fight infections. Track the roles and components of the blood clotting phases, from immediate actions to wound healing. The viability and metabolism of the body’s cells are dependent on adequate perfusion by the blood. In the systemic circulation, arterial blood transports components necessary for maintaining a relatively stable and constant cellular environment (e. Because2 the heart is a dual pump, the blood flowing in the pulmonary arteries carries O -poor blood to the lungs2 for oxygenation, and the aorta carries the oxygenated blood to the systemic circulation (see Chapter 11). The components carried by the blood are delivered to individual cells as it passes through an extensive network of thin-walled capillaries. Blood also plays a role in maintaining body temperature and in fighting infection (see Chapter 10). Blood tests are common diagnostic tools to detect a wide variety of homeostatic imbalances. The cellular and plasma components of blood work in concert to perform four major roles: transport substances, regulate hemostasis, maintain a stable internal environment by regulation of temperature and pH, and aid in resisting infection or disease via the immune system. Transport As the primary means of long-distance transport in the body, blood carries an abundance of important substances including electrolytes, amino acids, sugar, proteins, lipids, minerals, hormones, and waste products. Depending upon the solubility of an individual substance, it can be transported as freely dissolved in solution (plasma), bound to a specific carrier protein (e. For example, about half of circulating calcium is in 2+ its free form (Ca ), whereas the other half is complexed to albumin or anions. Hemostasis Complex and efficient mechanisms have evolved to prevent blood loss from a damaged vessel. The failure to stop bleed after injury is called hemorrhage and can result in life-threatening blood loss if not controlled by the physiological mechanisms that constitute normal hemostasis. Homeostasis Homeostasis, as a physiological term, means maintaining a relatively steady state to create an optimal internal environment. The blood system plays a pivotal role in preserving homeostasis by maintaining pH and temperature. Plasma proteins form an immediately available buffer system to modulate the acid equivalents produced by most metabolic reactions. In addition, blood carries excess acid and base equivalents to organs such as the kidney and lungs for elimination. The rapidly circulating blood is an excellent conduit for transporting the heat generated by metabolic reactions and, therefore, plays a major role in thermoregulation by sequestering blood in the core as a result of vasoconstriction when ambient temperature is low or by dissipating heat by peripheral vasodilation when the environment is hot or the body has generated internal heat (e. Although the unbroken skin and mucous membranes act as barriers to the entry of infectious agents into the body, microbes can penetrate or bypass these frontline defenses. In most cases, the blood’s defense system is efficient enough to eliminate the pathogens or to prevent their spreading before they can cause substantial bodily harm.

Many may require long-term care at home wars or need to be looked after permanently in facilities insurgencies proven 250 mg keppra medications errors. Investigation Conscious patients without neck symptoms Clinical diagnostic routines have never been found to have an unstable cervical When examining the skull buy keppra toronto medicine 0025-7974, eye and orbit: spine or have subsequently progressed to develop neurological deterioration buy keppra medicine tablets. It is, therefore, unnec- inspect and palpate the cranium essary to go to great lengths to maintain spinal cord palpate the orbital margins protection in a conscious patient who has not had test visual acuity a head injury and does not complain of neck pain. There is no need for further investi- test the red reflex gations if all neck movements are full and pain-free check for diplopia and eye movements and there are no neurological signs in the limbs. Difficulties arise in the unconscious patient, espe- When examining the nose check for: cially if they are intubated and ventilated. In these circumstances the management options include: lacerations contusions Treat the cervical spine as if it is unstable until epistaxis the patient regains full consciousness. When examining the maxilla check for: A stiff collar is effective unless there is overt bleed- ing from a wound in the neck. Displaced unstable mid-face asymmetry, deformity or swelling injuries are treated by halo traction until surgical missing teeth stabilization can be undertaken (see Chapter 10). Oropharyngeal and nasopharyngeal airways may laceration over the point of the chin need to be inserted if the soft palate and tongue pain (pre-auricular tenderness) are displaced backwards by a posteriorly displaced swelling Le Fort fracture or a mandibular fracture. The dis- reduced jaw opening placement may have to be manually disimpacted malocclusion before an endotracheal tube can be passed. Imaging Occipital plain radiographs are useful for diag- nosing maxillary, zygomatic and orbital fractures. Nasal X-rays, plain X-rays and orthopantomo- graphs are required for fractures of the nose and (A) mandible. Posteroanterior plain X-rays of the mandible may be needed to reveal condylar fractures. Angiography may be used to diagnose and man- age major bleeding by therapeutic embolization of the external carotid artery or its branches. Once the airway has is no mobility or malocclusion but, if these are been secured, the source of haemorrhage can be present, open reduction and fixation by micro- confirmed by catheter angiography and stopped by plates is indicated. Complications of maxillofacial injuries Fractures The complications include: Most fractures are now treated by open explo- Airway obstruction at the time of injury or ration, reduction and fixation with microplates. Surgery should be carried out as soon as possible Malreduction causing facial deformity, (Fig 6. Untreated blowout fractures can tures of the maxilla and mandible can be made in cause persistent diplopia and ophthalmoplegia. The orbit can be reached through the Dental malocclusion can follow inadequate conjunctiva and fractures of the upper maxilla and treatment of maxillary and mandibular fractures. Nasal fractures These are usually treated by closed Infection (osteomyelitis is rare). Zygomatic fractures Undisplaced fractures do not Delayed union and non-union are rare but require treatment. If a broken tooth is found, a chest temporal reduction or, in complex or comminuted X-ray should be considered as it might have fractures, by open reduction and fixation with been inhaled. Undisplaced maxillary fractures These can be treated with analgesia and a soft diet. Closed reduction and external fixation Zone 1 is between the thoracic outlet and the is sometimes still used (Fig 6. All deep neck wounds used to be explored but Open reduction and internal fixation is required this approach has been challenged in recent years if the condyle is dislocated and associated with with improved imaging and endoscopy. These can be subdivided into penetrating, blast and A fractured rib may cause a pneumothorax blunt injuries. In conflicts, haemo thorax if a chest wall vessel (intercostal or Fractures of the ribs 133 Investigation Clinical diagnostic indicators Fractured ribs are suspected from the history of injury and pain in the chest wall on inspiration. Blood tests Blood gas levels should be measured if the patient is showing air hunger. Imaging An erect chest X-ray should be taken, with addi- tional rib views if there are areas of local tenderness, to confirm the diagnosis. Oral or intramuscular analgesics may be supplemented by local infiltration around the origins of the intercostal nerves, just below the fractures, with a long-acting local anaesthetic agent such as marcaine 0. Blood gas mon- internal mammary) is torn or a lung vessel pierced itoring, repeated chest radiographs and physiother- (Fig 6. An underlying lung contusion must always be Supplemental oxygen should be given by a mask. The pain may take several weeks to settle, even The loss of chest wall rigidity can cause severe res- in uncomplicated rib fractures, and may persist piratory distress (Fig 6. Management In the past, many patients were treated by endotra- cheal intubation and positive pressure ventilation until the fractured ribs began to unite, but it is now recognized that this is often overtreatment. In many cases conservative management with pain relief (see above), supplemental oxygen, physiotherapy and careful monitoring is sufficient, but intubation and ventilation may become indicated if the patient shows signs of increasing respiratory distress and respiratory failure (reduced oxygen saturation). A chest X-ray should be taken to confirm Prognosis The prognosis is usually good but a severe the diagnosis (Fig 6. Clinical diagnostic indicators A thoracostomy tube should be inserted if there Patients with a simple large closed pneumothorax are signs of respiratory distress. Small pneu- and a chest X-ray shows the lung to be fully mothoraces may cause no physical signs and are expanded. Sucking wounds must be covered with an occlu- Open pneumothoraces are caused by pene- sive dressing to prevent further air entering the trating injuries and may present with a sucking pleural cavity. Tension pneumothoraces must be treated A tension pneumothorax causes respiratory distress, urgently by the immediate insertion of a tube to Lung contusion (direct injury) 135 relieve the tension. In an emergency situation, out- side of hospital, any hollow tube will suffice, prefer- ably one whose open end can be held under water to prevent the re-entry of air. A few patients will develop a continuous air leak, which suggests there is major bronchial dam- age (a bronchopleural fistula) or the tube has been incorrectly placed in the lung. Applying suction to the underwater seal drain may close the leak, but some patients require open or endoscopic closure of the fistula by direct suture. A tracheal injury, a very rare event, may require direct surgical repair, preferably after endotracheal intubation. Both failing to keep up with the blood loss and surgery to are associated with a reduced air entry to the lung stop the bleeding becomes essential. A large haemothorax The bleeding may be coming from the chest may cause dyspnoea and be associated with the wall (intercostal or internal mammary arteries) or signs of hypovolaemic shock. Emergency The haemoglobin and the blood gas saturations thoracotomy with pneumonectomy can occasion- should be measured. Prognosis Most patients with a straightforward Imaging haemothorax recover completely, but a failure to An erect chest X-ray will confirm the diagnosis remove all the clotted blood can leave a layer of (Fig 6. In these cir- cumstances the layer of fibrin around the lung Management may have to be removed by an operation called Intravenous catheters should be inserted.

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A hernia measurement of a patient’s fluid balance and has may recur or a joint may continue to be painful buy keppra in india treatment variance. It not only allows measurement of the urinary Psychosis output but in the early postoperative phase avoids patients having to suffer the discomfort of getting Patients may undergo a subtle personality change out of bed to pass urine order keppra with visa treatment quotes. It also prevents post- or show features of mild dementia buy cheap keppra on line symptoms meaning, particularly operative retention, but may of course only delay it after cardiac bypass surgery. It may be caused by cell debris coming from pubic route may be better for males with symptoms the bypass system causing cerebral micro-infarcts. Urethral It may occur following other major surgery or catheterization is not without complications. A catheter is a potential route for the entry of infection, particularly in the Fatigue immunocompromised patient liable to hospital Patients feel tired after surgery. It is not psychological for there reduced by the use of catheters made of silicone. There is no treatment Methods used to monitor the acutely ill of proven efficacy but reducing surgical trauma by surgical patient employing minimally invasive techniques makes Urinary catheter recovery significantly more rapid. After open removal of the gall bladder the usual time to com- Central venous pressure catheter plete recovery is about 2 months. After the same Pulse oximetry operation done laparoscopically, full function usu- Measurement of cardiac output ally returns in 3 weeks. It reflects venous return and hence gives a Immediate basic guide to the adequacy of circulating volume Pneumothorax and haemothorax replacement. The catheter is inserted into the subcla- Arterial cannulation vian or internal jugular vein in the neck, and advanced into the superior vena cava where the pressure is Damage to the brachial plexus approximately equal to the right atrial pressure. A needle is inserted into the vein, often Thrombosis facilitated by ultrasound guidance. A guide wire is passed through the needle into Fracture and migration of the catheter the vein and the needle removed. The catheter is then passed over the guide wire, sometimes after the passage of a dilator. It has no value procedure and major complications may occur in diagnosing the cause of a change in oxygenation, which are occasionally fatal (Table 2. Full sterile techniques must be employed during its insertion, Measurement of cardiac output and the catheter connected to a closed system. Infection is more likely if the catheter is used to give One of the principal objects of peri/postoperative parenteral nutrition. Cardiac output indicates how well the heart is Sepsis should be suspected if the patient gives performing this function, and its accurate measure- any of the general indications of infection such ment is fundamental to cardiovascular assessment. The technique is falling out of favour because The pulse oximeter is a non-invasive method of during its insertion the catheter may rupture the determining oxygen saturation The device is essen- pulmonary artery and cause fatal cardiac tampon- tially a colorimeter. There is also no evidence that its routine use wavelengths whose absorption changes with the improves patient survival. It is entirely safe and widely used because it Ultrasound methods employ the Doppler allows continuous monitoring of both the pulse rate effect to measure arterial blood flow. Because the and arterial oxygenation and so gives an indication oesophagus is closely related to the descending 36 Principles and methods of management aorta, a probe passed orally can be positioned adja- and from the lungs during respiration. Although the method measures aortic be estimated/guessed but is usually assumed to be blood flow, this can be used to calculate the cardiac between 0. You do not diarrhoea treat a patient by taking multiple physiological sequestration into the bowel lumen when measurements: you collect the data to select the distended by obstruction or paralysis as in correct management (Table 2. Fluid balance Extracellular fluid depletion is commonly To maintain optimum body function, any fluid lost referred to as dehydration, a well-established must be replaced with the same volume of the same but highly inappropriate term because it actually composition. A patient losing more fluid than has been taken The extracellular fluid volume in the average in is said to be in negative fluid balance. When more adult is only 15L, and when a significant amount fluid has been given than has been lost the patient is in positive fluid balance. A patient severely depleted of fluid characteristic clinical features of: because of an intestinal obstruction may need many litres of crystalloid to restore their fluid balance. Potassium depletion Most of the body’s potassium is A traditional analogy is that between a plum and inside cells, but potassium depletion is common in prune, which is in fact a reasonable comparison, surgical patients because potassium is lost by vomit- although a more contemporary equivalent might ing, the aspiration of gastric secretions and diarrhoea. The colloquial expression ‘dry as a crisp’ is depletion, the accompanying aldosterone response also to the point. As only 2 per cent of body potassium is in the Fluid replacement extracellular fluid and circulation, measurement of serum potassium is not a good guide to potas- Table 2. Decide on clinical grounds which fluid has been lost Acid/base balance The body is very sensitive to Replace this fluid intravenously: changes in pH beyond the normal range of 7. There colloid with plasma expander must therefore be a balance between the gains and extracellular fluid with isotonic electrolytes losses of acid and base. Monitor the response by: Acids are gained from carbon dioxide, produc- tion from protein metabolism, loss of bicarbonate clinical assessment – the patient will feel and in the urine and faeces, and by the ingestion of acids look better in food. Acids are used in the metabolism of some routine measurement of pulse rate and blood organic anions and are lost in the urine. The instruments currently in use measure pH and the partial pressure of oxygen and A normal individual of about 70–80 kG (11–12 carbon dioxide. By using the Henderson–Hasselbach stones) requires about 2–3 L of fluid a day if they equation, a calculation is made of the base excess. Most of this will be Acidosis and alkalosis may be metabolic or replaced as isotonic 5% dextrose (2 L), and half a respiratory in origin, usually the former in the sur- litre of normal saline will provide enough salt. Analysis and treatment of acid/base 38 Principles and methods of management imbalance are complex and not considered further Parenteral feeding in this text. Parenteral feeding is required when the intestine cannot absorb food, either because a significant Nutrition length of it has been removed, it has been short- circuited or it has lost function. This is known to affect wound is changed regularly or through a long-term central healing and to suppress the immune response, but catheter. Commercially available solutions include it is surprisingly difficult to demonstrate its effect appropriate amounts of amino acids, lipids and on outcome. The tain the nutrition of a surgical patient who cannot complications include: eat normally. If the patient culture medium for bacteria is unable to eat normally other routes for nutrition metabolic problems: particularly electrolyte and must be sought, and it is a matter of clinical judge- acid/base imbalance and impairment of liver ment when to do so. Enteral feeding is to be preferred to parenteral Feeding may be enteral, where the gut is used, feeding whenever possible because: or parenteral, where nutrient solutions are given intravenously. Sometimes same amount of food substitute given enterally the gut itself is normal but the patient cannot swal- is more effective than the same amount given low, as with a patient undergoing mechanical venti- parenterally. On other occasions there will be obstruction or injury to the upper gastrointestinal tract. Access The significant problems and complications to the small bowel with a fine bore tube may be: listed above associated with parenteral feeding have been overcome in specialized units. Intravenous nasogastric lines made with non-irritant silicone material can nasojejunal, with X-ray guidance during function for many years if infection is avoided by placement rigorous aseptic regimens. Various isotonic liquid diets are available con- taining balanced proportions of protein, fat and car- Blood transfusion bohydrate, with vitamins and trace elements.

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Changes in myocardial contactilit can have important consequences on the oxygen requirement for basal metabolism discount 250 mg keppra amex medicine you can order online, isovolumic wall tension generation generic 500mg keppra with mastercard treatment x time interaction, and external work order keppra online treatment 5th metatarsal stress fracture. Heart muscle cells use more energy in rapidly developing a given tension and shortening by a given amount than in doing the same thing more slowly. Also, with increased contractility, more energy is expended in active Ca2+ transport. The net result of these influences is often referred to as the "energy wasting" effect of increased contractility. The heart rate is one of the most important determinants of myocardial oxygen consumption because the energy cost per minute must equal the energy cost per beat times the number of beats per minute. In general, it has been found that it is more effcient (ie, less oxygen is required) to achieve a given cardiac output with the low heart rate and high stroke volume than with the high heart rate and low stroke volume. This again appears to be related to the relatively high energy cost of the pressure development phase of the cardiac cycle. This is a remarkable, highly efcient, adaptable, and long-lasting pump that we, despite our best efforts, are unable to duplicate with any signifcant degree of success. As might be expected, support of this pump is highly dependent upon main­ tenance of coronary fow to the ventricular wall. In this book, we have ignored the extracellular structures of the heart, that is, the fbrous valves, the connective tissue frame (cardiac skeleton) that functions to electrically isolate the atria from the ventricles, and the extracellular matrix that forms a dynamic scaffolding surrounding the contractile cells. These structures are made primarily of collagen from the fbroblasts and not only maintain the structural integrity of the heart but appear to participate importantly in dynamic adaptations to changing conditions. There is current interest in the influence of the extracellular matrix components on cardiac behavior. Heart sounds associated with valve movements and detected on auscultation can be used to identify the beginnings of diastolic and systolic phases of the cardiac cycle. The events of a single ventricular cardiac cycle can be displayed as records of elec­ trical, mechanical, pressure, sound, or fow changes against time or as a record of volume against pressure. Cardiac output is defned as the amount of blood pumped by either of the ventri­ cles per minute and is determined by the product of the heart rate and stroke • volume. Stroke volume can be altered by changes in ventricular preload (flling), ventricular afterload (arterial pressure), and/or cardiac muscle contractility. A cardiac function curve describes the relationship between ventricular flling and cardiac output and can be shifted up (left) or down (right) by changes in sympa­ • thetic activity to the heart or by changes in cardiac muscle contractility. Energy for cardiac muscle contraction is derived primarily from aerobic metabolic pathways such that myocardial oxygen consumption is tightly related to cardiac • work. If pulmonary artery pressure is 24/8 mm Hg (systolic/diastolic), what are the respective systolic and diastolic pressures of the right ventricle? Because pulmonary artery pressure is so much lower than aortic pressure, the right ventricle has a larger stroke volume than the left ventricle. In which direction will cardiac output change if central venous pressure is lowered while cardiac sympathetic tone is increased? Increases in sympathetic neural activity to the heart will result in an increase in stroke volume by causing a decrease in end-systolc volume for any given end­ diastolic volume. Four of these conditions exist during the same phase of the cardiac cycle and one does not. Some of these are noninvasive (eg, auscultation of the chest to evaluate valve function, electro­ cardiography to evaluate electrical characteristics, and various imaging techniques to assess mechanical pumping action) and others require some invasive instru­ mentation. This chapter provides a brief overview of some of these commonly used clinical tools. Visual or computer-aided analysis of such images provides information useful in clinically evaluating cardiac function. Tey can also provide estimates of heart chamber volumes at diferent times in the cardiac cycle that are used to assess cardiac function. Echocardiogaphy is the most widely used of the cardiac imaging techniques cur­ rently available. This noninvasive technique is based on the fact that sound waves refect back toward the source when encountering abrupt changes in the density of the medium through which they travel. A transducer, placed at specifed loca­ tions on the chest, generates pulses of ultrasonic waves and detects refected waves that bounce off the cardiac tissue interfaces. The longer the time between the transmission of the wave and the arrival of the refection, the deeper the structure is in the thorax. Such information can be reconstructed by computer in various ways to produce a continuous image of the heart and its chambers throughout the cardiac cycle. Doppler echocardiography can provide additional information about blood flow velocity and direction across the cardiac valves. Cardiac angography involves the placement of catheters into the right or left ventricle and injection of radiopaque contrast medium during high-speed x-ray flming (cinera­ diography). A gamma camera is used to obtain images collected at (ie, gated to) different times in the cardiac cycle. End-Systolic Pressure-Volume Relationship The end-systolic pressure-volume relationship can be used to assess car­ diac contractility. End-systolic volume for a given cardiac cycle is esti­ mated by one of the imaging techniques described above, whereas end-systolic pressure for that cardiac cycle can be obtained from the arterial pres­ sure recorded at the point of closure of the aortic valve (the incisura). Values for several diferent cardiac cycles may be obtained during infusion of a vasoconstric­ tor (which increases afterload), and the data plotted as in Figure 4-1 in the context of overall ventricular pressure-volume loops. As shown, increases in myocardial contractility are associated with a leftward rotation in the end-systolic pressure­ volume relationship. Decreases in contractility (as may be caused by heart disease) are associated with a downward shift of the line, discussed further in Chapter 11. Tis method of assessing cardiac function is particularly important because it pro­ vides an estimate of contractility that is independent of the end-diastolic volume (preload). The effect of increased contractility on the left ventricular end-systolic pressure-volume relationship. Tus, only alterations in con­ tractility will cause shifts in the end-systolic pressure-volume relationship. Thus, it is possible to get a reasonable clinical estimate of the slope of the end-systolic pressure-volume relationship (read "myocardial contractility") from a single measurement of end-systolic pressure and volume. This avoids the need to do multiple tests with vasodilator or vasoconstrictor infusions. Measurement of Cardiac Output Pickprncile: The most accurate (but unfortunately somewhat invasive) way of measuring how much blood is actually pumped by the heart per minute is by the use of the Fick principle described in Chapter 1. Recall that the amount of a substance consumed by an organ or tissues, xc, is equal to what goes in minus what goes out, which is the arterial-venous concentration dif­ ference in the substance ([X. Generally, the sample for mixed venous blood oxygen measurement must be taken from venous catheters positioned in the right ventricle or the pulmonary artery to ensure that it is a well­ mixed sample of venous blood from alsystemic organs. The calculation of cardiac output from the Fick principle is best illustrated by an example. Suppose that a patient is consuming 250 mL of02 per minute when his or her systemic arterial blood contains 200 mL of 02 per liter and the right ventricular blood contains 150 mL of02 per liter. This means that, on an aver­ age, each liter of blood loses 50 mL of02 as it passes through the systemic organs. In order for 250 mL of 02 to be consumed per minute, 5 L of blood must pass through the systemic circulation each minute: 250 mL Ozfmin 200 - 150 mL OiL blood Q = 5 L blood/min Indicator dilution techniques: Another method of estimating cardiac out­ put is to determine how much a given substance is diluted by the blood that passes through the heart in a given period of time.