Perfusion PDF

What is perfusion? In simple terms, perfusion is the flow of blood through a particular capillary bed within an organ or tissue. Therefore, tissues can be adequately perfused or inadequately perfused, depending on the amount of blood flowing to them. However, in practice, it is not this simple. Given...

What is perfusion? In simple terms, perfusion is the flow of blood through a particular capillary bed within an organ or tissue. Therefore, tissues can be adequately perfused or inadequately perfused, depending on the amount of blood flowing to them. However, in practice, it is not this simple. Given that the role of the circulatory system is to deliver oxygen and nutrients (at the same time as removing waste products), it becomes evident that cells receiving adequate circulation but blood which is depleted of oxygen or nutrients (e.g. due to hypoxaemia or hypoglycaemia) would not function effectively. Consequently, in this chapter we will discuss the interplay that exists between the circulatory, respiratory and digestive systems to allow adequate perfusion with quality blood, as well as provide examples of how inadequacies in these systems can lead to adverse outcomes (see Table 2.1). Let's start by considering the factors that lead to normal perfusion. Table 2.1 Factors affecting delivery of oxygen and nutrients to cells Factors leading to normal perfusion The continuous circulation of oxygen and nutrient-rich blood in a healthy individual requires a series of well-orchestrated events across multiple organ systems as explained in Figure 2.1. Let's commence our discussions by considering the blood returning to the right side of the heart from the body (systemic circulation). This blood is oxygen-poor (deoxygenated) and must be pumped to the lungs. FIGURE 2.1 Factors affecting perfusion quality. The lungs With each breath we take, air is drawn into the alveoli of the lungs, allowing diffusion of oxygen into the red blood cells (RBC) that are concurrently passing through the adjacent alveolar capillaries. It takes an RBC approximately 0.75 seconds to pass through the alveolar capillary at rest, but only 0.25 seconds for diffusion of gases to occur (West & Luks, 2016). As you can see, in a healthy individual, gas exchange is efficient! The blood Once oxygen diffuses into the blood it passes into RBCs and binds with haemoglobin (Hb). The amount of oxygen capable of being transported by the cardiovascular system is dependent on the amount of Hb present in the blood (Marieb & Hoehn, 2016). Oxygenated blood is returned to the left side of the heart for subsequent delivery to the systemic circulation. The heart The left ventricle of the heart pumps oxygenated blood to the tissues via the systemic circulation. Blood flow through this system is dependent on the difference in pressure within vessels across two points (i.e. the pressure in the arteries is greater than the veins, therefore driving blood flow). However, pressure itself is dependent on the resistance to blood flow as well as the amount of blood carried in the vessels. As a result, the left ventricle must generate large amounts of pressure in the arteries to promote efficient circulation of blood. In order to do this, the heart can beat faster (increasing heart rate) or stronger (via increasing the force of contraction which increases the volume of blood ejected with each contraction; this is known as stroke volume). Alterations in either heart rate or stroke volume lead to changes in the volume of blood pumped from the left ventricle in each minute (known as cardiac output): cardiac output (CO) = heart rate (HR) × stroke volume (SV). The blood vessels Oxygenated blood leaving the heart travels via arteries and arterioles to reach the capillaries: the site of gas and nutrient exchange at the level of the tissues. Once nutrients have been offloaded and waste collected, deoxygenated blood travels back to the heart via the veins. The level of perfusion for each organ is tightly regulated to maintain homeostasis. Smooth muscle in the walls of the arterioles can constrict or dilate the vessel, increasing or decreasing resistance and thereby directing the flow of blood to areas in need. The liver Although we have spoken about blood flow to different organs for the purposes of maintaining homeostasis, some tissues also contribute to blood via the release of hormones or nutrients (in addition to waste). For example, the liver receives oxygenated blood from the systemic arterial circulation but also receives nutrient-rich blood directly from the gastrointestinal tract (Marieb & Hoehn, 2016). Depending on the demands of the body, the liver responds to pancreatic hormones by either extracting glucose from the blood and subsequently storing it for later use, or alternatively, releasing stored glucose back into the bloodstream to provide energy for cellular processes. The cells At the level of the tissues, oxygen diffuses across the capillary walls, through the interstitial space and into the cells. With the help of insulin (released by the pancreas), glucose also enters the cells, allowing metabolic processes to continue. Without oxygen and glucose, cells are unable to function, therefore demonstrating the critical need for adequate tissue perfusion. REFLECTIVE BOX Exercise represents a physiological stress to your body. Consider how your body responds when you exercise. What happens to your heart rate, rate and depth of breathing, and blood flow to the skin? How do these changes maintain blood flow and oxygenation? How would your glucose levels be maintained in the absence of additional food sources? Disturbances of perfusion Owing to the complexity of the cardiovascular, respiratory and digestive systems, multiple factors must align to allow for adequate perfusion of tissues. Disturbances in any of these factors will result in decreased supply of oxygen and glucose to the tissue, as well as poor removal of waste (see Table 2.1). The lungs The ability of the lungs to provide a continuous supply of oxygen to the blood is an essential component of maintaining normal cell function. A decreased ability to oxygenate the blood can occur because of problems with ventilation (the ability to move gas in and out of the alveoli), external respiration (the ability of oxygen to diffuse from the alveoli to the blood) or both. Issues with ventilation include: choking and airway obstruction narrowed airways due to asthma, croup and anaphylaxis an inability to inspire due to spinal cord injury or physical damage of the chest wall or diaphragm. Issues with external respiration include: decreased surface area for gas exchange (e.g. chronic obstructive pulmonary disease) increased distance for oxygen diffusion (e.g. acute pulmonary oedema) altered pulmonary blood supply (e.g. pulmonary embolism) changes to atmospheric pressure that occur with diving and aviation (i.e. altitude). The blood Decreases in blood volume (hypovolaemia) or haemoglobin content (anaemia) result in inadequately perfused tissue. Causes of hypovolaemia include: injuries that cause significant bleeding (burns, musculoskeletal injuries, postpartum haemorrhage) conditions that promote movement of fluid from blood vessels to the interstitial space, thereby decreasing circulating blood volume (burns, anaphylaxis). Causes of anaemia include: diseases that cause a decrease in RBCs (kidney disease, B12 deficiency, iron deficiency) undetected haemorrhage such as a persistent bowel bleed. The heart The critical role of the heart in maintaining cardiac output, and therefore the pressure to drive blood flow, is dependent on its ability to regulate heart rate and achieve adequate stroke volume (determined by contractility, ejection fraction and venous return). Disturbances to heart rate Under normal circumstances, heart rate is tightly controlled via the autonomic nervous system to maintain homeostasis. While alterations in parasympathetic/sympathetic innervation have marked effects on heart rate, circulating hormone (e.g. adrenaline, thyroxine) levels, ion concentrations (e.g. Ca2+, K+) and temperature (e.g. fever) also play a role in determining heart rate. Abnormally fast (tachycardia) or slow (bradycardia) heart rates can lead to decreased cardiac output. Bradycardia. Significant decreases in heart rate result in insufficient blood being pumped into the circulatory system per minute (i.e. decreased cardiac output), thereby decreasing delivery of oxygen and glucose. Tachycardia. Significant increases in heart rate (e.g. 200 bpm) reduce the time for the ventricles to fill, thereby reducing the volume of blood available for ejection with each contraction. Consequently, stroke volume is decreased, and despite the high heart rate, cardiac output can be compromised. In the absence of compensatory mechanisms, reductions in cardiac output ultimately lead to decreased perfusion. Disturbances in stroke volume Stroke volume is determined by subtracting the volume of blood within the ventricle at the end of ventricular systole (contraction) from the volume of blood present prior to contraction (i.e. at the end of diastole when maximal filling has occurred); this represents the volume of blood ejected with each ventricular contraction (Marieb & Hoehn, 2016). For maximal efficiency, the heart must beat in a coordinated fashion. Injury to the tissues of the heart, changes in venous return or systemic blood pressure can all lead to decreases in the efficiency of ventricular contraction. REFLECTIVE BOX Approximately 30% of Australians over the age of 25 have hypertension (McCance & Huether, 2015). Consider what would happen to afterload in the case of high blood pressure. How would this affect stroke volume? What compensatory mechanisms might be required to maintain cardiac output in these patients? Alterations in rhythm. Injury to the heart myocardial tissue or pacemaker cells can alter its regular rate and rhythm, as well as its ability to contract in a coordinated fashion, thus leading to decreased stroke volume and cardiac output (e.g. atrial fibrillation, atrial flu er). Decreased preload. Reductions in the volume of blood returning to the heart (e.g. decreased blood volume; decreased filling time; obstruction of venous return; vasodilation due to anaphylaxis) not only alter the volume of blood available for ejection with each contraction, but also decrease the contractility of the heart, resulting in a less forceful ventricular contraction and a lower percentage of ventricular volume being ejected (known as ejection fraction). Decreased contractility. In addition to a decreased preload (described above), decreased sympathetic innervation and altered ion (K+, Ca2+) or hormone levels (e.g. thyroxine, adrenalin) can all have effects on the force generated by the heart with each contraction. Increased afterload. Afterload refers to the pressure in the great arteries which the ventricle must overcome in order to open the aortic/pulmonary valve to eject blood. In cases of hypertension, afterload increases, leading to decreased ejection fractions. The blood vessels The layer of smooth muscle present in the walls of blood vessels (in particular arteries and arterioles) provides a mechanism by which the diameter of the blood vessels can be altered (Marieb & Hoehn, 2016). Contracting the smooth muscle allows the vessel to constrict, increasing resistance to blood flow and maintaining pressure in the case of fluid (blood) loss. In contrast, relaxation of the smooth muscle layer allows the vessel to dilate. Both vasoconstriction and vasodilation are regulated by the autonomic nervous system. However, the smooth muscle layer is also responsive to hormones such as adrenaline and substances released during the inflammatory process. Causes of vasoconstriction include: sympathetic nervous system innervation adrenaline, noradrenaline release. Causes of vasodilation include: decreased sympathetic innervation inflammatory substances such as histamine (anaphylaxis) and nitric oxide drugs such as glyceryl trinitrate and morphine low levels of oxygen and/or high levels of CO2 in the systemic system, and high levels of oxygen in the pulmonary system. The liver In combination with the pancreas and the gastrointestinal (GI) system, the liver plays a vital role in determining blood glucose levels. Consequently, injury or disease affecting any one of these three organs can result in disturbances of glucose levels. The pancreatic hormones insulin and glucagon determine the uptake or release of glucose by the liver respectively. Diabetes and pancreatitis affect the amount of insulin and/or glucagon released from the pancreas but can also affect tissue sensitivity to these hormones. Intentional or accidental mismanagement of diabetic medications can cause hypoglycaemia. Chronic liver and GI diseases do not commonly present as health emergencies but they may complicate the diagnosis and management of other health emergencies. Assessment of perfusion The body's ability to compensate for inadequate perfusion poses a number of problems for the paramedic. At the extreme end of the scale—no perfusion—the clinical picture is clear: the patient will be unconscious, pulseless and cool to touch. However, the speed at which this state develops, and the range of symptoms that present as the patient passes from adequate perfusion to no perfusion, are less clear. Nearly all perfusion assessment tools use a combination of blood pressure, heart rate, skin and conscious state to categorise the severity of the condition. Of all of these, blood pressure can be the least reliable indicator of perfusion status. For example, patients who are normally hypertensive can suffer inadequate perfusion even when their systolic and diastolic pressures are within normal range. Without a pre-event measure of blood pressure, it can be difficult to evaluate the significance of a reading during a health crisis (however, an initial recording is useful in se ing a baseline with which subsequent measurements can be compared). The body's response of raising the heart rate and diverting blood flow to essential organs also 'protects’ the blood pressure until there is no more capacity for compensation. Hypotension is therefore generally considered a late sign of inadequate perfusion. An early sign of inadequate perfusion is often an increased heart rate (Craft et al., 2015). In response to sympathetic innervation, the increase in heart rate can support cardiac output in the presence of decreased blood volume (and therefore, decreased stroke volume). In the field, this tachycardia needs to be distinguished from anxiety and a response to pain; consequently, matching the tachycardia with the broader clinical picture is important. Assessing the skin provides additional information; cool, pale and clammy skin indicates shunting of the blood from non-essential organs (due to sympathetic vasoconstriction) and is a reliable indicator that the tachycardia is not simply a response to pain but more likely to be due inadequate perfusion. Conscious state is the last of the four perfusion indicators that are usually assessed. In the se ing of hypovolaemia, an altered conscious state is a late indicator, as the brain preferentially receives blood as the body responds to blood loss. However, the brain is sensitive to hypoxia and hypoglycaemia. Changes in these parameters often alter the conscious state long before changes occur to the skin. Remember, though, that the body is adept at maintaining its perfusion and the progression from normal to extremely ill may not be apparent in a single assessment. It will, however, become obvious if a series of accurate assessments are performed over time. This highlights the importance of undertaking regular observations for any patient you suspect could develop perfusion problems. Principles of medical management of perfusion With an understanding of the causes of inadequate perfusion, combined with an accurate patient assessment, paramedics should be able to effectively identify and manage patients with perfusion issues. Ventilate As per rule 1 (air must go in and out) and the primary survey, always ensure the patient is ventilating adequately before you start looking for other issues. For example, hypoxia will initially raise the heart rate but, if prolonged, the heart rate may slow and concurrently reduce perfusion. Ensuring adequate ventilation can be as simple as positioning the patient or providing jaw support. Alternatively, it can be as complex as decompressing a tension pneumothorax. Do not move to the circulation (C) until you have resolved any issues with airway (A) and breathing (B). The heart Disturbances of heart rate and rhythm should be identified during the vital signs survey and, if associated with inadequate perfusion, managed as a priority. Intensive care paramedics across Australia and New Zealand carry a range of antiarrhythmic medications such as atropine, adrenaline, verapamil, amiodarone and adenosine. The use of external defibrillators to electrically revert dangerous arrhythmias and conduct external pacing is also common practice. In addition, intensive care paramedics administer a range of medications to improve ventricular contractility. This group of medications (known as inotropes) can increase the ejection fraction and improve cardiac output. The liver The brain has a high metabolic demand and no mechanism for storing significant amounts of glucose or oxygen (McCance & Huether, 2015). As a result, the conscious state decreases rapidly if supplies of either glucose or oxygen are compromised. It is unusual for inadequate blood supply to be the cause of the problem when a patient is unconscious but has palpable distal pulses. If you have resolved any ventilation issues and have an unconscious patient with distal pulses, consider whether the blood being delivered to the brain is carrying sufficient glucose. Volume, inotrope, pressor (VIP) Restoring lost blood volume with intravenous fluids is widely supported by ambulance guidelines across Australia and New Zealand. If other causes of inadequate perfusion have been eliminated or treated and the patient remains poorly perfused, replacing lost volume is the first step in managing the patient. In most se ings an initial dose of 20 mL/kg of isotonic crystalloid is recommended, followed by a repeat if the patient remains inadequately perfused. The aim is to increase preload back to an optimum level, thus ensuring that the heart receives optimum filling and stretch pressures, thereby restoring contractility and stroke volume. If the patient remains inadequately perfused despite improved filling and pressures, the standard treatment is to commence inotrope therapy. Adrenaline, noradrenaline or dopamine all promote varying degrees of vasoconstriction and increase circulatory pressures but should not be commenced until normal intravascular volumes have been restored. In patients with low or absent vascular tone, normal preload will not be restored by volume alone and vasopressors (adrenaline, noradrenaline or even metaraminol) have to be used to improve venous return. This 'fill then squeeze’ approach aims to ensure there is sufficient circulating volume before vasoconstriction restricts blood flow to some organs. Paramedics in Australia and New Zealand do not commonly use selective pressors such as metaraminol. Unlike the inotrope family of drugs, which have effects on both the heart and the blood vessels, the pressors produce only potent vasoconstriction. There are a number of controversies surrounding the best fluid to use to restore perfusion (isotonic, hypertonic, crystalloid, colloid), how much should be administered and even whether it should be administered if there is a suspicion the patient may still be bleeding. This area is likely to see significant changes in guidelines over the next decade as data is collected and assessed. Summary The term 'perfusion’ describes the ability of the cardiovascular system to supply the body's cells with adequate oxygenation and nutrition, while also removing wastes. Perfusion is often used synonymously with the terms 'pressure’ or 'blood flow’, but it is important to keep in mind that neither of these terms is adequate in describing the critical requirements of the body's cells for oxygen and glucose. Assessing perfusion involves a number of factors including heart rate, blood pressure, conscious state and circulation to the skin. Regardless of the cause, inadequate perfusion usually triggers a sympathetic nervous system response. However, there is no heart rate or blood pressure that is exclusively indicative of adequate/inadequate perfusion; individual anatomy and physiology can alter the delivery of nutrients to cells as pressures fluctuate. Consequently, rates and pressures outside the normal range should always be considered pathological until proven otherwise.