Cardiac structures

Myocardial anatomy and physiology

The heart wall consists of three major tissue layers. These are the endocardium, a thin layer of endothelial cells in direct contract with the blood, the myocardium, the middle layer of the heart and the epicardium, the outer layer of the heart comprised of the visceral pericardium.

The heart is divided into the right and left side by a muscular structure known as the septum. The function of the right heart is to deliver unoxygenated blood from the body to the lungs. The left heart delivers oxygenated blood from the lungs to the body.

Blood enters the right side of the heart via the superior vena cava and the inferior vena cava. Blood leaves the right heart via the pulmonary artery. Blood enters the left heart via four pulmonary veins and leaves via the aorta. The right side of the heart has thinner musculature because it protects its volume against minimal resistance in the pulmonary circulation (PVR) The left side of the heart has thicker musculature because it protects its volume against greater resistance in the peripheral circulation (SVR).

Each side of the heart has two sets of valves. These valves separate and further divide each side of the heart into the receiving chamber (atrium) and the ejecting chamber (ventricle).

On the right we have the atrioventricular (tricuspid) and the semilunar (pulmonary)

valves. On the left we have the atrioventricular (mitral/bicuspid) and the semilunar

(aortic) valves. Both atrioventricular valves are supported by chordae tendonae

which are attached to papillary muscles. The papillary muscles extend from the

ventricular wall and contract with the ventricular wall allowing valve closure.

The semilunar valves separate the ventricles from their outflow vessels. The

semilunar valves appear cup like, each possessing three cusps, without any

supporting structures. The aortic valve contains the origin of the coronary arteries.

Selected blood tests

Haemoglobin

Normal values Female 12.0-16.0 g/dl Male 14.0-17.0 g/dl

Haemoglobin serves as the vehicle for the transportation of oxygen and carbon dioxide. It also serves as an important buffer in the extracellular fluid. Haemoglobin determination is part of a complete blood count.

Clinical implications include

· decreased haemoglobin is found in anaemia, hyperthyroidism, cirrhosis of the liver, severe haemorrhage, haemolytic reactions and various systemic diseases

· increased haemoglobin is found in chronic obstructive pulmonary disease, congestive heart failure and polycythemia vera

· variance in levels of haemoglobin occurs after transfusions and haemorrhage

· haemoglobin and HCT give valuable information and should always be interpreted in conjunction with other laboratory findings

Red blood cell indices

These indices define the size and haemoglobin content of the red blood cell. On the basis of the red blood cell indices, the erythrocytes can be characterised as normal or deficient (macrocytic, normocytic)

Normal values MCV 82-98 f l The MCV results are a basis of classification used in an evaluation of anaemia.

Red Blood Cell Count

Normal values Men 4.2-5.4 Female 3.6-5.0

Clinical implications

· decreased RBC values occur if there is a reduction in the number of circulating red blood cells, in the amount of haemoglobin or in the value of packed cell volume (HcT)

· decreased RBC values also occur in destruction of red cells (CPB and mechanical valve prosthesis)

· relative decrease in RBC values can be due to dehydration, overuse of diuretics, stress

Complete blood picture

A basic screening test which gives valuable information about the haematological and other body systems, prognosis, response to treatment and recovery. The CBP consists of a series of tests that determine the number, variety, percentage, concentration and quality of blood cells (WBC, Differential, RBC, HCT, Hb, MCV, Platelet count)

White blood cell count

Normal value 5-10ml

WBC or leukocytes fight infection and defend the body by phagocythosis in which the leukocytes actually encapsulate foreign organisms. Specific patterns of leukocyte response can be expected in different types of diseases.

· the differential count identifies persons with increase susceptibility to infection

· leukocytosis occurs in acute infections in which the degree of increase of WCC depends on the severity of the infection

· leukopenia occurs in overwhelming sepsis

Platelet count

Normal values 140,000- 400,000

Platelet activity is necessary for blood clotting, vascular integrity and vasoconstriction and clot formation. The life span of a platelet is about 7.5 days. Abnormal platelet count and function can be due to sepsis, cardio pulmonary bypass, disseminated intravadscular coagulopathy, haemorrhage, liver disease, use of certain drugs and aspirin.

APTT

Activated partial thromboplastin time Normal values 21-35 seconds

APTT is prolonged due to heparin therapy, vitamin K deficiency, liver disease and DIC. APTT > 100 secs signifies spontaneous bleeding

ACT

Automated coagulation time is a test carried out to ascertain whether intraoperative heparin has been adequately reversed. Normal values 140-150 seconds. A routine bedside procedure carried out in instances of UWSD blood loss > 200 mls/hr and for those patients receiving pump blood transfusion in the recovery setting. Reversal with protamine can be titrated immediately according to the ACT results (25 mg Protamine Sulphate for ACT 150 – 200 sec and 50 mg Protamine Sulphate for ACT > 200 sec). Routinely used in coronary artery bypass procedure and PTCA.

Prothrombin time

Recommended INR (international normal ratio) in AF 2.5, in mechanical valves 3.5.

Routinely carried out on patients requiring warfarin therapy. It is essential that the INR result is interpreted prior to the next dose and titrated accordingly. The INR is measured daily until a therapeutic range is reached. On discharge from hospital the INR may be measured only once every 3-6 months. Prothrombin is a protein, produced by the liver that acts in the clotting of blood. Prothrombin production depends on adequate vitamin K intake and absorption. During the clotting process, prothrombin is converted to thrombin.

DIC

· Disseminated intravascular coagulation is an acquired haemorhagic syndrome characterised by uncontrolled formation and deposition of fibrin. Causes include

· septiceamia

· cirrhosis of the live

· incompatible blood transfusion.

Paradoxically treatment for uncontrolled bleeding is heparin administration. The heparin blocks thrombin formation, which then blocks consumption of the other clotting factors and results in haemostasis. There are 5 primary screening tests

· platelet count

· bleeding time

· PTT

· PT

· fibrinogen level

Magnesium

Normal values 1.3 – 2.1 mEq/l

Decreased levels evidenced in

· prolonged P-R and Q-T intervals

· flattening of T-waves

· premature ventricular contractions

· ventricular tachycardia and fibrillation

Potassium

Normal value 3.5 – 5.0 mEq/l

Potassium is the principle electrolyte of intracellular fluid and the primary buffer within the cell itself. 90% is intracellular. The kidneys do not preserve potassium. Potassium plays a major role in nerve conduction

· muscle function

· acid-base balance

· osmotic pressure.

Along with calcium and magnesium, potassium controls the rate and force of the heart and therefore cardiac output. Values of < 2.5-mEq/l lead to ventricular fibrillation > 75-mEq/l lead to sine waves. In the cardiothoracic setting decreased levels are associated with

· shifting of potassium into the cell due to IV fluid administration without potassium supplements

· cardiopulmonary bypass

· stress.

Increased levels are associated with

· renal failure

· cell damage

· metabolic acidosis

Sodium

Normal values 135-145 mEq/l

Hyponatraemia reflects a relative excess of body water, <120-mEq/l weakness, and 90-105 mEq/l severe neurologic symptoms. Hypernatraemia primarily reflects dehydration, >155 mEq/l cardiovascular and renal symptoms, >160 155 mEq/l heart failure.

Creatinine

Normal values 0.6 – 1.2 mg/dl

A disorder of kidney function reduces excretion of creatinine (by-product of metabolism), resulting in increased blood levels. Increased levels occur in

· impaired renal function

· congestive heart failure

· shock

· dehydration

· rhabdomyolysis.

Elevated creatinine levels have implications for nephrotoxic medications.

Drug monitoring

Therapeutic drug monitoring is a reliable and practical approach to managing patients’ drugs therapy. Determination of drug levels is especially important when the potential for drug toxicity is significant or when inadequate or undesirable response follows the use of a standard dose. It provides an accurate means of estimating drug requirements. For some drugs, monitoring is routinely useful (digoxin, theophylline, warfarin) and for others it can be helpful in certain situations (antibiotics in the presence of renal failure)

Blood cultures

Normal value negative for pathogens

Indications

· bacteraemia

· septicaemia

· post-operative shock

· unexplained fever > 38°C.

If suspecting endocarditis, take three separate blood cultures 2hrs apart. Handle blood according to universal precautions. After disinfecting do not palpate the venipuncture site. Change needles once after collecting sample, prior to filling the aerobic bottle and again prior to filling the anaerobic bottle. If all cultures and gram stains are negative, the blood culture may be reported as a no growth after a three-day incubation.

Urine culture

Normal value negative for pathogens

Urine specimen for culture should be taken from the designated port on the indwelling catheter drainage system after disinfecting the port with alcohol and allowing it to dry.

At least 3-5 mls should be taken and transported to the lab ASAP.

Sputum cultures

Normal value negative for pathogens

A sputum specimen comes from deep within the bronchi and is collected using an inline sputum trap and a non-touch technique. Do not refrigerate and transport to the lab ASAP

Wound cultures

Clinical specimens taken from wounds can harbour a number of pathogens. Pathogenicicity is dependent on the number of pathogens present. Clinically significant pathogens are likely to be located in pus, necrotic tissue, wound discharge and pleural fluid and should be taken if wounds look inflamed or exudate present.

Cardiac enzymes

CK is an enzyme found in greater concentration in the heart and skeletal muscle and is used as a specific index of injury to the myocardium. With myocardial infarction the rise starts about 4-6 hours post attack and reaches the peak at 24 hours. CK, AST and CK-MB peak at 24 hrs, LDH peaks at 48 hrs. Because of its late peak at 48 hrs LDH may not be necessary if CK levels were sent promptly. More recently Troponin T and Troponin I have been used as markers of myocardial injury. (Troponin 1> 1.9 and Troponin T > 0.02 indicate AMI).

doctrine of medicine

Wednesday, September 2, 2009