Effect of Heart Rate on Duration of Cardiac Cycle

Cardiac Cycle

The cardiac events that occur from the beginning of one heartbeat to the beginning of the next are called the cardiac cycle. Each cycle is initiated by spontaneous generation of an action potential in the sinus node. This node is located in the superior lateral wall of the right atrium near the opening of the superior vena cava, and the action potential travels from here rapidly through both atria and then through the A-V bundle into the ventricles. Because of this special arrangement of the conducting system from the atria into the ventricles, there is a delay of more than 0.1 seconds during passage of the cardiac impulse from the atria into the ventricles. This allows the atria to contract ahead of ventricular contraction, thereby pumping blood into the ventricles before the strong ventricular contraction begins. Thus, the atria act as primer pumps for the ventricles, and the ventricles, in turn, provide the major source of power for moving blood through the body’s vascular system.

Shows the different events during the cardiac cycle for the left side of the heart. The top three curves show the pressure changes in the aorta, left ventricle, and left atrium, respectively. The fourth curve depicts the changes in left ventricular volume, the fifth the electrocardiogram, and the sixth a phonocardiogram, which is a recording of the sounds produced by the heart-mainly by the heart valves-as it pumps. It is especially important that the reader study in detail this figure and understand the causes of all the events shown.

When heart rate increases, the duration of each cardiac cycle decreases, including the contraction and relaxation phases. The duration of the action potential and the period of contraction (systole) also decrease, but not by as great a percentage as does the relaxation phase (diastole). At a normal heart rate of 72 beats/min, systole comprises about 0.4 of the entire cardiac cycle. At three times the normal heart rate, systole is about 0.65 of the entire cardiac cycle. This means that the heart beating at a very fast rate does not remain relaxed long enough to allow complete filling of the cardiac chambers before the next contraction.

Pharyngitis – Basic Information, Clinical Presentation, And Its Treatment

Basic Information

• Outpatient visits for pharyngitis account for 1% to 2% of all office visits.
• Most common cause is viral in adults, with a much higher incidence of group A streptococci in children.
• Some cases may be associated with evidence of more systemic infection (Epstein-Barr virus [EBV], acute HIV infection); severe pharyngitis (Neiserria gonorrohoeae or group A β-hemolytic streptococci [GABHS]); adult patients with a history of rheumatic fever managed differently, with lower threshold for prescription of antibiotics.
  1/ Viral infection causes most adult cases (~80%).
  2/ Rhinovirus (20%) is most common viral cause.
  3/ Coronavirus (5–10%), adenovirus (5%), herpes simplex (2– 4%) less common.
  4/ Uncommon causes include parainfluenza (2%), influenza (1%), EBV (<1%), cytomegalovirus (<1%), acute HIV type 1 (<1%), coxsackievirus (<1%).
• Bacterial infection less common in adults than children.
• Streptococcus pyogenes most common bacterial cause in adults (5–10%).
  1/ Other streptococci less common, usually group G or C.
  2/ Rare bacterial causes include N. gonorrhoeae (<1%), Corynebacterium diphtheriae (<1%), Arcanobacterium haemolyticum (often associated with rash <1%), Chlamydophila pneumoniae (1%), Mycoplasma pneumoniae (<1%).
• Many cases unknown and presumed viral.
  
  
  
  

Clinical Presentation

• Typical presentations include sore throat and malaise with possible fever or cervical lymphadenopathy.
• Severe sore throat with inability to swallow secretions or associated dyspnea should be evaluated in an emergency department setting; may indicate epiglottitis.
• Dehydration in severe cases may require IV hydration.
• Red, beefy tonsils with exudates may have either bacterial or viral causes (i.e., presence of exudate is not specific for bacterial cause.
• Primary infection with EBV (infectious mononucleosis) may present with fever, sore throat, and lymphadenopathy (anterior and posterior cervical lymph nodes—may be generalized and include splenomegaly) and is easily confused with GABHS.
  1/ In infectious mononucleosis, laboratory abnormalities may include predominance of lymphocytes or atypical lymphocytes. In 90% of adult cases, the aspartate aminotransferase, alanine aminotransferase, or lactate deyhydrogenase level is elevated to at least two to three times normal.
  2/ Prescription of amoxicillin for mistakenly believed or secondary concurrent GABHS predictably yields diffuse, pruritic, maculopapular rash in 95% to 100% of patients (rash does not mean patient is amoxicillin allergic for future dosing.

Diagnosis

Because the signs and symptoms of group A streptococcal and other viral presentations overlap, physicians are generally unable to include or exclude the diagnosis of streptococcal pharyngitis on epidemiologic and clinical grounds; therefore, laboratory testing should be done to determine whether group A streptococci are present in the pharynx.

• Throat culture gold standard (90% sensitive)
  • False positives may result from carrier state
• Rapid strep tests
  1/ Throat swab detecting carbohydrate antigen
  2/ Sensitivity 80% to 90% in adults but highly specific
  3/ If positive test, treat as GABHS; no further culture required
  4/ In children and adolescents, negative tests should be confirmed by standard culture. It is unclear whether this is necessary in adults.
• Sensitivity and specificity of clinical presentation 50% to 75% for GABHS.
• The production of heterophile antibodies used to diagnose infectious mononucleosis (that are not directed against EBV but agglutinate either horse or sheep red blood cells [RBCs]) occurs in 90% of cases and is detected by blood testing with commercial kits (e.g., Monospot, Meridian Bioscience, Cincinnati, OH).
  1/ Detection of anti-EBV capsid immunoglobulin M (IgM) antibodies typically done if heterophilic antibodies are negative but EBV still suspected (~10% of cases).
  2/ Anti-EBV immunoglobulin G (IgG) may be present at presentation in new infection or with preexisting infection and has less clinical utility in diagnosis of acute infection.
  
  
  
  

Treatment

• Viral: Generally benign, self-limited illness remedied by​​​ rest, hydration, nonsteroidal drugs for pain or fever, and saltwater gargles
• GABHS: Historically, treatment is given to avoid complications of acute rheumatic fever and may prevent suppurative complications such as tonsillar abscess. However, acute rheumatic fever is now rare in adults, and the main use of antibiotics offers shorter duration of illness (16–24 hours) if given within 72 hours of symptom onset.
  1/ All GABHS strains remain penicillin sensitive.
  2/ Penicillin (PCN) V standard for adults, 250 mg four times daily or 500 mg twice daily orally; long-acting intramuscular PCN given as one dose (1.2 million units benzathine ± procaine PCN G).
  3/ Erythromycin 250 mg four times daily or 500 mg twice daily orally for PCN-allergic patients.