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BioPac 6 Principles of Electrocardiography I Elements of the

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					BioPac 6: Principles of Electrocardiography I: Elements of the Electrocardiogram (ECG)

      Receiving Chambers
           o Right Atrium
           o Left Atrium
      Pumping chambers
           o Riht ventricle
           o Left Ventricle
      Dominant Tissue
           o Myocardium – cardiac muscle tissue
      Alternate contractions of atrial mycocardium and then ventricular myocardium are called
       heartbeats or cardiac cycles.
           o Heart beat is myogenic – the signal for the heart to beat comes from within the heart
               itself and not from an external source (i.e. nerve cell)
                     Does have motor nerves that INFLUENCE things like rate of contraction or
                        strength of contraction.
                     No role in the genesis of the heartbeat.
                     Extrinsic nerves cut or heart removed from body it would still beat
                     Ability to initiate and undergo a cardiac cycle or heartbeat by itself without any
                        stimulation from the rest of the body – inherent rhythmicity or automaticity
           o Before contraction, electric current passes through myocardial fibers. Conduction
               System or pacemaker system consists of:
                     Sinoatrial (SA) node – near junction of right atrium and superior vena cava
                             Impulse that initiates contraction originates here
                             Pacemaker
                     Internodal pathways – impulse from SA node transmitted through both atria
                        along these two path ways stimulating atrial muscle to contract
                     Interatrial pathways – Above.
                     Atrioventricular (AV) node –impulse then spread here which is part of the
                        junctional tissue between right atrium and the ventricle
                             Relays the electric impulse toward the ventricles after a slight delay
                                 which allows the atria time to contract before the ventricles do.
                     Bundle of His – common bundle of specialized conductive fibers lying along the
                        upper part of the interventricular septum - branches
                     Right bundle branches – carries impulse to right ventricles
                     Left bundle branches – carries impulse to left ventricles
                     Purkinje fiber network – bundle branches subdivide into small conducting fibers
                        (purkinje) – relay the electric impulse directly to ventricular muscle, stimulating
                        the ventricles to contract
           o In case of damage to the SA node the AV node may take over but the rate of firing is less
               than normal
           o Cardiac Cycle
                 Depolarization and repolarization of the SA node
                 Depolarization and repolarization of atrial muscle
                 Depolarization and repolarization of AV node and bundle
                 Depolarization and Repolarization of the Purkinje network
                 Depolarization and Repolarization of ventricular muscle
   The electrical and mechanical device that records the electrical activity of each cardiac cycle –
    electrocardiograph
        o Study of electrocardiograph and interpretation of electrocardiograms (records made by
            an electrocardiograph) is called electrocardiography
   Phases of the ECG complex
        o Isoelectric line (baseline) – point of departure for the P, Q, R, S, and T waves.
        o P Wave – the depolarization of atrial muscle as a wave of negativity spreads from the SA
            node toward the ventricles (upright)
        o P-R interval – measured from beginning of P wave to the beginning of QRS complex
                 Interval between activation of the SA node and the AV node
                 Abnormal lengthening suggests interference with conduction to the ventricles
        o P-R Segment - from end of the P wave to beginning of QRS complex
                 Interval between atrial depolarization and ventrical depolarization
                 No external potentials recorded
        o QRS complex – the spread of excitation through the ventricular myocardium resulting in
            depolarization of ventricular muscle
                 Repolarization of atrial muscle occurs during this phase
                 Abnormal lengthening suggests interference with the spread of excitation
                     through ventricular muscle
                          Purkinje failure
                          Myocardial infarction
        o S-T segment – interval between the end of the S wave and the beginning of the T wave
                 Period during which the ventricles are more or less uniformly excited
                 Indicates an isolectric state
                 Can diagnose abnormalities of ventricular repolarization
        o T wave – restoration of ventricular myocardium to resting state
        o Q-T interval – measured from beginning of QRS complex to the end of the T wave
                 Time of electrical systole when the ventricular beat is generated
   Sinus Arrhythmia – slowing-down-speeding-up rhythm correlated with the respiratory cycle
        o Reflect heart rate adjustments made by systemic arterial and systemic venous pressure
            receptor (baroreceptor) reflexes in response to the cycling of intrathoracic pressure
                 Inspiratory muscles contract, pressure in thorax (intrathoracic pressure)
                     decreases, thoracic veins expand causing a momentary drop in venous pressure,
                     venous return, cardiac output, and systemic arterial blood pressure
                 Carotid sinus reflex decreases heart rate in response to a rise in carotid arterial
                     blood pressure, but the momentary drop in systemic arterial blood pressure
                      during inspiration reduces the frequency of carotid baroreceptor firing causing a
                      momentary increase in heart rate
                    Inspiratory muscles relax, resting expiration passively occurs. Intrathoracic
                      pressure increases causing compression of veins increasing venous pressure and
                      venous return. Systemic venous baroreceptors increase heart rate but the
                      increase is temporary because it increases cardiac output and stystemic arterial
                      blood pressure which increases carotid baroreceptor firing causing heart rate to
                      decrease.
      Heart rates above 100 bpm – tachycardias
      Heartbeats below 60 bpm – brachycardias

Biopac 10 – The electrocardiogram and the Peripheral Pressure Pulse

      Systole – contraction
      Diastole – relaxation
      Heartbeat – cardiac cycle
      Left ventricle pumps blood to systemic arterial circulation producing a peripheral pressure pulse
           o Right ventricle pumps to lungs
      Electrical activity of the Ventricles marked by QRS complex
           o Precedes systole
           o Systole begins at R wave peak and ends at end of the T wave
                     T wave – repolarization of ventricles occurs when ventricles are in systole
                     Diastole begins at end of systole and lasts until the next R wave peak
      Stroke volume – volume of blood pushed into arteries by contraction of the ventricles
           o Left ventricle  aorta
           o Right ventricle  lungs
      Pumping action initiates a pressure wave transmitted via the arterial walls
           o Pressure increases with systole and decreases with diastole
           o Stiffness of vessel walls help transmit the pressure wave
           o Stiffer the walls faster the transmission but the more work required by the heart to
                move the same blood volume because elastic recoil is reduced
           o Actual blood flow is slower than the transmission of the pressure wave
           o Wave transmitted to the periphery – there is a pulse of increased blood volume
                     Study of blood volume changes by using volume displacement techniques –
                         plethysmography
                               Device – plethysmograph
                               Record of volume change vs. time – plethysmogram
                                     o Photoelectric transducer – shining a beam of light through skin
                                          and measuring the amount of light that is reflected
                                               Blood absorbs light directly proportional to blood
                                                  density
                                               The greater blood density the greater light absorption,
                                                and the less light reflection.

Bipoac 11 – The Respiratory Cycle

      Alternating processes of inspiration and expiration
           o Inspiration
                    Diaphragm and external intercostals contract
                    Increases volume in thorax and lungs
                    Intrapulmonic volume increases, intrapulmonic pressure decreases below
                       atmospheric pressure and air rushes into the lung
           o Expiration
                    Inspiratory muscles relax
                    Volume of thorax and lungs is reduced
                    Increased intrapulmonic pressure above atmospheric pressure forcing air out of
                       lungs
                    Expiration is passive
                            Becomes active dependent upon contraction of expiratory muscles that
                               pull down the rib cage and compress the lungs
      Volumes
           o Tidal Volume (TV) – specific volume of air drawn into the system and then pushed back
               out
                    Varies in proportion to the depth of inspiration
                    Quiet, normal breathing – eupnea – 500 mL
           o Pulmonary Ventilation (PV) – product of tidal volume and respiratory rate
                    Minute respiratory volume (MRV)
                    7.5 L/min
           o Pulmonary ventilation includes ventilating a portion of respiratory passageways such as
               the pharynx, larynx, trachea, primary bronchi, and other parts that play no direct role in
               gas exchange
                    Anatomical Dead Space (ADS) – 150 mL
                    Alveolar ventilation 0 rate of ventilating parts of lungs that play direct role in gas
                       exchange with the blood
                            Subtracting anatomical dead space from tidal volume and multiplying by
                               respiratory rate: AV=(TV-ADS)xRR
      Rate and strength of contraction are controlled by primary respiratory centers
           o In medulla oblongata
           o Inherently rhythmic
           o Eupnea
                    Expiratory centers limits and inhibits the inspiratory center producing a passive
                       expiration
                    Inspiratory center acts to produce an active inspiration
       o    Increase in respiratory rate and depth can be done be neural centers or peripheral
            receptors
                 chemoreceptors in aortic and carotid bodies
                          sense changes in arterial PCO2, [H+], and PO2
                                 o increased carbon dioxide, Hydrogen ions, and/or decreased
                                     oxygen increases rate and depth of respiration
                                 o decreased carbon dioxide, hydrogen ions, or increased oxygen
                                     tends to decrease respiratory rate and depth
                          Alveolar hyperventilation  respiratory alkalosis – resultant elevated
                             blood pH is called alkalemia
                          Alveolar hypoventilation  respiratory acidosis – resultant decreased
                             pH is academia
                 stretch receptors in joints, muscles, and tendons
                 somatic sensory receptors for pain and thermal stimuli
   Pneumograph transducer – measures ventilation by recording the rate and depth of the
    breathing cycle

				
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posted:11/1/2011
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