Cardiac physiology

Cardiac physiology

1. Action potential

   • Phase 0
     • Rapid DepolarizatioN
       • • Na⁺ Influx (very rapid and short-lived) • L-type Ca2+ channels open
       • The membrane potential rapidly shifts from negative to positive about +30 mV.
       • Triggered once RMP (approximately -70 mV)
       • Fast voltage-gated Na+
         • Class I
           • Na Channel blockers
             • Class Ia – disopyramide, quinidine, procainamide
               • “double quarter pounder”
             • Class Ib – lidocaine, mexiletine, tocainide
               • lettuce, mayo and tomato
             • Class Ic – flecainide, propafenone
               • “…and fries please”
   • Phase 1
     • Early repolarization
       • • Na⁺ channel close. • Brief K+ efflux
       • This is referred to as a ‘notch’
   • Phase 2
     • Plateau phase
       • • Ca+2 Influx (slow and long-lasting flow) •K+ efflux
         • Class IV
           • Ca Channel blockers
             • verapamil, diltiazem
       • Ca2+ entry during AP 25%, The rest from SR (Ca2+-induced Ca2+ release)
   • Phase 3
     • Rapid repolarization
       • • Massive K+ efflux • Ca+2 channel close. • Na⁺/K⁺-ATPase restoring Na⁺ efflux and K+ influx
         • Class III
           • K Channel blockers
             • amiodarone, sotalol, dofetilide, dronedarone
   • Phase 4
     • Resting phase
       • RMP restored by Na+/K+ ATPase and Na+/ Ca2+
       • RMP is approximately -90 mV
       • Na+ and Ca2+ channels are closed in the resting phase
       • Class ii
         • Beta blokcers
   • Resting membrane potenital
     • SAN
       • RMP is - 60 mV
       • Threshold potential - 40 mV,
       • upstroke of the AP is slow, as it is not due to activation of fast Na+ channels like cardiac myocytes, but instead slow L-type Ca2+ channels
       • Factors that affect pacemaker potential are called chronotropic agents.
     • Myocyte
       • -90 mV
       • Threashold potential -65 mV
     • maintained by Na/K pump

2. Pressures

   • Atrial pressure
     • Central venous pressure (CVP)
       • Measurment
         • CVP is 0-8 cmH20 (0-6 mmHg) in a spontaneously breathing patient.
         • The tip of the catheter should be in the junction between SVC and the RA.
         • Fluid challenge (250-500 ml crystalloid) causing an increase in CVP that is not sustained for > 10 minutes ➝ hypovolaemia
         • Transducer should be at level of RA at 4th intercostal space MAL
         • The pressure in the vena cava at the level of the heart (the central venous pressure) is usually close to 0 mmHg.
       • A wave
         • *Atrial contraction
       • C wave
         • tri*Cuspid bulging from ventricular *Contraction
       • X descent
         • atrial rela*Xation
       • V wave
         • *Venous filling of the atrium
       • Y descent
         • atrial empt*Ying = Ventricular filling (sometimes misleading in questions)
       • Factors affecting CVP
         • Skeletal muscle pump
           • ↑ VR
             • ↑Excercise
               • Veins in the arms and legs are surrounded by skeletal muscles. They also have one-way valves in their walls that only open for upward flow.the muscles contract and squeeze blood in the veins upward, toward the heart. The one-way valves prevent blood from flowing down again when the muscles relax.
         • Respiratory pump
           • ↑ VR
             • INcrease during INspiration
               • ↓ intra-thoracic pressure
                 • During inspiration, the diaphragm moves down, expanding the thoracic cavity➝ ↓ intra-thoracic pressure➝expansion of the lungs➝ ↓pressure is transmitted across the walls of the heart➝↓ lowering RA pressure ➝ ↑venous return.
               • ↑ intra-abdominal pressure
                 • The descent of the diaphragm ➝↑ abdominal pressure. As the inferior vena cava passes through both abdominal and thoracic cavities➝ increase intra-abdominal pressure together with a decrease in thoracic pressure ➝ squeeze the blood upward, toward the heart➝ ↑venous return.
             • Positive pressure ventilation / PEEP.
           • ↓VR
             • Negative pressure ventilation
             • During expiration.
         • Vena cava compression.
           • Valsalva maneuver and pregnancy
         • ↑Sympathetic activity ➝↑ atrial activity ➝↑ heart rate (Tachycardia) ➝↑ Atrial filling
         • Blood volume
           • ↑ VR
             • Hypervolemia
           • ↓ VR
             • Hypovolemia and disrubtive shock
         • Pulmonary Outflow
           • Pulmonary emblosim,
         • Venous Retun = Venous pressure - RA pressure / venous resistance
         • Gravity
           • Gravity (Postural hypotension) causes blood to pool in veins in the legs. Central venous pressure (CVP) falls, causing a fall in stroke volume and cardiac output

3. ECG

   • P wave
     • Duration < 0.12 s (<120ms or 3 small squares)
     • Amplitude < 2.5 mm (0.25mV) in the limb leads < 1.5 mm (0.15mV) in the precordial leads
   • Digoxin toxicity
     • Downsloping ST depression with a characteristic “Salvador Dali sagging” appearance
   • Sgarbossa criteria
   • Wolff-Parkinson-White syndrome
     • *W*P*W
       • *Wide QRS complex
       • *PR interval shortened
       • *Wave (Delta wave)
     • Types
       • Type A
       • Type B
   • LBBB
   • RBBB
   • AV block

4. MAP

   • MAP = [(2 x DBP) + SBP] / 3
   • MAP = DBP + [(SBP - DBP) / 3]
   • MAP = (CO X SVR) + CVP
   • Noradrenaline is the first-line choice of vasopressor used to maintain MAP in sepsis.
   • total blood volume in the circulatory system of a healthy adult is about 5 L.
   • (MAP) = (CO) x (TPR).

5. Pulse pressure

   • Pulse pressure = SBP-DBP
     • approximately 30-40 mmHg
   • Narrow pulse pressure
     • Less than 25% of the systolic value
       • ↓CO
       • A.S
       • Cardiac Tamponade
       • CHF
   • Wide pulse pressure
     • Greater than 60 mmHg
       • AR
       • Aortic Dissection
       • Aortic root aneurysm
       • Athersosclerosis (major vessels)
       • Hyperthyroidsim

6. Conducting system

   • SAN 60-100 BPM
     • AVN 40-60 BPM
       • Annulus fibrous
         • Bundle of His
           • Right Bundle of His
           • Left Bundle Of His
             • Pukinji fibers
           • 20-40 BPM
         • delays the impulse for about 120 ms, allowing time for atrial contraction to complete ventricular filling.
     • Located at the junction of SVC and RA
     • +
       • Sympathetic
     • -
       • Parasympathetic

7. Heart sounds

   • S1
     • Start of Systole
       • (Isovolumetric ventricular contraction ) pressure building up aganist closed valves
         • AV valves closure
           • QRS complex
       • Ventricular ejection
         • No heart sounds
           • ST segment on ECG
   • S2
     • End of Systole
       • Isovolumetric ventricular relaxation
         • Semilunar valves closure
           • Iso-electric line
   • S3
     • Early Diastole
       • ventricular filling phase
         • Iso-elecrtic line
   • s4
     • Late Diastole
       • Atrial systole
         • Filling of an abnormally stiff ventricle in atrial systole
         • P wave PR interval

8. ECG Deflections during Cardiac Cycle

   • P wave
     • Atrial depolarisation
   • QRS complex
     • Ventricular depolarisation
   • T wave
     • Ventricular depolarisation

9. Cardiac muscle

   • Epicardium
     • layer of mesothelial cells
   • Myocardium
     • left side has more muscle than the right
   • Endocardium
     • anti-thrombogenic surface
   • Epic my end

10. Vascular system

    • Artery
      • Thick-walled with elastic tissue and smooth muscle. damp out oscillations in pressure produced by pumping of the heart; stiff arteries (e.g. age, atherosclerosis) result in larger oscillations.
    • Arteriole
      • More muscle and are resistance vessels, The major regulator of vascular resistance (control tissue blood flow through constriction).
      • 65 mmHg
    • Microciculation
      • no smooth muscle or valves and which provide the exchange surface between blood and tissues.
      • 25 mmHg,
      • Fluid filtered by the microcirculation (about 8 L per day) is returned to the circulation by the lymphatic system
    • venules
      • join into larger venules
    • Veins
      • -Veins have a larger diameter than equivalent arteries and provide less resistance. -Thin distensible walls and contain about 70% of total blood volume at any one time.
      • 15 mmHg

11. Circulation

    • Pulmonary circulation
      • Metabolic
        • Hypoxia
          • Hypoxic pulmonary vasoconstriction
            • blood vessels are constricted and blood is diverted to areas of the lung that are better ventilated,thus maintaining optimal ventilation-perfusion matching.
            • Persistent hypoxia ➝f pulmonary hypertension and right-sided heart failure (cor pulmonale).
        • Acidemia
        • hypercapnia.
      • Autonomic nerves
        • Sympathetic
          • vasconstrictrion
        • Parasympathetic
          • vasodilatation
    • Cerebral circulation
      • 15% of total CO = 750 millilitres per minute
      • 2X as high in Grey matter vs white matter) Remember :Grey is Greater
      • CBF = CPP / CVR
      • CPP = MAP- ICP
      • cerebral blood flow can maintain a constant flow for blood pressures (MAP) between 50 and 170 mmHg
      • Hypercapnia ↑ CO2 → Relaxation of cerebral blood vessels and ↑ CBF ↑ ICP
    • Coronary Circulartion
      • High metabolic demand, High capillary density ➝ extract an unusually large fraction (about 70%) of oxygen from the blood.
      • Adenosine
        • Vasodilation
      • K+
      • hypoxia
      • Nitric oxide
      • Hypercapnia
    • Skeletal Muscle Circulation
      • 15 - 20% of total CO, up to > 80% during exercise
      • major contribution to PVR and sympathetic regulation and baroreceptor reflex.
      • At rest most capillaries are not perfused as their arterioles are constricted
      • Capillaries are recruited during exercise by

12. Vascular Tone Regulation

    • vasoconsrtictors
      • bind to G-protein coupled receptors
        • ↑ intracellular [Ca2+] ➝vascular ➝smooth muscle contraction.
          • Noradrenaline
            • NEAT V
          • Endothelin
          • Angiotensin II
          • Thromboxane A2
          • Vasopressin (ADH)
    • Vasodilators
      • ↑ cyclic guanosine monophosphate (cGMP)
        • nitric oxide
          • stimulated by
            • local mediators such as bradykinin, histamine and serotonin
            • neurotransmitters (e.g. substance P).
            • Increased flow (shear stress)
      • ↑ cyclic adenosine monophosphate (cAMP)
        • prostacyclin and Beta-adrenergic receptor agonists
      • ↓ intracellular [Ca2+] ➝vascular ➝smooth muscle dilation
    • Ca Cause contraction

13. Baroreceptor reflex

    • Detect MAP
      • Aortic
      • Carotid

14. Inotropic effect

    • Positive
      • Sympathetic stimulation
        • Noradrenaline
          • β1 Receptors
            • L-type Ca2+
              • Factors that ↑ intracellular [Ca2+] ➝ ↑ cardiac contractility
      • Cardiac glycosides (e.g. digoxin)
        • Cardiac glycosides :Direct inhibition of Na+/K+ ATPase ➜ indirect inhibition of Na+/Ca2+ exchanger. ↑ intracellular [Ca2+] ➜ positive inotropy.
          • Dixogin toxicity
            • ↑ by : ↑Ca ↓Mg ↓O ↓K
      • ↑ HR
    • Negative
      • Acidosis
        • because H+ competes for Ca2+ binding sites.
      • Parasympathetic stimulation
      • CCB

15. Frank Sterling curve

    • Force of contraction is proportional to end diastolic length of cardiac muscle fiber (preload)
      • Preload
        • ↑
          • Right shift
        • ↓
          • Left shift
      • Contractility
        • ↑
          • Upward shift
        • ↓
          • Downward shift
      • Afterload
        • ↑
          • downwards and to the right
        • ↓
          • upwards and to the left

16. Oedema

    • Increased capillary hydrostatic pressure
      • increased venous pressures
    • Decreased plasma oncotic pressure
    • Increased capillary permeability (leading to reduced oncotic pressure gradient)
    • Lymphatic obstruction

17. Capillary Filtration

    • hydrostatic pressure along capillary > plasma oncotic pressure ➝ small net filtration of fluid from the capillary into the interstitial space.
    • Plasma colloid osmotic pressure > interstitial colloid osmotic pressure
    • iltration of fluid out of the capillaries is slightly greater than absorption of fluid into the capillaries

18. EDV

    • (EDV) is usually about 120 - 140 mL

19. The arterial pulse waveform

    • The systolic phase (The anacrotic limb)
    • The diastolic phase(The dicrotic limb)
    • The dicrotic notch, which represents the closure of the aortic valve
    • The anacrotic notch is a pathological finding and indicates severe aortic stenosis or hypertrophic obstructive cardiomyopathy. It occurs before the opening of the aortic valve.

20. Capillary Permeability

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