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Chapter 14 Heart: Cardiovascular Physiology Part 3

Exam 3 will be on Monday November 21 Will cover chapters 11, 12, 13, 14 May cover more, depends on how far we get Bring Scantron, #2 pencils

Electrocardiogram (ECG or EKG) Recordings of the electrical activity of the heart First ECG: 1887 Walter Einthoven, a Dutch physiologist figured all of this out and named everything He also came up with Einthoven's Triangle (next slide) – Hypothetical triangle – leads are placed on both arms and the left leg

Figure 14-19

Figure 14-22

Electrocardiogram (ECG or EKG) An ECG is recorded from one lead at a time One electrode acts as the positive and the other as the negative In lead I, left arm electrode is positive, right arm electrode is negative

Figure 14-19

Electrocardiogram (ECG or EKG) An ECG tracing shows the summed electrical potentials generated by the heart cells Atrial and ventricular depolarization and repolarization (electrical events) are shown on the ECG Because depolarization initiates muscle contraction, the electrical events of the heart can be associated with the mechanical events (contraction/relaxation) of the heart

Electrocardiogram (ECG or EKG) Cardiac Cycle A single contraction-relaxation cycle of the heart Major components to an ECG – Waves • Deflections above or below the baseline – Segments • Sections of baseline between two waves – Intervals • Combinations of waves and segments

Figure 14-20

Electrocardiogram (ECG or EKG) P wave – Depolarization of the atria QRS complex – The progressive wave of ventricular depolarization T wave – Repolarization of the ventricles Note: Atrial repolarization isn't shown as a separate wave – It is part of the QRS complex

Figure 14-20

ECG (fig. 14-21, p. 493) Mechanical events lag slightly behind electrical events P wave – Atrial contraction begins during latter part of P wave, continues during PR segment QRS complex – Ventricular contraction begins just after the Q wave, continues through the T wave

Figure 14-21, overview

ECG is a good diagnostic tool: – Quick, painless, non-invasive But interpretation of results can be quite complicated Interpretation of results: 1. Heart rate • Timed from beginning of one P wave to beginning of next P wave • • Normal resting heart rate: 60-100 beats/min Often slower in trained athletes

Heart rate – Tachycardia: faster than normal – Bradycardia: slower than normal 2. Heart Rhythm – Regular (occurs at regular intervals) – Irregular or Arrhythmia • Can range from a benign extra beat to fibrillation • “dropped beats” caused by ventricles not getting their signal to contract • Premature ventricular contractions (PVCs) – A pacemaker other than the SA node jumps in and fires out of sequence

3. Are all normal waves present and recognizable?

– See examples in fig. 14-23, p. 495 4. Does a QRS complex follow each P wave; is the P R segment constant in length?

– If not, then a problem with signal conduction through the AV node may be present 5. Look for subtle changes: – For example: Alterations in shape or duration of waves or segments

Long QT syndrome (LQTS): Results from structural abnormalities in the potassium channels of the heart, Predisposes affected persons to an accelerated heart rhythm (arrhythmia) Can lead to sudden loss of consciousness and may cause sudden cardiac death in teenagers and young adults who are faced with stressors ranging from exercise to loud sounds.

Normal ECG

Long QT syndrome (LQTS)

With training and experience, it becomes possible to see, in the ECG, heart conditions such as: – Enlargement of the heart – Tissue damage from ischemia (lack of adequate blood flow and oxygen to a tissue) – Changes in conduction velocity – Etc.

Figure 14-23

Cardiac Cycle (fig. 14-24, p. 496) Cardiac cycle has 2 phases: Systole and Diastole Systole – Time interval during which cardiac muscle contracts Diastole – Time interval during which cardiac muscle relaxes

Figure 14-24, overview

Cardiac Cycle (fig. 14-24, p. 496) 1. Heart at rest: atrial and ventricular diastole – Both atria and ventricles are relaxed – Atria are filling with blood from veins – Ventricles have just completed a contraction – As ventricles relax, the AV valves open – Blood flows by gravity from atria to ventricles

Cardiac Cycle (fig. 14-24, p. 496) 2. Completion of ventricular filling: atrial systole – Most blood flows to the ventricles via gravity – The last 20% is squeezed down into ventricles when the atria contract (normal person at rest) – During exercise, atrial contraction can play a bigger role in ventricular filling – Atrial contraction (systole) begins after the depolarization wave has swept across the atria

Cardiac Cycle (fig. 14-24, p. 496) 2. Completion of ventricular filling: atrial systole (con't) During atrial contraction, a small amount of blood is forced back into the veins since they don't have one way valves This backward flow can be felt as a pulse in the jugular vein (normal person, lying with head and chest elevated about 30 degrees)

Cardiac Cycle (fig. 14-24, p. 496) 3. Early ventricular contraction and first heart sound Ventricular contraction begins as the spiral bands of muscle squeeze the blood upward, from apex towards the base of the heart AV valves are forced closed by blood pushing up against them First heart sound (S 1 or “lub”) comes from vibrations after AV valves have closed

Cardiac Cycle (fig. 14-24, p. 496) Isovolumic ventricular contraction Similar to an isometric contraction (like squeezing a water balloon) Both AV and semilunar valves are closed during this part —blood has nowhere to go At the same time, the atria are repolarizing and relaxing

Cardiac Cycle (fig. 14-24, p. 496) 4. Ventricular ejection As the ventricles continue to contract (from part 3), they soon generate enough pressure to open the semilunar valves and push blood into the arteries The pressure generated by ventricular contraction b4comes the driving force for blood flow At the same time, AV valves remain closed, atria are filling

5. Ventricular relaxation and the second heart sound At the end of ventricular ejection, Ventricles begin to repolarize and relax Ventricular pressure decreases Once it falls below the pressure in the arteries, blood begins to flow backward into the heart The backflow fills the cusps of the semilunar valves and forces them closed Second heart sound (S Heart sound: lub-dup 2 or “dup”)

Isovolumic ventricular relaxation Semilunar valves close, ventricles become sealed chambers again AV valves are still closed also When ventricular relaxation causes ventricular pressure to fall below atrial pressure, the AV valves open Blood rushes into the ventricles and the cycle starts over again

Figure 14-24, overview

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