Physio B Lab Test 2

what is the cardiac axis?

the general direction of electrical impulses as they travel through the heart

what is normal cardiac axis?

from upper right to lower left

how do you determine an axis deviation?

observing the deflection patterns on Leads I, II, and III/avF

what is right axis deviation?

when the heart is anatomically or electrically deviated to the right

what do the leads look like in right axis deviation?

Lead I has a negative deflection pattern (the electrical vector moves toward the right arm electrode) while Leads II and III still deflect upwards

what conditions cause right axis deviation?

common in children and tall thin adults; thoracic or spinal deformity, enlarged heart

what is left axis deviation?

when the heart is anatomically or electrically deviated to the left resulting in an electrical vector toward the left side of the body

what do the leads look like in left axis deviation?

Lead I deflects upward, while Lead II and III/avF have a negative deflection pattern

what conditions cause left axis deviation?

normal in short and stocky individuals, emphysema, hyperkalaemia, tricuspid atresia

summarize the lead deflections for the different cardiac axes

1. Normal = positive for all leads

2. Right axis deviation = negative for Lead I, positive for Leads II & III

3. Left axis deviation = positive for Lead I, negative for Leads II & III

how to determine heart rate

note that 1 tiny box = 1mm = 0.04 seconds (1/25 of a second) and 1 big box = 5mm = 0.20 seconds

if you count using big boxes, the HR will be equal to 300/# of big boxes

if you count using tiny boxes, the HR will be equal to 1500/# of tiny boxes

what is the PR interval?

the time interval from the beginning of the P wave until the beginning of the QRS complex

what is the normal range of PR interval?

0.12 sec (3mm/3 tiny boxes) to 0.2 sec (5 mm/1 big box)

what does a prolonged PR interval indicate (>0.2 sec)?

heart block

what does an abnormally short PR interval (<0.12 sec) indicate?

less blood flow to ventricles and lower cardiac output

review what the intrinsic conduction system looks like

SA node —> AV node —> Bundle of His —> Bundle branches —> Purkinje fibers

Electrical signal goes through SA node; atria depolarize; signal travels through the septum into both ventricles, depolarizing them

what are some causes of heart block?

• age-related wear and tear to heart muscle

• diseases and conditions which can cause scarring or damage to the electrical system of the heart

• scar tissue from a previous heart surgery, especially in children

  • side effect of medications (like a calcium channel or beta blocker)

what is an SA block (junctional rhythm)?

• when the AV node ends up becoming the pacemaker since the SA node no longer works (at the junction between the atria and ventricles)

• as a result, the electrical signal sent by the AV nod escapes out the top and bottom at the same time

• the P wave and QRS occur at the same time, so the P wave ends up being obscured by the QRS wave

what is a first degree AV block?

• when there is a delay in the transmission of the impulse from the atria to the ventricles; this is reflected in the PR interval

• you can identify a first degree AV block by its prolonged PR interval (>0.20 sec) in all the leads

• the PR interval will be the same duration for each rhythm

what is a second degree AV block Mobitz I and Wenckebach?

• Type I demonstrates an increasing PR interval until the QRS is dropped

• you can usually see the P wave, but its impulse does not immediately initiate the ventricular response

what is a second degree AV block Mobitz II?

• Where there are multiple P waves prior to the QRS complex

• This is a fixed rate and will be consistent

  • You can either have a Mobitz II, 2:1 or 3:1

What is a third degree AV block (complete heart block)?

• This is where the atria and ventricles have no association

• The atria are stimulated by the SA node

• But the impulses are not conducted to the ventricles due to there being a block at the AV node

• Instead, another pacemaker becomes active below the block to generate the QRS complex (either bundle of His or in the ventricle)

• Hence, this produces no discernable pattern between the timing of the P waves and QRS waves

• Sometimes the P waves may be masked

what are bundle branch blocks?

when the bundle branch in either the left or right ventricle is blocked

how does the heart work around the bundle branch blocks?

it stimulates the ventricle through a tissue pathway leading to a delay in the ventricular contraction

what are some causes of bundle branch blocks?

• congenital heart disease

• cardiomyopathy

• myocardial infarction (heart attack)

• coronary artery disease

• heart failure

• hypertension

• myocarditis

• heart valve disease

RBBB specific:

• chronic obstructive pulmonary disease

• pulmonary embolism

• pulmonary hypertension

which leads will you use to look at bundle branch blocks?

chest leads (V1-V6)

what do you see on the leads if there is an RBBB?

V1 or V2 will have a second R wave representing the separate stimulation of the right ventricle

what do you see on the leads if there is an LBBB?

V5 or V6 will have a second R wave representing the stimulation of the left ventricle; note that the R waves can slur into each other and create a wide QRS complex

what is the right AV valve?

tricuspid valve

what is the left AV valve?

bicuspid (mitral) valve

which valves are parachute valves?

AV valves

which valves are hip-pocket valves?

semilunar valves

what is hypertrophy?

an increase in mass attributable to increase in cell size

hypertrophy refers to elevated mass in

ventricles

enlargement refers to elevated mass in

atria

what is the Frank-Starling Law of the Heart?

this states that the larger volume of blood in a chamber, the cardiac muscle fibers stretch, leading to an increased force of contraction and over time hypertrophy/enlargement

what does the horizontal axis on an ECG strip represent?

time of electrical activity (1mm = 0.04 sec)

what does the vertical axis on an ECG strip represent?

magnitude of electrical activity (1mm = 0.1 mV)

what is the normal size of a P wave?

less than 2.5 mm high (0.25 mV) and 2.5 mm wide (0.1 sec)

which leads do you use to diagnose atrial enlargement?

Lead II, III, and avF

what is the telltale sign of RAE?

a tall P wave (2.5 mm or more) in 2 out of the 3 inferior leads = RAE

what are the causes of RAE?

tricuspid stenosis or prolapse

what is the telltale sign of LAE?

a wide P wave (2.5 mm or more) in 2 out of the 3 inferior leads = LAE

what are the causes of LAE?

bicuspid stenosis/prolapse

what is another way to diagnose LAE?

examine Lead I for a wide P wave and V1 for a biphasic P wave

which leads do you use to diagnose ventricular hypertrophy?

V1 & V5 OR V2 & V6

what tells you that RVH is present?

opposite deflection pattern; this is because the electrical vector will be directed to the right side, resulting in a positive deflection in the first two leads and a negative deflection in the last two leads

what is the cause of RVH?

pulmonary valve stenosis/prolapse

what tells you that LVH is present?

increased magnitude in the deflection pattern; specifically, if the S wave of V1/V2 and R wave of V5/V6 is greater than or equal to 35 mm (3.5 mV), LVH is present

what are the causes of LVH?

normal adaptation to exercise, aortic valve stenosis/prolapse