SECTION IV: Cardiology Laboratory

Timothty M. Hoffman

BLOOD PRESSURE MEASUREMENT

Accurate measurement of the blood pressure depends on selection of an appropriate cuff size. The recommended width of the cuff is 40% to 50% of the circumference of the measured extremity. If the cuff size is too small, then the blood pressure will be overestimated, whereas if it is too large, the blood pressure will be underestimated. Auscultation of the diastolic component of the blood pressure exhibits two endpoints: Korotkoff phases IV and V, respectively. In children, phase IV (i.e., the point of diminution or muffling of the Korotkoff sound) is generally considered a more accurate representation of the diastolic pressure. Phase V (i.e., the disappearance of the Korotkoff sounds) should be considered the diastolic endpoint if it falls within 6 mm Hg of phase IV. Standard blood pressure measurements for children from birth to 18 years of age are shown in the figure Standard Blood Pressure Measurements in Accordance with Age and Gender, below.

Pulsus Paradoxus

Pulsus paradoxus, a decrease of more than 10 mm Hg in the systolic blood pressure during inspiration, may be associated with cardiac tamponade (e.g., as a result of pericardial effusion), constrictive pericarditis, or severe respiratory compromise (e.g., asthma exacerbation).

Hint: Pulsus paradoxus must not be confused with pulsus alternans (i.e., a decrease in the systolic pressure on alternate contractions that indicates left ventricular failure).

Using a sphygmomanometer, the clinician inflates the cuff until the pressure is 20 mm Hg above the systolic pressure. She then slowly deflates the bladder until the first Korotkoff sound is heard independent of the respiratory cycle—this is the first data point. She continues to slowly deflate the bladder until the Korotkoff phase I sound is noted in all respiratory cycles (in inspiration and expiration)—the second data point. The difference between the points is considered the reduction in systolic pressure during inspiration and is abnormal if it is greater than 10 mm Hg. The figure Pulsus Paradoxus schematically depicts pulsus paradoxus.

HYPEROXITEST

The hyperoxitest can be useful for differentiating cardiac and pulmonary causes of cyanosis, and is one of the first evaluations performed when confronted with a cyanotic newborn. Cyanosis usually becomes apparent at a mean capillary concentration of 3 to 4 g/dL of reduced hemoglobin, a concentration that corresponds to an oxygen saturation of 70% to 80%.

In infants with cyanosis and hypoxemia, the arterial oxygen tension (PaO2) can range from 10 to 60 mm Hg. In an infant with a pulmonary cause for the cyanosis, administration of 100% oxygen increases the PaO2 to a level significantly greater than 150 mm Hg. In an infant with cardiac cyanosis, the PaO2 does not increase beyond 150 mm Hg in response to the administration of 100% oxygen. In patients with congenital heart disease, this phenomenon is attributable to right-to-left shunting (i.e., the mixing of unoxygenated blood and oxygenated blood within the circulatory system). Right-to-left shunting may be either intracardiac or extracardiac in nature.

ELECTROCARDIOGRAPHY

The electrocardiogram (ECG) provides data concerning the following:

Rhythm

Rhythm diagnosis is beyond the scope of this chapter.

Rate

The heart rate is determined accurately by dividing 60 by the RR interval (measured in seconds). For example, if the RR interval is 0.4 second (ten small boxes), the heart rate is 60 divided by 0.4, or 150 beats/min.

Intervals

See the figure A Normal Electrocardiogram Showing Waveforms and Intervals. Interval standards include the PR, QRS, and QT intervals. The normal heart rate, PR interval, and QRS duration vary with age (see the table Heart Rate, PR Intervals and ORS Duration.

Heart Rate, PR Intervals and QRS Duration

PR Interval

First-degree atrioventricular block is characterized by a PR interval that is greater than the standard range for age. A short PR interval associated with a delta wave is indicative of Wolff-Parkinson-White syndrome.

QRS Interval

The QRS duration represents the intraventricular conduction time and is normally less than 0.09 second (in children younger than 4 years) or 0.1 second (in children older than 4 years). A QRS duration greater than normal is identified as a bundle branch block:

QT Interval

The QT measurement is corrected for heart rate using the following formula:

The QTc interval is less than 0.45 second for infants younger than 6 months, and less than 0.44 second for children.

Axis

The frontal axis should be determined for the P, QRS, and T waves from the limb leads (I, II, III, aVR, aVL, aVF). The hexaxial reference system (see the figure Hexaxial Reference System, below) is usually used. The normal axis falls between 0 and 90 degrees.

The axis is equal to the direction of the largest positive force on depolarization. A cursory way to determine the frontal axis is to note the complex that is isoelectric and locate the two planes perpendicular to it. The perpendicular plane with the greatest positive deflection is indicative of the axis.

P-Wave Axis

The location of the P-wave axis determines the origin of an atrial-derived rhythm:

Hint: Classic “mirror image” dextrocardia is associated with a P-wave axis of 90 to 180 degrees in conjunction with high-amplitude forces in the right chest leads and low voltage in the left chest leads (i.e., lead V₄R has a greater amplitude than lead V₄).

QRS Axis

The QRS axis value is age-specific (see the table Age-Specific QRS Axis Values, below), but may correlate with congenital heart disease in certain clinical settings. For example, a north-west axis or left axis deviation (-30 to -120 degrees) may correlate with an endocardial cushion defect or tricuspid atresia. Left axis deviation is always abnormal in a newborn.

Age-Specific QRS Axis Values

T-Wave Axis

The T-wave axis helps determine strain associated with ventricular hypertrophy. A T-wave axis that is 90 degrees different from the QRS axis suggests strain.

Hypertrophy

Right Atrial Hypertrophy

Diagnostic findings on ECG include a peaked P wave that exceeds 2.5 mm in any lead (often best seen in lead II).

Left Atrial Hypertrophy

A P wave with a notched contour and a duration of more than 0.08 second in lead II is diagnostic. Alternatively, a biphasic P wave in lead V₁ or V_3R with a terminal inverted portion that measures 1 × 1 mm will suffice to make the diagnosis.

Right Ventricular Hypertrophy

The table Normal Range of Values for R and S Waves in Leads V₁ and V₆ (above) summarizes the normal measurements of R and S waves in leads V₁ and V₆ by age. Right ventricular hypertrophy can be diagnosed if any one of the following is seen:

Normal Range of Values (5th–95th Percentile) for R and S Waves in Leads V₁ and V₆

Left Ventricular Hypertrophy

Left ventricular hypertrophy can be diagnosed if any one of the following is seen:

Wave Changes

CHEST ROENTGENOGRAPHY

When examining a chest radiograph, one must comment on the cardiac size, organ situs, and the pulmonary vascular markings (normal, increased, or decreased).

Cardiac Size

Cardiac size is determined by estimating the cardiothoracic ratio. The ratio is calculated by dividing the largest diameter of the heart by the largest internal diameter of the thorax. If the result is greater than 0.5, cardiomegaly is present.

Organ Situs

The normal cardiac silhouette in the anterior-posterior and lateral views is depicted in the figure Normal Cardiac Silhouette, right.

Pulmonary Vascular Markings

Pulmonary vascular markings reflect the appearance of the pulmonary arteries and veins on a chest radiograph over the lung fields. These markings can be increased in states of excessive pulmonary arterial flow [e.g., ventricular septal defect (VSD), atrial septal defect (ASD), patent ductus arteriosus (PDA)], as well as in states characterized by excessive pulmonary venous flow (e.g., congestive heart failure, conditions characterized by pulmonary edema or pulmonary venous obstruction). On the radiograph, increased pulmonary vasculature is noted when cephalization occurs and the vascular shadows extend more than two-thirds across the lung field.

Decreased pulmonary vasculature markings are manifested radiographically as hyperlucency of the lung field and a paucity of pulmonary vasculature, as seen in patients with tetralogy of Fallot.

Hint: The following signs, seen on the anterior-posterior projection, offer clues to the diagnosis:

A “boot-shaped” heart is often seen in newborns with tetralogy of Fallot.

An “egg on a string” is noted in newborns with a narrow mediastinum owing to absence of a large thymus, and is associated with transposition of the great arteries.

A “snowman sign” is associated with supracardiac total anomalous pulmonary venous return. The left vertical vein, innominate vein, and the superior vena cava form the superior aspect of the snowman.

ECHOCARDIOGRAPHY

In pediatric patients, echocardiography is performed in a stepwise fashion to obtain subcostal views, apical four-chamber views, parasternal views, and suprasternal views (see the figure Echocardiographic Series).

M-Mode Echocardiography

A parasternal short-axis view using M-mode echocardiography reveals a cross section of the left ventricle and, therefore, can be used to estimate cardiac dimensions. Most commonly, it is used to obtain a shortening fraction (SF), calculated in the following manner:

The normal value for the SF is generally considered to be 28% to 38%, independent of age.

Doppler Echocardiography

Doppler echocardiography detects a frequency shift that reflects the direction and velocity of blood flow. Doppler echocardiography is used to detect valvular insufficiency or stenosis and abnormal vasculature flow patterns.

CARDIAC CATHETERIZATION

Cardiac catheterization allows sampling of oximetric and hemodynamic data. The normal pressures and oxygen saturations for children are depicted in the figure Normal Pressures, Mean Pressures, and Oxygen Saturations for Children on the next page. Cardiac catheterization, an invasive procedure, is often used in conjunction with angiography to confirm the diagnosis and physiology of certain acquired and congenital heart diseases. The technique also has therapeutic applications, such as patent ductus arteriosus coil embolization, coil embolization of aortopulmonary collaterals, pulmonary artery angioplasty and stent placement, and balloon valvuloplasty of semilunar valvular stenosis.

Shunts

Data obtained from cardiac catheterization can be used to calculate the degree and direction of an intracardiac or extracardiac shunt. The calculation is based on the Fick principle, using oxygen as the indicator.

The oxygen content equals the dissolved oxygen (which is usually negligible) plus the oxygen capacity [hemoglobin (g/dL) × 1.36 mL O₂/dL × 10] multiplied by the oxygen saturation (as a percentage).

Flow (Q) is the oxygen consumption divided by the arteriovenous oxygen content difference:

where

In order to calculate the amount of a shunt, one needs to calculate the effective pulmonary blood flow (Qp eff ):

A left-to-right shunt is the pulmonary flow less the effective pulmonary flow (Qp - Qp eff), and a right-to-left shunt is the systemic flow less the effective pulmonary flow (Qs - Qp eff).

Resistance

Systemic and pulmonary vascular resistance can also be calculated using the catheterization data. This calculation is based on the Ohm law (essentially, the pressure change across the vascular bed divided by flow equals resistance):

where

A pulmonary resistance (Rp) of 2.5 Wood units or less is considered within the normal range; however, no vascular bed is rigid, and variations in flow can affect the result obtained.

Copyright © 2000 Lippincott Williams & Wilkins
M. William Schwartz, Louis M. Bell, Jr., Peter M. Bingham, Esther K. Chung, David F. Friedman and Andrew E. Mulberg, The 5 Minute Pediatric Consult