Detection of an intracardiac shunt

The presence of a shunt can be determined either invasively or noninvasively (e.g., radionuclide studies, magnetic resonance imaging [MRI], or Doppler echocardiography). In current practice, many shunts are suspected based on physical exam or electrocardiogram (ECG), diagnosed at echocardiography and then quantified using right heart catheterization.

There are several methods by which an intracardiac shunt can be detected, localized, and quantified in the catheterization laboratory. The indicator dilution method is of historical interest, but is rarely used outside of research studies. It involves injecting a substance, such as indocyanine green, into the venous systems or a right heart chamber and then monitoring its appearance in the systemic circulation. Dye curve measurements are very accurate, but are slow and require specialized equipment that is rarely available in modern catheterization laboratories. Contrast angiography, in which contrast dye is injected into the higher pressure chamber of a suspected shunt (e.g., left ventricle), is occasionally used to identify a VSD or PDA. It has limited ability to diagnose ASD and is a poor tool for quantifying the size of a shunt.

Oximetry, or measurement of the oxygen saturations in various locations in the venous system and the right heart (“oxygen saturation run”), is the most frequently used invasive technique due to its simplicity and reliance on readily available equipment.

Oxygen saturation run

The oxygen saturation run is performed as a catheter is passed through the venous system, right heart, and pulmonary circulation. The samples need to be acquired with the patient breathing room air or a gas mixture containing no more than a maximum of 30% oxygen [1]. Saturation data may be inaccurate in patients breathing more than 30% oxygen, since a significant amount of oxygen may be present in dissolved form in the pulmonary venous sample. Dissolved oxygen is not factored into calculations when saturations are used and thus pulmonary flow will be overestimated and the amount of shunt exaggerated.

The oxygen saturation run includes saturations from the superior vena cava (SVC), inferior vena cava (IVC), right heart, pulmonary artery, and arterial circulation. Pulmonary capillary wedge and/or left atrial samples are also sometimes obtained. Samples can be obtained as the right heart catheter is advanced, although many clinicians choose to place the catheter in the pulmonary artery first and then obtain samples as the catheter is withdrawn. This latter technique can improve the speed of obtaining samples, which can become an issue if there is difficulty advancing the catheter into the pulmonary artery.

Blood samples should be taken from the locations listed in Table 7.1 and oxygen saturations determined. IVC saturation varies depending on where the sample is obtained, and the sampling site should be at the level of the diaphragm to ensure that hepatic venous blood is taken into account. An arterial blood sample should be collected at the same time as the venous samples are being obtained.

Multiple samples may have to be taken in various chambers to ensure accuracy; however, it is important to note that variability in oxygen saturation (in the absence of shunt) decreases as blood flows through the heart. Oxygen content varies by approximately 2 mL O₂/100 mL blood in the right atrium (RA), 1 mL O₂/100 mL blood in the right ventricle (RV), and 0.5 mL O₂/100 mL blood in the pulmonary artery (PA) [2]. These values were determined by direct measurement of oxygen content in the days prior to the development of oximetry. Oxygen content can be approximated by the following formula:

In a patient with a hemoglobin of 15 mg/dL, a difference in saturation of 5% is approximately equal to a difference in oxygen content of 1 mL O₂/100 mL blood.

The goal of the oxygen saturation run is to measure differences in oxygen saturation in various chambers of the heart. There are several practical factors that must be kept in mind to ensure accurate results:

• The oxygen saturation method assumes that the body is in a steady state during the collection of samples. To ensure that the results are as accurate as possible, the samples must be collected as close in time as good technique permits. For this reason, many physicians perform a retrograde oxygen saturation run in which they begin with the catheter in the PA.
  
• Blood samples should not be withdrawn into the syringe too rapidly. Matta et al. found that rapid aspiration increased oxygen saturations [3].
  
• Complete mixing of blood is assumed in all chambers.
  
• There has to be adequate flushing of the catheter between samples.

Limitations of using oximetry to detect and quantify intracardiac shunts

Using the saturation “step‐up” method to detect and quantify intracardiac shunts has some limitations that should be noted. First, because of the inherent variability in determining oxygen saturations, the oximetry method of quantifying shunts loses accuracy when determining small shunts or in the presence of high cardiac output (which decreases AVO₂ difference). Second, the magnitude of the step‐up varies with the oxygen‐carrying capacity of blood and the cardiac output. Saturation step‐ups are increased if hemoglobin concentration is low or cardiac output is low. As Shepherd et al. [4] note, a 5% step‐up occurs with a shunt flow of 3400 mL/min when systemic blood flow is 7.5 L/min, a shunt flow of 1300 mL/min when systemic blood flow is 5 L/min, and a shunt flow of only 285 mL/min if systemic blood flow is 2.5 L/min.

Third, the relationship between the magnitude of step‐up and the shunt flow is nonlinear and, with increasing left‐to‐right shunting, a given change in shunt flow produces less of a change in the saturation step‐up. In contrast, quantification of shunt size using Q_p/Q_s is not affected by hemoglobin concentration and the relationship between Q_p/Q_s and shunt flow is linear. Q_p/Q_s is sensitive to cardiac output and some investigators advocate using exercise in patients with low cardiac output to improve accuracy.

Diagnosis of intracardiac shunts at right heart catheterization

Patients are often referred for cardiac catheterization to confirm and quantify a shunt that has been detected by physical exam and/or imaging. Occasionally, however, a patient will have a shunt that is unsuspected prior to right heart catheterization. These patients can be identified by routinely measuring oxygen saturation in the PA and the RA. Hillis et al. found that the difference (mean ± SD) between RA and PA saturations in 980 patients without intracardiac shunts was 2.3% ± 1.7%. In this same population, the difference (mean ± SD) between SVC and RA saturations was 3.9% ± 2.4%. Using threshold values of 8.7% between SVC and RA and 5.7% between RA and PA as “cut‐offs” (mean + 2 SD) to identify patients with intracardiac shunts had excellent sensitivity and specificity [5]. Similarly, differences in oxygen saturation of 8% between SVC and PA and 5% between RV and PA can be used to detect intracardiac shunts.

Quantifying a left‐to‐right shunt

Quantifying a left‐to‐right shunt is generally done in two ways. The ratio of pulmonary blood flow versus the systemic blood flow can be calculated, termed the Q_P/Q_S, where Q_P is the pulmonary blood flow and Q_S is the systemic flow. Alternatively, the actual flow of the shunt can be calculated. This is the difference between the pulmonary blood flow and the systemic blood flow (these two are equal in a normal heart).

The pulmonary (Q_P) and systemic (Q_S) blood flows are calculated using the Fick method of estimating cardiac output (Table 7.2). The only difference between the pulmonary flow equation and the systemic flow equation is in the arterial and venous saturations used. The Q_P and Q_S equations are shown as well.

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