**3. Computed tomography assessment of right ventricular dysfunction**

Contrast-enhanced pulmonary CT angiography is increasingly used for first-line imaging in patients suspected of PE. This method allows the direct visualization of emboli, as well as providing information regarding the status of the right heart. Several methods have been suggested for the quantitative assessment of RV dysfunction by CT.

Fig. 1. Echocardiographic findings of pulmonary embolism in the parasternal long-axis (A)

apical four-chamber views)

paradoxical septal motion The interventricular septum bulges toward the left ventricle

Pulmonary artery dilation Main pulmonary artery >2.5 cm on parasternal short-axis

Table 1. Abnormal echocardiographic findings in patients with pulmonary embolism

**3. Computed tomography assessment of right ventricular dysfunction** 

Contrast-enhanced pulmonary CT angiography is increasingly used for first-line imaging in patients suspected of PE. This method allows the direct visualization of emboli, as well as providing information regarding the status of the right heart. Several methods have been

velocity >2.6 m/s Direct evidence of pulmonary hypertension

suggested for the quantitative assessment of RV dysfunction by CT.

The ratio of the right ventricular end-diastolic area to left ventricular end-diastolic area exceeds the upper limit of normal (>0.6 on parasternal long-axis views and >0.9 on

Subcostal view, diameter >2 cm with <50% respiratory variability; indirect sign of increased central venous pressure

Hypokinesis of the free wall but preserved apical kinesis

views; indirect sign of pulmonary hypertension

RV: right ventricle; LV: left ventricle; Ao: aorta; LA: left atrium

**Abnormal Finding Description** 

and short-axis (B) views.

Right ventricular dilatation

Septal flattening and

Reduced respiratory variability of the dilated inferior vena cava

Tricuspid regurgitation jet

Right ventricular regional systolic wall motion abnormalities

#### **3.1 Computed tomography findings of right ventricular dysfunction 3.1.1 RV dilation (RV/LV ratio)**

Similar to echocardiography, contrast-enhanced CT allows assessment of the right-to-left ventricular ratio. RV and LV diameters are assessed on each single image at the plane of maximal visualization of the ventricular cavities, usually at the mitral valve plane for LV and the tricuspid valve level for RV, between the inner surface endocardial border of the free wall and the surface of the interventricular septum (Fig. 2).

Fig. 2. Transverse contrast-enhanced CT scan showing maximum minor axis measurements of the right ventricle (A) and left ventricle (B). RV/LV ratio = 2

The RV/LV minor axis ratio is widely accepted as a measure of RV dilatation on CT, however, the cut-off values of RV/LV ratio used for RV dysfunction vary among reports. Ghuysen et al suggested an RV/LV ratio >1.5 indicates a severe episode of PE (Ghuysen et al., 2005), Araoz et al suggested an RV/LV ratio >1 was associated with a 3.6-fold increased risk of admission to the intensive care unit (Araoz et al., 2003), and in another study, the same threshold was shown to be a significant risk factor for mortality within 3 months, with an RV/LV ratio ≤1.0 having a PPV of 10.1% (95% CI: 2.9%, 17.4%) and an NPV of 100% (95% CI: 94.3%, 100%) for an uneventful outcome (van der Meer et al., 2005). An RV/LV ratio >0.9 on reconstructed CT four-chamber views has been associated with a poorer prognosis in patients with PE (Schoepf et al., 2004), with an NPV of 92.3% and a PPV of 15.6% for 30-day mortality, and a hazard ratio for predicting 30-day mortality of 5.17 (95% CI, 1.63 – 16.35; P=0.005).

Concerns have arisen regarding whether non-gated CT may be inaccurate in measuring ventricular chamber size because the images are acquired in different phases of the cardiac cycle. However ECG-gated CT scan is not always available, and is time-consuming. Thus, a ECG-gated CT scan is impractical in an emergency situation. In addition, recent findings

Risk Stratification of Submassive Pulmonary Embolism:

**3.1.3 Obstruction index** 

index of 55%

**3.1.4 Pulmonary artery diameter measurement** 

The Role of Chest Computed Tomography as an Alternative to Echocardiography 175

The PA obstruction index, or the percentage of vascular obstruction of the pulmonary arterial tree caused by PE, may be calculated as Σ (n × d) expressed as percentage vascular obstruction ([Σ (n × d)/40] × 100), where n is the value of the proximal clot site that equals the number of segmental branches arising distally, and d is the degree of obstruction, with partial obstruction scored as 1 and complete obstruction as 2. Values for n range from a minimum of 1 (obstruction of one segment) to a maximum of 20 (obstruction of both right and left pulmonary arteries) (Qanadli et al., 2001). With this scoring system, the maximum obstruction score is 40 (thrombus completely obstructing the pulmonary trunk), which corresponds to a 100% obstruction index. Using a cutoff of 60%, 83% of the patients with an index >60% died, whereas 98% of patients with a lower index remained alive (Wu et al., 2004). Patients with an obstruction index ≥ 40% were found to be at an 11.2-fold (95% CI: 1.3, 93.6) increased risk of dying from PE (van der Meer et al., 2005). However this index may

Another obstruction index, the pulmonary embolism severity index (PESI), was developed to estimate 30-day mortality in patients with acute PE. This index has also been used to identify patients with a low mortality risk who may be suitable for outpatient management of acute PE (Aujesky et al., 2007). The PESI contains 11 differently weighted baseline clinical parameters and is relatively complicated to administer and score. A simplified version of the PESI (sPESI) was therefore developed for ease of application. The sPESI showed similar prognostic accuracy and clinical utility as the PESI, although its use made it easier to

Fig. 4. Transverse contrast-enhanced chest computed tomographic scan showing pulmonary emboli (arrows) in both main pulmonary arteries (PAs). This patient had a PA obstruction

A pulmonary artery (PA) diameter greater than 30 mm indicates a PA pressure greater than 20 mmHg (Kuriyama et al., 1984). Moreover, the diameter of the central PA has been significantly correlated with the severity of PE (Collomb et al., 2003). In other studies, however, the diameter of the main PA and the ratio of the diameters of the main PA and the

not be practical for routine application without the aid of radiologists.

identify patients at low-risk of adverse outcomes (Jimenez et al., 2010) (Table 2).

have indicated that the benefits from a separate ECG-gated CT scan for the evaluation of RV ventricular diameter are minimal and do not justify its routine clinical use instead of the standard measurements of the minor axis (Lu et al., 2009).

Although most studies have indicated that CT assessments of RV dilatation contribute to the risk stratification of patients with PE, two recent meta-analyses have found that CT findings of RV dilatation have limited prognostic importance for mortality among patients with nonhigh-risk PE (Coutance et al., 2011; Sanchez et al., 2008), and that the greatest value of this method appears to be the identification of low-risk patients based on the lack of RV dilatation. These analyses suggested that measurements made on the four-chamber view are more reliable than traditional measurements made on the minor axis. However, most of these studies were of retrospective design and in small numbers of patients with generally undefined clinical presentations. Hence, any conclusions about the usefulness of this marker must be treated with some caution, although future large clinical studies and standardized definitions of RV dilatation will be required in this patient subset.

#### **3.1.2 Interventricular septal straightening/bowing**

If RV afterload suddenly increases, the interventricular septum, which normally bows toward the RV, may shift toward the LV because of its confinement within the pericardium. This phenomenon is readily visible on helical CT pulmonary angiography as straightening or bowing of the interventricular septum.

Leftward bowing of the interventricular septum on CT has been related to severe PA obstruction (Fig. 3). This bowing was found to strongly predict admission to the intensive care unit for PE, but was not associated with in-hospital mortality (Araoz et al., 2003). Thus, this sign is likely not an indicator of outcome and is not specific for PE (van der Meer et al., 2005). This bowing has also been observed in patients with chronic pulmonary artery hypertension, although, in the latter condition, the RV wall is usually thickened (>6 mm), whereas, in acute PE, the RV wall thickness is usually normal.

Fig. 3. Ventricular septal bowing (arrow) into the left ventricular lumen
