Zero reference level for right heart catheterisation

Where is the ideal ZRL?

Every pressure reading during a catheter investigation is a difference between the pressure at the chosen ZRL and in the “chamber or vessel” where the fluid-filled catheter tip is located, provided there is no obstruction and no significant flow within the catheter. According to one classical physiological theory going back to the end of the 19th century [11], the ZRL should be set at the level of the “physiologic zero point” or “hydrostatic indifferent point” which represents the location in the cardiovascular system where the central venous pressure is tightly regulated, changing little if at all during the volume shifts caused by changes in the position [12, 13]. This point may be found at the junction of “phlebostatic levels” [14] and was supposed to be within the right atrium or in the right ventricle next to the tricuspid valve, but in any case at the level of the right atrium in the supine patient. Accordingly, it has been widely accepted that in the case of PAP and venous pressure measurements, the ideal ZRL should be set at the level of the right atrium [6, 9, 15–17]. However, in the search for the optimal ZRL, it may also be adequate to refer to the level of the left atrium [18, 19], particularly if the diastolic function of the left ventricle is to be analysed.

How to define the level of the right atrium?

There were several efforts to define well recognisable external points which may help to set the ZRL at the level of the right atrium in the supine position. However, there is considerable heterogeneity among the methods, which may be classified into the following approaches: 1) fixed distance from the anterior surface of the chest; 2) fixed distance from the table level; 3) a measure relative to the anterio-posterior diameter of the chest; and 4) individual determination by echocardiography or other imaging technique.

The first method was suggested by Moritz and von Tabora [6] on the basis of cadaver examinations, and has the advantage of being easy to perform, well reproducible and insensitive to softer underlayment. According to most studies, the ZRL was set at 5 cm below the surface of the thorax. The method was mainly criticised because in individuals with large thoracic diameters this ZRL may be too high, resulting in too low pressure readings [9].

According to the second approach, the ZRL is mostly set at 10 cm from the table level. This approach was also based on cadaver examinations and suggested by Lyons et al. [9]. Based on their measurements, normal subjects had the smallest variability by using this approach. The method is widely accepted, easy to perform, but has also been criticised by several authors [10, 20] and comparison studies. The main argument against this method was that in patients with large thoracic diameters the ZRL may be below the level of the heart.

The third approach provides a measure relative to the anterio-posterior diameter of the chest. Several options have been suggested, but most frequently the mid-chest level [8, 18], the 1/3 to 2/3 ratio of the thoracic diameter [7, 21, 22], the mid-axillary line [23–25] and the anterior axillary line [26, 27] were used. Comparison studies found these approaches to provide more accurate ZRLs as compared to fixed distances from the anterior thorax surface or the spine [10]. One disadvantage is that the determination of the ZRL is more difficult and may lead to errors and considerable variability, when the method is applied by different and/or inexperienced personal [28].

A fourth method was suggested by Courtois et al. [29], based upon individual determination of the ZRL by echocardiography. Although the use of the ultrasound approach may appear complex for everyday routine, it might enhance diagnostic accuracy in centres that use the readings for scientific reasons.

In this retrospective study, we used CT images in order to define the true level of the right atrium. This allowed for an unequivocal determination of the centre of the right and left atrium in every single patient. We consider both as possible reference ZRLs. ZRL methods that employed the antero-posterior thoracic diameter appeared to be more accurate as compared to ZRLs with a fixed distance from either the anterior body surface or the table level. The best agreement with the right atrium was found, both in patients with and without PH, when the ZRL “1/3 thoracic diameter below anterior thorax surface” was used, while the ZRL at 10 cm above table level quite poorly predicted the right atrial level. This may be due to the fact that we generally found larger thoracic diameters in our patients compared to most historic cohorts. As thoracic diameter was correlated with BMI, this may be explained by the increase of BMI in the population during the past decades. It may be mentioned that, in the REVEAL registry, the BMI of PH patients was even higher than in our cohort [30]; therefore, the impact of ZRL definition would have even been larger than in our study. For the left atrial level, the mid-thoracic line was most adequate in nearly all our patients. This suggests that, for scientific questions related to left ventricular function, the ZRL at the mid-thoracic level might be ideal.

Clinical relevance

How do differences between ZRLs lead to pressure differences? As the specific gravities for blood and mercury are 1.055 and 13.6, a blood column of 1 cm is equivalent to a mercury column of 0.78 mm. Accordingly, by shifting the ZRL from the mid-atrial ZRL to “5 cm below anterior thorax surface”, the percentage of patients in our patient cohort with mean PAP ≥25 mmHg would decrease from 68% to 59%, while shifting the ZRL to the level “10 cm above table level” would increase this percentage to 80% (table 4). Similar changes may be observed regarding PAWP. PAWP readings are of special relevance for the diagnosis of pulmonary arterial hypertension, because readings >15 mmHg preclude this diagnosis [1]. Notably, in their description of left atrial pressures in normal subjects, Braunwald and co-workers set the ZRL at 5 cm below the anterior thorax surface and found that the normal left atrial pressure never exceeded 12 mmHg [31, 32]. However, other ZRLs, such as 10 cm above table level, would have resulted in significantly higher values [33]. Therefore, one of the major confounding factors for any definition of the “normal range of PAWP” is the definition of ZRL. This may be of special interest when the upper limit of PAWP is considered among the inclusion criteria of PAH studies, as the 15 mmHg cut-off value using the 1/3-thoracic diameter ZRL would be very close to the 12 mmHg cut-off level described by Braunwald et al. [31] using the ZRL 5 cm below the sternal angle.

Unfortunately, in different countries, different cities, and even within centres, different ZRL methods have been used and there is no consensus on a gold standard. This may lead to therapeutic consequences for patients, particularly in the obese population. Therefore, a uniform definition of ZRL should be established, in order to avoid discrepancies and misconceptions about the definitions of PH and elevated left heart pressures. Remarkably, independent of BMI and thoracic diameter, the reference line at 1/3 thoracic diameter remained representative for the right atrium and the one at 1/2 thoracic diameter for the left atrium in our study. When choosing the appropriate ZRL, besides physiological considerations, also the practicability and reproducibility of the method has to be taken into account [28]. This might speak in favour of the mid-thoracic level.

Limitations

In this manuscript, we focused on the influence of ZRL on pressure readings in the supine position. We did not address other important issues such as the role of in- and expiration [34], intrathoracic pressure and its changes caused by obstructive or restrictive lung diseases and by exercise. In addition, we did not address the question, where the ZRL should be placed in the upright or semi-upright positions. Based on the “phlebostatic axis” theory, in the sitting position, the use of the fourth intercostal space for ZRL may be the most widespread one [14]. We feel that all these mentioned factors are very important for the interpretation of pressure readings, but they cannot be addressed adequately, if there is no consensus on the ZRL in the supine position.

Our chest CT readings may have included the vena cava confluens as part of the right atrium in some patients, leading to a slight overestimation of right atrial diameters. This may have influenced the localisation of the posterior wall of the right atrium, causing a minor error in the determination of the centre of the right atrium.

The choice of the CT slice for the thoracic diameter measurement was based on the optimal presentation of the right or left atrium in the chest CT, and not on an anatomical landmark. The detected diameters showed only minimal differences (0.3±0.5 cm) between these two slices. Therefore, in the absence of major thoracic deformities, a significant effect of these differences appears unlikely.

The proportion of patients with changes in final diagnosis due to the change of ZRL is dependent on the examined population. This rate may have been different in a centre, where more patients with congestive heart failure or less patients with slightly elevated PAP values would have been investigated than in our study. This, however, does not question the necessity of standardisation.

Conclusion

The four most commonly used methods for ZRL setting result in significantly different thoracic blood pressure readings and may significantly influence the classification of PH patients. As long as there is no international standard, the method of zeroing should be provided in each study on haemodynamics.