The hemodynamic monitoring in the dialyzing patients during the dialysis sessions necessitates the CVP measurement. This is used to be done by central catheters in the IJV. The invasive nature of this procedure, especially in patients who do not have an indwelling catheter has directed the researchers for finding a non-invasive substitute.
The IJV US could be used for assessing the CVP and it has offered a simple and reliable alternative for the catheter measurement.
In this context, we used the IJV US for the CVP measurement in two different methods based on the literature review, including the collapsing point (method 1) of the IJV and the cross-sectional area measurements for the IJV and the CCA (method 2).
For method 1; our results are consistent with the Kerleroux et al. study, which had exclusively enrolled the hemodialysis patients like those in our study, but their sample size was much smaller than ours (22 patients) and they had reported a significant correlation between CVPni and CVPi with P < 0.0001 [10].
Siva et al. had also reported a highly significant positive correlation between CVPni and CVPi (ρ = 0.004) in their study population (44 patients) using method 1 [11].
Congruence with Xing et al. was also present, where they had used the same principle (in method 1) as ours for CVPni measurement. However, they used echocardiography in their patients for more accurate detection of the right atrium center instead of using the five cm additive estimation [12]. Despite being a more accurate method, but it would significantly decrease the merit of being a simpler and less time-consuming procedure, consequently, it requires more training for the operator when compared to the other methods used in our study and Kerleroux et al., Siva et al. studies [10, 11]. Nonetheless, Xing et al. had also reported a similar significant positive correlation between CVPni and CVPi in both preoperative measurements (r = 0.90; ρ < 0.01) and in postoperative measurements (r = 0.93; ρ < 0.01) for their patients (118 patients) [12].
For method 2; our results are concordant with those of Hossein-Nejad et al. who performed their study on 52 non-ventilated patients and also reported a highly significant positive correlation between IJV/CCA cross-sectional area ratio and CVPi (r = 0.728, p < 0.0001 at inspiration, and r = 0.736, p < 0.0001 at expiration), while the AUC for the ROC curve was 0.882 for predicting patients with CVPi < 10cmH20. They calculated a cut-off point (= 2) for the prediction of CVP ≥ 10cmH20, and they found a significant correlation between the IJV area and CVPi with no significant correlation between CCA area and CVPi [13]; however, our sample size is almost double theirs.
Bailey et al. had also documented similar results to ours. They had concluded that the IJV/CCA cross-sectional ratio could predict the value of CVP. Their preliminary results suggested that if the IJV/CCA cross-sectional area ratio was at least 2, then the CVP seemed to be ≥ 8 mmHg which is nearly close to 10 cmH20 (ρ < 0.001). It was a pilot study that was conducted in the pediatric burn population with a small sample size including only six patients [14].
Bano and Canuad had performed their study on 49 ventilated and non-ventilated patients. However, they measured IJV/CCA diameter ratio instead of cross-sectional area ratio and found a significant positive correlation between IJV/CCA diameter ratio and CVPi in only non-ventilated patients at end-expiration (r = 0.439, n = 24, ρ = 0.032), and calculated a cuff-off (IJV/CCA) diameter ratio (= 1.75) for predicting CVP ≥ 10 cm H20 [15], which is matching with our results. However, they found no significant correlation between the IJV/CCA diameter ratio and CVP in non-ventilated patients at inspiration (r = 0.308, n = 24, p = 0.143) and in ventilated patients at both inspiration and expiration (r = 0.343, n = 25, p = 0.094 and r = 0.346, n = 25, p = 0.094, respectively). Our study was performed on non-ventilated patients and all our measurements were taken at end-expiration, so our results are concordant with theirs regarding their sub-group of non-ventilated patients who were being examined at end-expiration.
Donahue et al. had utilized only the IJV diameter and IJV cross-sectional areas in both supine and 35° reverse Trendelenburg positions at both end-inspiration and end-expiration. They depicted a significant difference in IJV diameter in patients with a CVP < 10 or ≥ 10 cm H2O and a significant positive correlation was present between the IJV end-expiratory diameter and CVP (r = 0.82) in the supine position [16]. Their results are in line with ours as we also had found a significant correlation between the IJV area and CVPi (ρ < 0.001).
A contradiction with the results of Elsadek et al. who had performed their study on smaller sample size (16 pediatrics patients) and had detected a poor correlation between IJV diameter or IJV area and the volume status (as predicted by left ventricular end-diastolic area) (LVEDA) [17]. However, the smaller sample size, the different age groups, and most importantly the use of LVEDA as a predictor of volume status instead of CVPi are all considered as probable causes for this contradiction.
Comparison and combination of both methods were then performed in terms of novelty; to our knowledge, our study was the first one to include and compare these two methods. Both methods show overall comparable accuracy for differentiation of patients with CVPi < 10cmH20 and ≥ 10cmH20 (87.10% for method 1 and 87.85% for method 2). Upon using both methods together for the same patient we had achieved a higher accuracy level approaching 89.25% which is better than using either method 1 or method 2 that had an accuracy of 87.10% and 87.85%, respectively. Thus, we propose the application of both methods together is recommended to improve the confidence in the acquired measurements; moreover, the experience with both methods could be beneficial when one of the two methods is not applicable or feasible for use in one patient.
It worth mentioning that one of the strengths of this study is the homogeneity of the studied sample (Non-ventilated adult hemodialysis patients) but on the other hand, it is considered one of its limitations as it is not applicable for ventilated patients.
Some additional limitations were met in this work including.
For Method 1, its limitation was the inability to measure the extremes of CVP as no collapse point for the IJV was detected along its neck course. In our study, the exact measurements of CVPni could not be obtained in nine patients with very high CVPi (mean CVP for these 9 patients was 19.278 cmH20) as the IJV was distended all-through its neck course, moreover, the measurements were not feasible in another four patients who had a low CVPi (mean CVP for these 4 patients was 2.25cmH20) as the IJV was collapsed all-through its neck course. Nevertheless, this limitation is of little clinical importance because it could still detect if the CVP is low (below 5cmH20) or very high.
For Method 2, it only gives an estimate and not a direct reading of the CVP, however, in agreement with other studies [13, 15, 16]; we found that method 2 was able to accurately differentiate between CVPi < 10cmH20 and ≥ 10cmH20, and this is considered as an important issue during resuscitation of the critically ill patients. Further studies on larger samples are recommended to find out IJV/CCA cross-sectional area ratios that are well correlated with the exact CVPi values.
Finally, we recommend (from our experience) US measurement of CVP as the first method of choice in patients with a dialysis arteriovenous fistula, to avoid insertion of central venous catheters that could carry a risk of central venous thrombosis and deprives the patient of future central access by either Mahurkar or Permacath in case of a dysfunctional fistula.