Comparison of intrathoracic and intra-abdominal measurements of central venous pressure

factor downregulates an endothelial nitric oxide synthase mRNA by shortening its half-life. Circ Res 1993; 73: 205-09. 27 Kirchhofer D, Tschopp TB, Hadvary P, Baumgartner HR. Endothelial cells stimulated with tumor necrosis factor-alpha express varying amounts of tissue factor resulting in inhomogenous fibrin deposition in a native blood flow system. Effects of thrombin inhibitors. J Clin Invest

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28 Dunkman

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myocardium. Br J Pharmacol 1992; 105: 575-80. Pinsky DJ, Cai B, Yang X, Rodriguez C, Sciacca RR, Cannon PJ. The lethal effects of cytokine-induced nitric oxide on cardiac myocytes are blocked by nitric oxide synthase antagonism or transforming growth factor beta. J Clin Invest 1995; 95: 677-85.

Comparison of intrathoracic and intra-abdominal measurements of central

venous

pressure

G M Joynt, C D Gomersall, T A

Buckley, T E Oh,

RJ

Young, R C Freebairn Introduction

Summary Background

Complications

can

arise

from

standard

intrathoracic central venous pressure (CVP) measurements in critically ill, mechanically ventilated patients. We have assessed the feasibility of catheterisation by the femoral route to measure CVP in the abdomen (ACVP). We compared measurements by the standard jugular or subclavian route (TCVP) with simultaneous ACVP measurements by the femoral route. Methods Between June, 1994 and May, 1995, we recruited 20 critically ill adult patients with various disorders; all patients already had a TCVP line in situ. We placed a femoral catheter in the inferior vena cava close to the right atrium under electrocardiographic guidance. The catheter position was confirmed (and corrected if necessary) by chest radiography. CVP was measured from both sites hourly for 6 h. Positive end-expiratory pressure, mean airway pressure, and intra-abdominal pressure were recorded simultaneously.

Central venous catheterisation and measurement of central venous pressure (CVP) are commonly used in the management of critically ill patients. The cannula is usually inserted through the internal jugular and subclavian veins, and CVP is measured in the superior vena cava proximal to the right atrium. 1,2 However, this approach can result in such complications as arterial puncture, pneumothorax, air embolism, venous thrombosis, thoracic-duct laceration, damage to the phrenic or recurrent laryngeal nerves, and brachial-plexus

injury.’-3

Interpretation

Catheters inserted through the femoral vein have been used successfully in acute resuscitation2,4,sand patients with burns.6 Studies in critically ill childrenand adults8 have shown that the femoral vein can be used for central venous cannulation with few complications. With the femoral approach CVP could be measured by placement of the catheter tip in the right atrium or in a major thoracic vein, but to position the catheter within, or to cross, the right atrium is dangerous and contrary to Food and Drug Administration recommendations.9 CVP measurement from the abdominal inferior vena cava accurately reflects superior-vena-cava pressure in animalslo and right-atrial pressure in infants and children undergoing cardiac catheterisation." At present, there is little clinical evidence on the use of this approach in critically ill adults. To assess the feasibility of CVP measurement by the femoral route, we compared intraabdominal central venous pressure (ACVP) with intrathoracic central venous pressure (TCVP) in a heterogeneous group of adult, ventilated, critically ill patients under clinical conditions.

Department of Anaethesia and Intensive Care, Prince of Wales Hospital, Chinese University of Hong Kong, Sha Tin, New Territories, Hong Kong (G M Joynt FFASA, C D Gomersall MRCP, T A Buckley MBChB, Prof T E Oh MD, R J Young MBBs, R C Freebairn MBChB) Correspondence to: Mr G M Joynt

Our study was approved by the Clinical Research Ethics Committee of the Chinese University of Hong Kong. All patients gave informed consent. Femoral catheterisation was undertaken only when thoracic routes of cannulation were impossible or the risk of complications was judged to be high. Eligible patients were those with coagulopathy, burns extending over venous access sites, or chronic illness with repeated central venous cannulations, and those undergoing mechanical ventilation that produced high airway pressures. From June, 1994 to May, 1995, we investigated all patients who were undergoing femoral catheterisation and who already had a line to measure TCVP in situ (n=20). Patients were excluded from the study if the existing TCVP line was thought to be a source of sepsis or if chest radiography did not confirm its correct

Findings One patient was excluded because radiography showed that the catheter position was incorrect. For 133 paired measurements of ACVP and TCVP in the remaining 19 patients, the mean difference was 0·45 mm Hg (SD 0·89; 95% Cl 0·30-0·60); the limits of agreement were -1·33 to 2·23 mm Hg (-1·63 to 2·53). We found a small tendency for the difference between ACVP and TCVP to increase as positive end-expiratory pressure and mean airway pressure increased; the difference was statistically, but not clinically, significant. Our study showed that for clinical purposes CVP can be measured by a femoral catheter placed in the abdominal inferior vena cava near the right atrium. This approach can replace standard TCVP measurements in critically ill, mechanically ventilated patients.

Methods

1155

51MV=synchromsed, intermittent mandatory ventilation; PS=pressure support ventilation; PCV=pressure central ventilation. *Catheter tip position in cm proximal to diaphragm. tCatheter depth estimated from

markings at skin insertion site. Table: Characteristics of patients and catheter placement catheter

position within the superior vena cava. All patients were placed in the supine position with a maximum head-up-tilt of 15°. Position of the patient and ventilator settings were not changed throughout the study. We inserted a long (40-70 cm) 7-Fr multilumen catheter with an electrocardiography adapter attached to the distal lumen by the Seldinger technique. 12 The femoral catheter was placed in the inferior vena cava close to the right atrium under electrocardiographic guidance.13 We identified the position of the catheter tip within the right atrium by monitoring changes in P wave configuration during insertion, and then by withdrawing the catheter to allow placement in the inferior vena cava, close to the right atrium. We confirmed the correct position of the catheter (and repositioned it if necessary) by chest radiography before CVP measurements were made. Catheter position and depth recorded. The existing TCVP line was left in situ throughout the investigation of the patient. CVP was measured and recorded from the monitor screen hourly for 6 h from both sites (at the same time but in random order) with the same disposable electronic transducer, fixed at the level of the right atrium (junction of mid-axillary line and fourth intercostal space). Two pairs of measurements were recorded: one pair (TCVP and

were

Figure 2: Sequential changes In ACVP plotted against simultaneous changes in TCVP Line of equality (y=x) is expected if two methods are in perfect agreement. measured manually with a cursor by freezing the the monitor at end-expiration; the other pair was obtained from the record of the algorithm-generated CVP readout. The observer did not know the site of recording at any time. Simultaneous measurements of positive end-expiratory pressure, mean airway pressure (from the ventilator), and intraabdominal pressure (from an indwelling urine bladder catheter") were taken.

ACVP) CVP

was

trace on

Results

patient was withdrawn from the study because radiography showed an incorrect catheter position. In the remaining 19 patients the line position was radiographically confirmed to be within 5 cm of the right atrium (mean 2-1cm; [SD 1-0]). Catheter placement under electrocardiographic guidance was successful in 13 of 19 cases. Incorrect positioning was caused by electrical interference that prevented adequate P wave and QRS definition or by absence of the expected P-wave changes. For these six patients (2-4, 7, 12, and 19) catheter-tip placement was achieved radiographically. Characteristics of the patients, diagnosis, mode at verification, and catheter position and depth are shown in the table. We compared 133 pairs of measurements in 19 patients. The algorithm-generated CVP measurements ranged from 1 to 26 mm Hg. Paired measurements of ACVP and TCVP were compared by the method of Bland and Altman 15 (figure 1). The mean difference between TCVP and ACVP was 0-45 mm Hg (SD 0-89; 95% CI 0-30-0-60). The limits of agreement were therefore -1-33 to 2-23 mm Hg (-1-63 to 2-53). There was a small but significant tendency for the One

difference between TCVP and ACVP

to

increase

as

positive end-expiratory pressure (p<0-001, r=0-334) mean airway pressure (p<0-05; r=0-217) increased (data not shown). Intra-abdominal pressure in the range

and

Figure 1156

1: Bland-Altman

plot comparing ACVP with TCVP

2-22 cm H2O had little effect on the difference between TCVP and ACVP. The ability of ACVP to reflect TCVP was assessed by plots of sequential changes in ACVP against the simultaneously measured changes in TCVP (figure 2).

Agreement between algorithm-generated and manually recorded TCVP was compared by the Bland and Altman method. The mean difference between algorithmgenerated and manually recorded CVP was 0-79 mm Hg (95% CI 0-58-1-0). The differences were distributed normally around the mean with a SD of 1-22 mm Hg. The limits of agreement were therefore -1·65 to 3-23 mm Hg (-2-0 to 3-58). Results of comparisons of manually recorded TCVP and ACVP were similar to the results of the algorithm-generated CVP.

Discussion Our results show that for clinical purposes CVP can be measured by a catheter placed in the abdominal inferior vena cava near the right atrium. TCVP readings are on average about 0-5 mm Hg higher than ACVP readings, and ACVP will be 2 mm Hg below or 1 mm Hg above TCVP in 95% of measurements. These differences are not clinically significant. The limits of agreement are acceptable for all values likely to be found in practice among patients with varied pathology and ventilatory requirements. Changes in CVP can be as important as absolute values and our data confirm that sequential changes in ACVP and TCVP are of similar magnitude. We found no clinically significant differences between manual readings and the results generated by the algorithm; the latter are easier to obtain, less timeconsuming, and more commonly used. Use of the femoral route has many advantages and is likely to become a more common approach for central venous catheterisation.16 Disorders such as septic shock, adult respiratory distress syndrome, and multiple organ dysfunction have become common, and require intravenous inotropes, intensive vascular monitoring, ventilation with high airway pressures, renal support, and intravenous nutrition. Patients may have long stays in the intensive care unit because of improvements in supportive measures. Under these circumstances multiple sites for cannulation are needed. Under high ventilatory pressures, iatrogenic pneumothorax can occur and coagulopathy can increase substantially the risk of bleeding complications. Femoral venous cannulation carries no risk of pneumothorax and the site is directly compressible should bleeding occur in patients with coagulopathy. The femoral route also increases the number of venous access sites. The femoral vein is a convenient site for cannulation during airway and respiratory emergencies, in patients with burns or with restricted venous access, and in patients who cannot lie flat. Furthermore, there is no risk of thoracic duct laceration and nerve damage is unlikely. There are also potential disadvantages of the femoral route-catheter-related deep-venous thrombosis and increased risk of infection. Although no comprehensive investigations on infection rates of femoral venous catheters have been done, a study of infection rates in long-term radial and femoral arterial lines showed similar rates of infection-related complications. 17 Among our patients no clinically obvious infections or thrombotic complications were observed. Since mechanical positive-pressure ventilation increases intrathoracic pressure, which affects pressure in the intrathoracic vessels, we looked for an effect of these ventilatory procedures on the difference between TCVP and ACVP. The effect was not clinically significant. Similarly, raised intra-abdominal pressure in patients with abdominal disorders might be expected to increase

pressure in the inferior

and lead to a greater discrepancy between ACVP and TCVP. Again, we found no evidence for such an effect. In supine patients, venous pressure measured in any intrathoracic vein reflects right atrial pressure within 1 mm Hg.11, We do not know whether measurements in the abdominal cavity also reflect right-atrial pressure this accurately. We confirmed the position of all femoral catheters close to the diaphragm and right atrium before starting measurements, and we do not know whether catheters placed in the abdominal inferior vena cava further from the right atrium would show the same agreement with TCVP. All our patients were already receiving some form of positive-pressure ventilation before we measured CVP, not by design but because the patients who met the criteria for femoral catheterisation were inevitably critically ill. Although there is no reason to suspect that the results would be different in patients who can breathe spontaneously, our findings should not be extrapolated to this group. We conclude that for clinical purposes CVP measured in the inferior vena cava close to the right atrium can replace standard TCVP measurements in mechanically ventilated, critically ill patients. vena cava

References

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1644-49. Swanson RS, Uhlig PN, Gross PL, McCabe CJ. Emergency intravenous access through the femoral vein. Am Emerg Med 1984; 13: 244-47. 6 Purdue GF, Hunt JL. Vascular access through the femoral vessels: indications and complications. J Burn Care Rehabil 1986; 7: 498-500. 7 Kanter RK, Zimmerman JJ, Strauss RH, Stoekel KA. Central venous catheter insertion by central vein: safety and effectiveness for the pediatric patient. Pediatrics 1986; 77: 842-47. 8 Williams JF, Seneff MG, Friedman BC, et al. Use of femoral venous catheters in critically ill adults: prospective study. Crit Care Med 1991; 19: 550-53. 9 Food and Drug Administration. Precautions necessary with central venous catheters. FDA Task Force. FDA Drug Bulletin July, 1989: 15-16. 10 Berg RA, Lloyd TR, Donnerstein RL. Accuracy of central venous pressure monitoring in the intra-abdominal inferior vena cava: a canine study. J Pediatr 1992; 120: 67-71. 11 Lloyd TR, Donnerstein RL, Berg RA. Accuracy of central venous pressure measurement from the abdominal inferior vena cava. Pediatrics 1992; 89: 506-08. 12 Seldinger SI. Catheter replacement of the needle in percutaneous arteriography. Acta Radiol 1953; 39: 368-76. 13 McGee WT, Ackerman BL, Rouben LR, Prasad VM, Bandi V, Mallory DL. Accurate placement of central venous catheters: a prospective, randomized, multicenter trial. Crit Care Med 1993; 21: 1118-23. 14 Iberti TJ, Lieber CE, Benjamin E. Determination of intra-abdominal pressure using a transurethral bladder catheter: clinical validation of the technique. Anesthesiology 1989; 70: 47-50. 15 Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; i: 307-10. 16 Seneff M. Central venous catheters. In: Rippe JM, Irwin RS, Alpert JS, Fink MP, eds. Intensive care medicine, 2nd ed. Boston: Little, Brown, and Company, 1991: 17-37. 17 Russell JA, Joel M, Hudson RJ, Mangano DT, Schlobohm RM. Prospective evaluation of radial and femoral artery catheterisation sites in critically ill patients. Crit Care Med 1983; 11: 936-39. 18 Guyton AC, Jones CE. Central venous pressure: physiological significance and clinical implications. Am Heart J 1973; 86: 431-37. 5

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