Are Radial and Aortic Pressures the Same on Vasopressors?
Are Radial and Aortic Pressures the Same on Vasopressors?
Abstract & Commentary
Synopsis: Radial artery pressures are unreliable and should not be used to manage drug infusion in patients receiving high doses of vasopressors. A group of 14 septic patients receiving an average of 85 µg/min of norepinephrine demonstrated a systolic pressure gradient of 57 mm Hg and a mean pressure gradient of 15 mm Hg, with radial artery pressure significantly less than aortic pressure.
Source: Dorman T, et al. Crit Care Med 1998 (10);26:1646-1649.
Suspecting a discrepancy between central and radial artery pressures, Dorman and colleagues studied 14 patients with septic shock who were receiving norepinephrine titrated to maintain a mean radial arterial blood pressure of at least 60 mm Hg. The study involved inserting a 30-cm, 16-gauge vascular catheter into the central aorta using the Seldinger technique from a femoral artery, and comparing simultaneous aortic and radial artery pressures. Nine men and five women averaging 57 years of age were studied. Septic shock was presumed present if the usual conditions were seen. Norepinephrine was begun if the mean radial arterial pressure was less than 60 mm Hg with a pulmonary artery wedge pressure greater than 12 mm Hg and a high cardiac output (average 7.1 L/min). After mean pressure was restored to 60 mm Hg or higher, the femoral catheter was inserted and aortic pressure measured. Using the same transducer and identical tubing, simultaneous determinations of systolic, diastolic, and mean pressures were then made.
The first recorded pressures obtained within 5 minutes of femoral line placement were averaged. (See Table 1.) The average dose of norepinephrine was 85.6 µg/min at the time of the measurement. All patients demonstrated a higher mean aortic than radial arterial pressure, while several had identical systolic pressures and one patient actually had a lower aortic diastolic pressure. Most patients had their radial artery catheter removed after the femoral line was placed. Two patients remained cannulated in both arteries over the several days while they were weaned from norepinephrine. In these two patients, the mean arterial gradient disappeared after the vasoconstrictor was discontinued.
| Table 1 | |||
| Recorded Average Pressures | |||
|
mm Hg |
mm Hg |
mm Hg |
|
| Aortic |
|
|
|
| Radial |
|
|
|
| Pressure Difference |
|
|
|
| ___________________________________________________________ | |||
Comment by Charles G. Durbin, Jr., MD, FCCM
Radial artery systolic pressure is normally higher and diastolic pressure lower than aortic pressure due to the effects of reflected and standing waves in the vascular tree. Hypertension, stiff vessels, and tachycardia increase these gradients. Mean arterial pressure, however, is unaffected by this wave phenomenon and radial and aortic mean pressures are usually close to each other. The reversed systolic pressure gradient and lower radial mean arterial pressure reported here have also been identified in patients during rewarming after cardiopulmonary bypass. In this circumstance it is thought to be due to extreme vasodilatation in the peripheral circulation, although the exact mechanism is not known.
Whatever the cause of this gradient, the clinical implications are clear: septic patients receiving high-dose norepinephrine should have their vasoactive drip titrated to aortic pressure rather than to the pressure measured in a radial or other peripheral artery. Patients in this study had the dose of norepinephrine rapidly reduced following aortic catheter insertion. Unanswered questions include: At what norepinephrine dose does the gradient become significant? Do other vasoconstrictive agents (i.e., phenylephrine, epinephrine) produce the same problem? Does the vasoconstrictor use gradient exist in other forms of shock?
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