CRITICAL CARE MEDICINE SECTION
Utilization of Arterial Lines in the Emergency Department Alex Yang, MD* and Elias Wan, MD FAAEM†
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onitoring and early goal-directed therapy when utilized for the critically ill patient in the emergency department has shown to reduce mortality. The placement of an arterial cannula into the radial, brachial, femoral, or dorsalis pedis artery allows for continuous and accurate monitoring of arterial blood pressure and can provide vital information that can affect the management of critical patients in the emergency department.1
The arterial pressure waveform Once in the artery, the arterial cannula is connected to a calibrated transducer that converts pressure readings to electrical signals that show up on the monitor as a waveform (Figure 1). The waveform consists of two phases—the systolic and diastolic—separated by the dicrotic notch. The systolic phase consists of the systolic upstroke which is initiated by the opening of the aortic valve, the peak systolic pressure, and the systolic decline. The diastolic phase consists of the dicrotic notch and the diastolic runoff. The dicrotic notch, which may represent the closure of the aortic valve, interrupts the arterial runoff phase with a small peak as the forward flow of blood is reflected upon the closed valve. The waveform then terminates with the diastolic runoff where the lowest point is marked by the minimum diastolic pressure.2
emergency department.3 A landmark study by Lehman et al. in the Beth Israel Deaconess Medical Center compared over 27,0000 simultaneously obtained blood pressure measurements in ICU patients and found that despite having similar mean arterial pressures (MAP), non-invasive systolic pressure readings (NIBP) were less sensitive in hypotensive patients compared to invasive measurements (IABP).4 A recent study also demonstrated that patients with hypotensive shock who were either on vasopressin or had elevated lactic acid of 4 millimole per liter (mmol/L) demonstrated clinically relevant MAP difference between IABP and NIBP.13 While MAP is the measurement of choice in the management of hypotensive patients on pressors, there are several populations of patients who require accurate continuous monitoring of systolic and mean arterial pressures, especially diseases with hypertensive emergency, which often can have more than >10mmHg difference between IABP and NIBP. Moreover, up to 30% can have clinically relevant differences that need intervention.14 These populations include patients with strict blood pressure goals such as patients with aortic dissections, spontaneous intracranial hemorrhages, and left ventricular assist devices (LVAD). Current guidelines recommend a strict blood pressure goal of 100-120 mmHg for patients with aortic dissections to limit the shearing forces on the initial aortic flap.5 Similarly, patients with spontaneous intracerebral hemorrhages have a strict goal of lower to 130-150 mmHg to minimize hemorrhage while also maintaining appropriate perfusion.6 Lastly, LVAD patients have a strict MAP goal of 70-80 mmHg to optimize cardiac support which is not measurable by traditional blood pressure cuffs due to the absence of pulsatile arterial flow.7
Pulse monitoring
Figure 1: The Arterial waveform Legend: A) Aortic valve opens B) Systolic upstroke C) Peak systolic pressure D) Systolic decline E) Aortic valve closes F) Dicrotic notch G) Diastolic runoff H) Minimum diastolic pressure
Blood pressure monitoring Accurate monitoring of blood pressure in critically ill patients was the primary indication in 41% of arterial lines placed in one pediatric
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COMMON SENSE JULY/AUGUST 2022
Invasive monitoring is also necessary to monitor pulsatility. This is particularly useful in the peri-arrest patient, and patients receiving extracorporeal membrane oxygenation (ECMO). In the peri-arrest patient, pulses are often soft and difficult to palpate and can determine whether the patient has a shockable rhythm.8 Monitoring pulsatility is also useful in ECMO patients to assess cardiac function. These patients tend to have abnormal pulses due to the collision between the pulsatile antegrade flow of blood from the heart and the continuous retrograde flow of blood supplied by ECMO. Using invasive monitoring, left ventricular function can be estimated based on whether the pulse is absent, decreased, or increased. Absent or decreased pulsatility in ECMO patients tend to suggest decreased left ventricular stroke volume and therefore decreased cardiac function, while increased pulsatility suggests myocardial recovery.9 Routinely, in peripheral cannulated Veno-Arterial ECMO setting the preferred site would be right radial as this would reflect the most distal sampling of oxygen delivery.
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