Susan R. Wilcox, MD
One of the most important concepts in resuscitation is volume responsiveness, or the ability of the cardiac output to increase in response to a fluid challenge. As compared to prior static measures of venous filling, such as central venous pressure (CVP) or pulmonary artery occlusion pressure (PAOP or wedge pressure), clinicians are increasingly turning to dynamic means of assessing patients’ volume responsiveness.1,2 One such measure is the pulse pressure variation (PPV).1,2
PPV is a reflection of cardiopulmonary interactions. As a patient breathes, both spontaneously and with mechanical ventilation, cardiac output varies. The more the cardiac output varies with respirations, the more likely that patient is to respond to a fluid bolus with an increase in cardiac output. Using this simple principle, clinicians can take advantage the common arterial line tracing to assess a patient’s volume responsiveness.
To measure the PPV in a given patient, that patient must have consistent and demonstrable cardiopulmonary interactions. This means that the patient must:
If all these conditions are met, the arterial waveform can be assessed for variation in a few simple steps.
1. To make measurements easier, condense the waveform to 6.25 mm/sec from the baseline of 25 mm/s. (This is easy to do, and is located under “speed” on most monitor settings.)
2. Identify the maximum (during inspiration) and the minimum (during exhalation) waveforms on the undulating pattern.
3. Using the cursor, find the systolic pressure value for the largest, or maximum, inspiratory waveform, then find the diastolic pressure. Take care to use the systolic and diastolic values on the same form. The difference in these two values is your pulse pressure. The following four figures have been captured from a monitor and enlarged for detail. The first figure below shows the value for the maximum systolic of 109 mmHg.
This figure shows the measurement of the diastolic of the maximum waveform, or 60 mmHg.
4. Repeat step 3 for the minimum form. Here, the systolic is 91 mmHg.
And the diastolic is 54 mmHg.
5. These steps will give you two pulse pressures. The pulse pressure variation is calculated as:
Therefore, in this case, the PPmax is 49, the PPmin is 37, for a mean PP of 43. This leads to a calculated PPV of 27.9%. (12/43 X 100)
Numerous studies have found that a PPV of > 12% is associated with volume responsiveness in the operating room and intensive care unit (ICU) alike. A meta-analysis of PPV to predict fluid responsiveness found a sensitivity and specificity of 0.89 and 0.88 respectively.3 A meta-analysis of 22 studies focused on ICU patients had similar findings, with a pooled sensitivity of 0.88 and pooled specificity was 0.89.2 To date, no study has specifically looked at PPV in the ED, and this is an area for future study.
The most common pitfalls in using PPV for clinical decision-making involve failing to recognize that the patient does not meet one or more of the required criteria listed above. If patients are breathing spontaneously, even while on assist control ventilation, they can vary their intrathoracic pressure and thereby cause exaggerated or blunted responses to the positive pressure breaths.2 Similarly, clinicians should note that the tidal volumes used in this assessment, of > 8mL/kg,2 are larger than are otherwise used (ideally, around 6mL/kg). In our practice, we temporarily increase the tidal volume just for the purposes of this assessment, to ensure that the patient is seeing sufficient positive pressure ventilation to impact hemodynamics. However, if these criteria are met, PPV is an easy, rapid means to assess a critically ill patient’s likelihood of volume responsiveness.