Kevin Kuruvilla, MD
Background
In patients with shock, improving end organ perfusion is the goal. To achieve this one must identify the type of shock and the volume status of the patient and whether the patient is in need of fluid resuscitation, diuresis, vasopressors, or ionotropes. Despite advances in critical care medicine, accurate and objective measurement of volume status remains uncertain and controversial. In patients with undifferentiated shock, in addition to invasive blood pressure monitoring, sometimes other hemodynamic parameters such as cardiac filling pressures, cardiac output and IVC collapsibility can be measured using combinations of the pulmonary artery catheter(PAC), ultrasound or echocardiography.
A couple points:
-We much more commonly have patients in the SICU and TICU with hemorrhagic or septic shock. Generally the most reasonable initial move that can be both somewhat therapeutic and diagnostic is a “fluid challenge” with one liter of crystalloid. Reassess the patient’s hemodynamic and urine output response to the fluid. Did heart rate decrease? Did MAP improve? Did UOP improve.
-Along these lines rarely are our patients ever pushed to far “to the right” on the Starling curve. So rarely will they benefit hemodynamically from early diuresis.
-Do not always try to normalize vital signs. A young patient with sepsis or after hemorrhage may perfuse vital organs well at a MAP of 55 or 60 mm while Surviving Sepsis guidelines suggest a goal MAP of 65 mm. Trying to attain this goal may come at the cost of increased fluid administration or pressor use. An elderly patient may not perfuse well at or above a MAP of 65 mm and may need further hemodynamic augmentation. It is uncertain what the goals should be for these patients and so they should be assessed individually.
Arterial pressure monitoring
Indirect methods (non-invasive) – Sphygmomanometer
This device is used most commonly to measure blood pressure in all clinical settings from a primary care office to the ICU. However blood pressure is often measured inaccurately with this method. Using a cuff of appropriate size is of paramount importance as using a cuff bladder which is too small for the upper arm can result in a falsely elevated blood pressure. Auscultation method (manual blood pressure) and oscillometric method (automatic blood pressure) are used to measure BP using a sphygmomanometer. Mean arterial pressure (MAP) is the most accurate measurement provided by the oscillometric method. Discrepancies have been observed in large numbers of cases when non-invasive pressure monitoring has been compared to direct intra-arterial measurements.
Direct methods – Arterial lines
Intrarterial pressure monitoring can be performed by arterial catheters placed in the radial, femoral, axillary, or brachial arteries. Strong consideration should be given to placement of an arterial line for patients in the ICU requiring a continuous infusion of vasopressors. Of note, recent studies have shown that arterial catheter related blood stream infections are comparable to central line associated blood stream infections (1.3%, 3.4/1000 catheter days vs. 2.7%, 5.9/1000 catheter days).
Full sterile barrier precautions should be used during placement of arterial lines. Radial and femoral arteries are preferred for placement of arterial lines as thrombosis of axillary and brachial arteries can be limb threatening.
Pulmonary artery catheters(PAC)
The PAC is a device that provides information regarding cardiac filling pressures, cardiac output(CO), systemic vascular resistance(SVR), and mixed venous oxygen content. It is typically introduced through a 8-9 Fr introducer sheath placed in the internal jugular or subclavian vein. Multiple variables measured using a PAC and their normal range are shown in the table below. The pulmonary artery wedge pressure(PAWP) is a surrogate upstream measure of the left ventricular filling pressure which is then used to estimate left ventricular end diastolic volume(LVEDV). There are a number of issues with this:
-variations among patients in left ventricular compliance-elderly in the US commonly with LVH with decreased compliance
-variations in pulmonary vascular resistance
-left sided valvular disease
In patients with undifferentiated shock, a PAC can be helpful though physician misinterpretation of measurements is a common problem. When a PAC is used, hemodynamic parameters should be trended frequently to determine what effect interventions have produced. Examples:
-A 70 year old woman with known CAD, HTN remains with a low MAP and UOP after a lengthy spine operation. Initial PAC numbers include a PAWP of 16 mm and SVO2 of 58%. Her blood loss was quoted by the spine service as “300 mL”. Two units of PRBC are given for presumed hypovolemia. What happens to her PCWP, SVO2, CO, and then UOP over the next hour?
-A 70 year old woman with known CAD, HTN remains with a low MAP and UOP after a brief spine operation with “minimal blood loss”. Her heart rate is 70. Her Hb is unchanged from her baseline postoperatively. Initial PAC numbers are a PAWP of 26 mm, SVO2 of 58%, and CI of 1.6 L/min/m2. A diagnosis of cardiogenic shock is made and norepinephrine infusion is started.
Unfortunately few randomized clinical trials have been done in SICUs. One from 2003 that looked at almost 2000 patients did not show a mortality difference:
Yet still in 2019 we cannot write off PACs as there may be a population that benefits from their use. Trials have not thoroughly protocolized management strategies nor hemodynamic targets. A lack of evidence does not equal a lack of utility.
| Parameter | Normal Range |
| Central venous pressure (CVP) | 0-5 mm Hg |
| Pulmonary artery wedge pressure (PAWP) | 6-12 mm Hh |
| Cardiac Index (CI) | 2.4-4.0 L/min/m2 |
| Parameter | Hypovolemic shock | Cardiogenic shock | Distributive shock |
| CVP or PAWP | Low | High | Low |
| CO | Low | Low | High |
| SVR | High | High | Low |
Ultrasound(US)/Echocardiography(ECHO)
Point of care US(POCUS) is evolving as an attractive option for non-invasive assessment of volume status in critically ill patients. This however has a similar issue to the PAC-it is a static measurement that cannot easily be followed continuously. One of the most studied US markers of volume responsiveness is respiratory variation in the inferior vena cava. IVC collapsibility has been reported as a reliable marker of fluid responsiveness in spontaneously breathing critically ill patients when indexed to a bioreactance system.
Multiple other studies have shown it to be no more accurate as a predictor of volume status than CVP, basically a “coin flip”. Dr. Paul Marik is particularly skeptical of its use.
ECHO can be used to estimate cardiac function, degree of ventricular filling and to assess for hypokinesis if cardiogenic shock is suspected. This requires more significant training or cardiology consultation at this time.
Other techniques to consider and worth review
Systolic pressure variation(SPV) and pulse pressure variation(PPV)
Carotid doppler peak velocity-has been assessed in sepsis with mixed results
Passive leg raise
Peri-patellar tissue oxygen monitoring