Arterial Waveform Analysis Devices

Image 1: From PiCCO @ Bedside, iOS Mobile app [1]. 

Due to the results of the March 2014 "ProCess Trial" indicating that early goal directed therapy in septic shock did not improve outcomes [2] , there has been less interest in using pulse contour analysis devices to optimize fluid management in the critically ill patient. 

The main reason for this is that these devices, even though less invasive then the pulmonary artery catheter still require at the minimum an arterial line and in addition (depending on device) a central venous catheter. 

However, before letting these devices collect dust on a shelf, I believe this tool can be still be very beneficial in two patient populations.

One, is the high risk surgical patient to optimize fluid management.

This strategy is currently termed "Goal directed Intra-operative fluid administration".

Fluid overload during surgery has been associated with [3]:

-Increased length of stay

-Bowel wall edema

-Edema organ dysfunction

-Adverse outcomes

Additionally, patients receiving fluid and hemodynamic optimization during surgery are at an decreased risk of renal impairment. 

The second type of patient that may benefit from minimally invasive cardiac output monitoring is the critically ill cardiovascular patient.

 With these patients the ICU physician is always questioning [4]:

-"Is the patient's pre-load sufficient to obtain adequate Cardiac output"?

-"Does the patient need volume (fluids) or forced diuresis"?

-"What is the patient's cardiac function"?

-"Does the patient need inotropes or pressors"?

In these patients'  not optimizing fluid management can be detrimental. 

TECHNOLOGY

All devices calculate cardiac output based on the traditional equation:

CO = SV X HR

CO = Cardiac Output

SV= Stroke volume

HR= Heart rate 

However, each device uses a proprietary method to calculate stroke volume or a modification of the Fick principle. Below is a table I created that compares and contrast three different pulse contour analysis devices.  

Image 2: Table comparing Pulse Contour Analysis Devices.

For more information on Arterial Waveform Analysis Devices one can visit the Society of Critical Care Medicine and take the online course "Less-Invasive Hemodynamic Monitoring" [5]. 

REFERENCE

1. PiCCO @ Beside, GE HealthCare iOS mobile app

2. Early Goal Directed Therapy Does Not Improve Outcomes in Septic Shock (ProCESS)

3. Optimization of the High Risk Surgical Patient. SCCM, lecture 2014

4. Transpulmonary Thermodilution. SCCM, lecture 2014. 

5. Less-Invasive Hemodynamic Monitoring. 

ASV Behind the Scenes

The advantage of using Adaptive Support Ventilation (ASV) is that it automatically enforces the three primary clinical goals of mechanical ventilation, which include safety, comfort, and liberation.

To accomplish these goals ASV uses rules to set lung protective boundaries. 

Many of these rules are out of view from the device operator and are not affected by operator selection of inputs. The rules fall into two categories 'hard rules' and 'soft rules' or a combination of both types.

A Hard Rule is a pre-set limit which is unaffected by the operators interaction or by measured patient lung mechanic values (a "behind the scene" action). An example of this is the minimum & maximum mandatory breath limits and I:E ratio limits.

A Soft Rule is a limit based on an operator input and/or measured values of the patients respiratory mechanics. An example of this is the minimum & maximum inspiratory pressure threshold, which is determined by the operator directly setting the PEEP (affects lower PIP threshold) and the ASV pressure limit (affects the high PIP limit).

These rules are similar to techniques used by the experienced device operator, in which one uses to optimize ventilator settings. Now the less experienced device operator can provide the same standard of care as a seasoned practitioner in regards to enforcing the clinical goals of mechanical ventilation.

In upcoming blog posts and YouTube videos I will be presenting the the ASV rules in more detail.

REFERENCE

Richey, S. (2010). Adaptive Support Ventilation: Guidelines  Standards for Using ASV.