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Hemodynamics: Are you getting the whole picture?

Posted by Janet Kalina Tate, BSN | Sep 18, 2018 12:17:34 PM

This is a blog about the factors that influence hemodynamics and the relationship between pressure and flow.  It discusses some common misconceptions about pressure and suggests evidence-based methods of evaluating volume and flow.  It also outlines a logical multi-level strategy for improving tissue perfusion.  Listen to the audioblog by clicking play below!

 

If BP is your only hemodynamic value, you are missing some of the important pieces of the hemodynamic puzzle!

There’s no question that blood pressure is the oldest, most commonly measured hemodynamic value. It’s readily available in almost every situation, and as clinicians it can inspire our urgency to take action.

In 1733, Rev. Stephen Hales was the first to quantitatively measure blood pressure by inserting a tube into a mare’s artery and observing the pulsing of blood in a 12ft. tube. Hales, was an English botanist, physiologist, and clergyman, and he published a manuscript entitled “Haemastaticks”. In it, he also wrote of measuring the capacity of the left ventricle of the heart, the output of the heart per minute, and the speed and resistance to flow of blood in the vessels. He also discovered various substances could be used to dilate or constrict the blood vessels…. all this in the mid-1700s!

lady close up

Happily, blood pressure measurement has become a lot easier in the past 250 years. However, to look at BP as the lone hemodynamic indicator is to see only one piece of the puzzle…one part of the whole picture…

You see… there could be danger lurking, or at least a lot more information available….and EARLY Detection is KEY!

lady zoom out
The function of a healthy cardiovascular system is to maintain adequate perfusion to all tissues

• Adequate oxygen delivery to all organs is an essential condition for the organs' health and removal of the byproducts of cellular metabolism completes the perfusion cycle.
• Adequate perfusion requires both adequate systemic arterial pressure and Cardiac Output

'It is a source of regret that the measurement of flow is so much more difficult than the measurement of pressure.  This has led to an undue interest in the blood pressure manometer.  Most organs, however, require flow rather than pressure." ~ Jarisch A, 1928.

• Actually, as blood flows through the vessels from arteries to capillaries to veins, the pressure within them is constantly decreasing…but flow is maintained and may increase as the surface area decreases in larger veins.

blood pressure
• In the 1970s and 80’s clinicians gained new perspective into the hemodynamics of patients as the pulmonary artery catheter (PAC) became rapidly adopted in ICU’s. It gave us the ability to measure pressures in various intracardiac locations and obtain cardiac output measurements, expressed in liters per minute. 

• But because the PAC has not been shown to improve outcomes, is expensive, risky, and requires a highly skilled physician to insert and interpret it, today it is only used in about 2% of our critical care populations

  • And there are those who would argue that the most negative impact of the PAC has been the way it affected the hemodynamic thinking of several generations of medical professionals.
    • Leading us to surmise that pressure and flow were synonymous.
    • ie: That CVP or PAoP could accurately reflect adequacy of ventricular preload 

• Today we understand that an elevated CVP could be the result of many different conditions including:

  • Right ventricular failure.
  • Tricuspid stenosis or regurgitation.
  • Pericardial effusion or constrictive pericarditis.
  • Superior vena caval obstruction.
  • Hyperdynamic circulation.
  • High PEEP setting
  • And/or Fluid overload.
• We now also have overwhelming evidence that CVP does not reliably predict volume status / preload 1,2

- Today, Dynamic assessments of volume responsiveness such as fluid challenge (FC) and passive leg raise (PLR) where alterations in flow are evaluated as a result of the challenge have replaced CVP as preferred methods to determine fluid responsiveness.

Pressure = Flow x Resistance, which, in hemodynamics, translates to
BP = CO x SVR

• There is no doubt that systemic arterial hypotension is dangerous. Some of the latest evidence shows that even just one minute below threshold, can increase the risks of acute cardiac and kidney injury.3  The Kidneys are exquisitely sensitive to hypotension because of the huge percentage of flow (22%) which is required to do their job of filtering and elimination. And it’s no surprise that the risk of injury increases dramatically with increasing time below threshold. 3 But it’s also no surprise that BP is not the earliest indicator of change.

salmasi

Intellectually, we know that BP is not the earliest indicator of changes in flow.

  • Unfortunately, static blood pressure values have often become the only hemodynamic value that we measure.
  • Traditional vital signs may not be the earliest predictors of compromise, and may lead to imprecise interventions
  • There are multiple mechanisms at play in the human body which help to stabilize and regulate Blood pressure
  • It’s only when those homeostatic and compensatory mechanisms fail, that BP goes outside an expected range.
    • the arterial waveform does reveal a virtual “picture” of each ejection,
  • and minimally invasive cardiac output monitoring technologies can further analyze these waveforms with specialized pulse contour algorithms to obtain a beat-to-beat flow measurements of Stroke Volume (SV) as well as other hemodynamic values such as CO and SVR.

Consider the factors that determine cardiac output stroke vol v2

pre aft cont block

In a healthy person, between their lowest O2 demand at rest and their highest oxygen demand during strenuous exercise such as running a marathon, their Cardiac Index may increase by 500% while their MAP changes only a little or perhaps not at all.

body run

In a critically ill patient, you can better manage perfusion if you have all the pieces to the puzzle:  Pressure & Flow.

Some clinicians will want to trust their physical assessment skills alone to “guesstimate” cardiac performance, blood flow and volume. However, studies have shown that Physicians' assessment of flow (ie cardiac index) and intravascular volume (ie preload responsiveness) in patients is incorrect more than half of the time.4

LiDCO monitor fluid
 
Goals of Dynamic Assessments of Preload
 
It has been observed that some clinicians will attempt to assess preload reserve/responsivene using a myriad of flawed methods:
  • For example, giving a fluid challenge of 1litre over an hour with the goal of improved MAP
    • Problem being, this infusion is too slow to test preload reserve/responsiveness
    • A litre could harm some patients with heart failure.
    • The goal should be to determine if SV improves with smaller, safer volumes.
  • or a leg raise as an intrinsic fluid challenge with improved cuff BP as the goal
    • Problem being, the leg lift is frequently performed with suboptimal technique (see how to give a passive leg raise)
    • The measurable change in cardiac performance related to the increased preload occurs too quickly to be measured by BP. The goal should be to measure changes in SV.

The Arterial pressure wave analysis technology from LiDCO is designed to quickly and safely provide advanced hemodynamic information to aid in critical patient management decisions

Arterial waveforms can be obtained

- minimally-invasive (via an arterial line) …..  or non-invasively (via finger cuffs)

MI_monitor_grey_textNI-1

LiDCO’s Pulse Power algorithm can analyze each arterial waveform to determine a beat-to-beat SV. In addition, it will calculate CO/CI, SVR, SVV, PPV, HR, variability parameters as well as oxygen delivery (DO2) and oxygen consumption (VO2). This allows for the various factors that contribute to FLOW to be measured and trended without the cost and risk associated with the PAC. And since the heart and vascular system are capable of regulating on a moment to moment basis, it been proven that SV and it’s variations can serve as realtime predictors of preload reserve/responsiveness and cardiac performance 

Here is a recent framework for a rational step-wise approach to managing Hemodynamics proposed by the Peri Operative Quality Initiative (POQI) Fluid subgroup of American Society of Enhanced Recovery (ASER) in 2016. 7

hemodynamics framework

It describes the logical progression of

  1. Fluid optimization followed by
  2. Vasoconstrictors when indicated & finally
  3. Inotropes to increase the hearts force of contraction

SO, Is your patient perfusing?

Minimally invasive hemodynamic monitoring allows you to safely determine which parameter may be out of balance, and in which direction. It provides you with immediate feedback and an opportunity for early intervention.

Optimizing preload is the first step, because both hypervolemia and hypovolemia each carry additional risks.

- If the Stroke Volume (SV) is out of range, we can consider adding fluid (crystalloid, colloid, or blood) or removing fluid (diuretics or dialysis). These treatments can be guided by variations or changes in SV

  • To make a dynamic assessment of preload reserve/responsiveness, perform a fluid challenge (FC) or a passive leg raise (PLR) and observe the changes in SV.
  • The goal is to measure the immediate changes in cardiac performance (SV) that can occur related to the rapidly increasing Preload.
  • Essentially, this is testing where your patient is on the Frank-Starling curve, and if cardiac performance can be enhanced by adding more preload.

    ultimate goal

Fluid challenge protocol orange version

  • Keep in mind that the maximum change in SV related to a volume challenge will generally be observed within seconds (PLR) to minutes (FC) and may not be sustained. Therefore, changes in BP are not good goals for determining preload reserve/responsiveness.
  • Beat-to beat measurements of SV or variation can therefore provide a precise reflection of the benefit or detriment of the additional volume/preload.
  • In patients on controlled mechanical ventilation with a regular cardiac rhythm, it is possible to measure the variations in hemodynamics caused by positive pressure ventilation. These breath-by-breath variations in Stroke Volume (SVV) or Pulse Pressure (PPV) can indicate if a patient would be likely to be preload responsive.5 Documentation of actual improvements in cardiac performance would still reference improvements in SV.
    SVV PPV graphic
  • And since approximately 64 percent of the total blood volume resides in systemic veins, any action like exercise, that increases the flow of blood through the veins will transiently increase venous return to the heart, and hence preload.
  • If the Systemic Vascular Resistance (SVR=LV afterload) is in the low range, we can use a vasoactive medication to constrict the vasculature in an effort to restore effective blood pressure and optimize vascular resistance.
    • There are several classes of vasopressors now available which each mimic different intrinsic homeostatic mechanisms. These can be used in combination to achieve a balanced effect.
      combination key
  • Finally, if additional support is required to maintain pressure and flow, we can focus on improving Contractility, by adding positive inotropic agents or removing negative inotropes.
  • Examples of Positive Inotropes: Dobutamine and Milrinone
  • Negative inotropes include: Beta-blockers, Calcium Channel Blockers, and some Antiarrhythmics


heart image with the text

BP= CO X SVR          CO= SV X HR

FLOW

PUMP

RESISTANCE

PRELOAD CONTRACTILITY AFTERLOAD
SV/EVENT RESPONSE MEASURE CO/CI MEASURE SVR

Possible interventions:

Possible interventions: Possible interventions:
ADD FLUID:
Crystalloid-colloid-blood
INOPRESSORS:
Norepinephrine, Epinephrine,
Dopamine
VASOPRESSORS:
Phenyepherine, Vasopressin
REMOVE FLUID:
Furosemide, CRRT
INODILATORS:
Dobutamine, Milrinone
VASODILATORS:
Nitroglycerin, Sodium Nitroprusside

Recently, the large international RELIEF trial compared restrictive vs liberal fluid regimens perioperatively in 3000 major abdominal surgical patients. 6 Their results, which contradicted their original hypothesis, showed the rate of acute Kidney injury (AKI) and surgical site infection (SSI) was actually higher in the restrictive fluid group. Demonstrating once again that individualized fluid management is most effective.

A logical multifaceted approaches to goal directed hemodynamic management In critical care or surgical patients should beginning with SV optimization, followed by the addition of a vasopressors for persistent hypotension with normal Cardiac Index (CI), and/or the addition of an Inotrope for normotension with low CI .

Summary

  • BP is important, but not enough to determine blood flow
  • Flow based parameters, such as Cardiac output and Stroke volume, can be obtained safely and reliably via arterial wave analysis 8
  • Knowing key flow parameters enables clinicians to manage a patient’s oxygen delivery more precisely than traditional vital signs alone.
  • Remember, it’s always easier to maintain than it is to recover from a profound decompensation

You know the Blood Pressure….. its time to

know the flow-1

Janet Kalina Tate is a Clinical Specialist with LiDCO, a 24-year breast cancer survivor,
and a fervent ambassador for early detection and intervention.
the best protection is early detection

Want to view the clinical evidence on hemodynamic monitoring?


References
1. Marik PE, Baram M, Vahid B. Does the central venous pressure predict fluid responsiveness? A systematic review of the literature and the tale of seven mares. Chest. 2008;134(1):172-178.
2. Marik PE, Cavallazzi R. Does the central venous pressure predict fluid responsiveness? An updated meta-analysis and a plea for some common sense. Crit Care Med. 2013;41(7):1774-1781.
3. Vafi Salmasi, Kamal Maheshwari, Dongsheng Yang, Edward J. Mascha, Asha Singh, Daniel I. Sessler, Andrea Kurz; Relationship between Intraoperative Hypotension, Defined by Either Reduction from Baseline or Absolute Thresholds, and Acute Kidney and Myocardial Injury after Noncardiac Surgery: A Retrospective Cohort Analysis. Anesthesiology 2017;126(1):47-65. doi: 10.1097/ALN.0000000000001432.
4. Physicians' estimates of cardiac index and intravascular volume based on clinical assessment versus transesophageal Doppler measurements obtained by critical care nurses.(PMID:12882064)
American Journal of Critical Care : an Official Publication, American Association of Critical-Care Nurses [01 Jul 2003, 12(4):336-342]
5. Michard F. Changes in arterial pressure during mechanical ventilation Anesthesiology. 2005 Aug;103(2):419-28
6. Myles,PS. Restrictive vs Liberal Fluid Therapy for Major Abdominal Surgery; N Engl J Med 2018; 378:2263-2274
7. Thiel RH et al; Joint Consensus statement on perioperative fluid management; Perioperative Medicine 2016; 5:24

Topics: education, hemodynamics, normal hemodynamic parameters, ICU, Systolic (SBP), Systolic (SBP), systolic, Diastolic, diastolic (DBP), stroke volume normal, stroke volume, Cardiac output normal, Systemic Vascular Resistance, Stroke Volume Index, Arterial pressure cardiac output, blood pressure, Adequate oxygen delivery, goal directed hemodynamic management, SV optimization, vasopressors, persistent hypotension,, Inotrope, normotension, acute Kidney injury, Contractility, Haemastaticks, systemic arterial pressure, CVP, Cardiac Index, Pulse Power algorithm, Vasoconstrictors, fluid responsiveness, central venous pressure, Goal Directed Therapy, Cardiac Index (CI)

Written by Janet Kalina Tate, BSN

Janet is a Clinical Specialist with LiDCO who is based in FL. She travels throughout the US providing clinical support and education for LiDCO customers and prospects. Janet began her career as a critical care nurse while in Chicago at both suburban and University hospitals in their cardiac and surgical ICUs. She has since contributed to several publications; presented at local, national and international conferences; and has served as a company liaison for clinicians on behalf of several other corporations as well. In her free time, Janet enjoys water sports, exploring the wilderness, exceptional food and spirits, Blues music, and time with family & friends.

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