Grocott and Gan have previously contributed an excellent and important article in this newsletter on goal-directed hemodynamic management for improving patient safety.¹ Appropriate hemodynamic monitoring and assessment are fundamental keys to improving patient safety by goal directed hemodynamic management. One of the basic problems that we frequently face is the lack of hemodynamic monitoring. This deficit is compounded by our being lulled into a false sense of security when patients have an “acceptable” blood pressure and pulse. Unfortunately it is not always possible to have a reliable hemodynamic assessment with blood pressure and pulse measurements alone, and we may be unaware of significant developing hemodynamic problems using these measurements. We need to be able to evaluate and prevent hemodynamic derangement at its onset in order to improve patient safety. This begins with the realization that blood pressure monitoring can be misleading and incomplete, and that blood flow data are also necessary to have complete hemodynamic understanding.

In any three-variable equation (A x B = C), if we know only one variable, we are very limited and cannot gain a meaningful understanding of the whole equation. If we know two variables, we can calculate the third and can gain the whole meaning of the equation. “Blood flow x vascular resistance = blood pressure” is a three-variable equation. It is impossible to gain an appreciation of these relationships knowing only one variable, the blood pressure. This can limit appropriate hemodynamic management.

We are so focused on blood pressure in the majority of “routine” patients that we sometimes forget how incomplete and misleading blood pressure can be. This lack of reliability can also extend to readings of central venous and pulmonary artery pressure. The pulmonary artery catheter allows a more complete hemodynamic appreciation because we can estimate blood flow from its thermodilution cardiac output capability. Without blood flow data we cannot know the true underlying hemodynamic state.

Transesophageal ultrasound hemodynamic monitoring can be very helpful because it is noninvasive and can be quickly applied. This method can also provide information regarding preload, afterload, and contractility through the measurement of ejection time, peak velocity, and acceleration. The small, dedicated ultrasound hemodynamic monitors, by design, offer the additional benefit of beat-to-beat, quantitative measurements that large ultrasound machines do not. The Hemosonicª (Arrow International, Reading, PA) and its predecessor, the Dynemo 3000, offer flow assessment using the most optimal ultrasound flow assessment techniques, in that they measure continuous velocity-time integrals and cross-sectional areas at the same anatomical location and at the same instant in time.

Blood flow and resistance can be considered “causative or defining factors” and blood pressure as their “result.” Because the causative/defining factors can fluctuate widely and in opposite directions, focusing on their result (blood pressure) can be misleading. If we have a means to see the underlying causative/defining factors at work and are able to follow how they change, we can then make diagnoses of hemodynamic derangement at a basic level. If we can treat these basic hemodynamic factors, we may optimize the hemodynamic state before the derangement manifests as a change in blood pressure and further compromises the patient. Transesophageal ultrasound monitoring allows rapid continuous assessment of basic hemodynamic function and a mechanism to attain/maintain hemodynamic stability.

One can noninvasively ascertain the underlying hemodynamic state in 85% of patients using monitors such as the HemoSonic.² This can be accomplished within 3 to 5 minutes from the inception of “thought to monitor flow” to the access of reliable hemodynamic data. Sub-optimal imaging may occur in approximately 10 to 15% of patients when the esophageal/aortic anatomy precludes a good ultrasound window or when aberrant tachyarrhythmias are present. This is similar to any other ultrasound technique. As with other ultrasound techniques there is an abundance of valuable information available once one has learned the technique of obtaining optimal images. This technique follows aortic blood flow and left ventricular function, but it is not a direct indicator of right ventricular function. So there are instances where right ventricular hemodynamics, or other aspects of cardiac function, need to be assessed more directly by additional means.Transesophageal ultrasound monitoring can also provide measurement of the components of flow as well as direct stroke volume assessment. The velocity time integral provides a significant amount of additional information. Peak velocity is useful as a general indication, while acceleration is a more specific indication of global left ventricular performance and myocardial contractility.^3,4 Maximum acceleration is the contractile index least affected by afterload conditions.⁵ Left ventricular ejection time (LVET) is useful to assess and follow preload conditions and outflow impedance conditions. LVET will increase with increases in preload and/or decreases in afterload. Conversely, LVET will decrease with decreases in preload and/or increases in afterload.^6,7 The ratio of pre-ejection period/left ventricular ejection time, the PEP/LVET ratio, increases with decreases in contractility and conversely decreases with increases in contractility.⁸ It is another method to follow contractile state changes.

Transesophageal ultrasound based hemodynamic monitoring also has been proposed as an alternative to the pulmonary artery catheter because it is free of the many potential serious risks of pulmonary artery catheterization.⁹ It has been used to direct intraoperative fluid administration, resulting in improved outcome and a significant reduction in hospital stay.^10,11 This is not the case with pulmonary artery catheter directed therapy, which has come under increased criticism because of its demonstrated lack of benefit for patients undergoing major surgery.¹²

Summary

• Blood pressure alone as a measure of hemodynamic state is incomplete and can be misleading.
• Adding blood flow assessment to blood pressure data allows real appreciation of the hemodynamic state.
• Appreciation of the hemodynamic state is fundamental and provides the opportunity to optimize hemodynamics for improving patient outcome and safety.
• Blood flow as assessed by transesophageal ultrasound hemodynamic monitoring is noninvasive and rapidly applied, providing important basic and useful hemodynamic information.
• Dedicated ultrasound hemodynamic monitors provide valuable, continuous, quantitative data, in real time, that the large ultrasound machines do not.
• Monitors that perform simultaneous continuous aortic diameter and velocity time integral calculations use optimal ultrasound flow assessment principles, allow rapid basic hemodynamic diagnosis, and can avoid complications and costs associated with pulmonary artery catheters.

Appropriate hemodynamic monitoring, assessment, and management are the keys to improving patient safety through hemodynamic optimization.

Dr. Corey is a staff anesthesiologist (Section on Cardiothoracic Anesthesia), Sharp Memorial Hospital, San Diego, CA.

Disclosure: Dr. Corey is currently a consultant to Arrow International.

References

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11. Sinclair S, James S, Singer M. Intraoperative intravascular volume optimization and length of hospital stay after repair of proximal femoral fracture: randomized controlled trial. BMJ 1997;315:909-12.
12. Polanczyk CA, Rohde LE, Goldman L, et al. Right heart catheterization and cardiac complications in patients undergoing noncardiac surgery. JAMA 2001;286:309-14.