Method and apparatus for measuring cardiac output via an extracorporeal cardiopulmonary support circuit

A method and apparatus for determining cardiac output in conjunction with flow through an extracorporeal circuit, wherein flow through an arterial line of the extracorporeal circuit is temporarily reversed and an indicator is passed through the cardiopulmonary circuit. A dilution curve is measured in the arterial line of the extracorporeal circuit during the reversed flow, and cardiac output is determined corresponding to the measured dilution curve.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

Not applicable.

REFERENCE TO A “SEQUENCE LISTING”

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to determining cardiac output of a patient, and more particularly, to determining the cardiac output of a patient through an extracorporeal cardiopulmonary support circuit, wherein flow through at least a portion of the extracorporeal cardiopulmonary support circuit is reversed and a measurement at the reversed flow is taken from which the cardiac output is determined.

2. Description of Related Art

Respiratory failure requiring pulmonary support affects in excess of 300,000 people in the United States per year. Approximately one-half of these patients suffer from adult respiratory distress syndrome (ARDS). Adult respiratory distress syndrome is an acute inflammatory lung disease with a mortality rate of 50%. This disease is characterized by increased capillary permeability resulting from the development of interstitial edema and alveolar flooding. For the vast majority of patients with ARDS, there is no specific treatment, or supportive therapy. Supportive therapy for ARDS focuses on mechanical ventilation. An alternative life support modality, such as extracorporeal oxygenation, can be a therapeutic option for acute respiratory failure in both infants and adults.

In addition, extracorporeal circulation (perfusion) is used for the most part in cardiac bypass surgery. In a total bypass, all the systemic venous return blood of the patient is diverted from entering the right side of the heart and into an extracorporeal circuit. In such application, the extracorporeal circuit includes a heart-lung machine that comprises a pumping function and an oxygenation function, completely taking over cardiopulmonary function for the patient, returning oxygenated blood to the aorta, downstream of the cardiopulmonary circuit. In a partial bypass only a portion of the blood is diverted to the extracorporeal circuit, the remaining flow passing to the heart, the lungs and from the lungs through heart to the systemic circulation.

A use of extracorporeal circulation as “extracorporeal life support” can include “extracorporeal membrane oxygenation” known by the respective acronyms of “ECLS” or “ECMO”, for simplicity herein called ECMO. As opposed to the more conventional extracorporeal circulation and substitution or assist of the cardiac function, ECMO connotes the application of such support to supply oxygenation where the native lungs may be compromised. This is especially useful for neonates, including premature birth babies, whose life is threatened because their immature lungs cannot provide adequate gas exchange. Another use is resuscitated drowning victims whose lungs are damaged and unable to supply adequate oxygenation without restorative healing. The extracorporeal circulation provides oxygenated blood to the lungs under the impetus of the patient's native heart and gives time to allow healing of the lungs to occur until the lungs can take over oxygenation. In excess of 1,000 ECMO procedures are conducted annually in the United States.

Another use of extracorporeal circulation is to provide heart support without supplementary oxygenation. For example, part of the blood flow bypasses the heart and instead passes through the extracorporeal circuit, thereby reducing a portion of the load on the heart.

While the applications and successes of extracorporeal circulation have been increasing, the need remains for limiting the duration of the extracorporeal circulation to a substantially as needed basis. The need exists for determining the as needed basis in terms of measuring patient performance during the extracorporeal circulation. A need exists for determining cardiac output during extracorporeal circulation. The need exist for determining cardiac output without requiring further intervention or cessation of treatment.

BRIEF SUMMARY OF THE INVENTION

One configuration provides a method and apparatus for determining cardiac output of a patient on extracorporeal circulation including, but not limited to extracorporeal life support. By monitoring the cardiac output, the extracorporeal circulation can be controlled to correspond to the capacity of the patient so as to minimize excessive extracorporeal circulation time. In addition, selected configurations can provide real time assessment of heart performance, and particularly as in response to a substantially contemporaneous or prior treatment.

The present method provides for measuring a patient cardiac output with an extracorporeal cardiopulmonary support device withdrawing blood from a venous portion of a patent vascular system through an extracorporeal venous line and delivering blood to an arterial portion of the patient vascular system through an arterial extracorporeal line, then temporarily reversing flow in the extracorporeal venous line and the extracorporeal arterial line for withdrawing the blood and a portion of the dilution indicator, measuring a dilution curve in the extracorporeal system and determining a cardiac output corresponding to the measured dilution curve. Preferably, the dilution curve is measured in the arterial line of the extracorporeal circuit.

The apparatus for determining the cardiac output includes means for reversing the flow in at least the arterial line of the extracorporeal circuit, means for introducing a dilution indicator to pass through the cardiopulmonary circuit, a sensor for measuring a dilution curve of the dilution indicator in the extracorporeal circuit, and preferably in the arterial line, and a controller for determining the cardiac output corresponding to the measured dilution curve.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, an extracorporeal circuit100is shown connected to a circulation system20.

The circulation system20is a human (or animal) circulatory system including blood, a vascular system, and a heart. For purposes of this description, the circulation system20is represented as a cardiopulmonary system30and a systemic system40connecting the cardiopulmonary system30to the tissues of the body. Specifically, the systemic system40passes the blood though the vascular system (arteries, veins, and capillaries42) throughout the body. The portal circulation is taken as part of the systemic system40.

The cardiopulmonary system30includes the right heart, the lungs and the left heart, as well as the vascular structure connecting the right heart to the lungs, the lungs to the left heart and some portion of the aorta and large veins located between the extracorporeal circuit and the right and left heart. That is, in theory the cardiopulmonary system30would include only the right heart, the lungs, the left heart and the vascular structure directly connecting the right heart to the lungs and the lungs to the left heart. However, in practice it is sometimes impracticable to operably connect the extracorporeal circuit100immediately adjacent the large vein at the right heart, or immediately adjacent the aorta at the left heart. Therefore, the cardiopulmonary system30often includes a limited length of the vein entering the right heart and the aorta exiting the left heart. For example, the extracorporeal circuit100can be connected to a femoral artery and femoral vein, thereby effectively extending the cardiopulmonary system30to such femoral artery or vein.

For cardiopulmonary and vascular systems, the term “upstream” of a given position refers to a direction against the flow of blood, and the term “downstream” of a given position is the direction of blood flow away from the given position. The “arterial” side or portion is that part in which oxygenated blood flows from the heart to the capillaries. The “venous” side or portion is that part in which blood flows from the capillaries to the heart and lungs (the cardiopulmonary system30).

The basic components of the extracorporeal circuit100for a conventional heart-lung machine include a venous line110, a venous reservoir (not shown), an oxygenator120and heat exchanger (not shown), a pump130, an arterial line filter (not shown), an arterial line140, a dilution sensor146in the arterial line and a controller160

Generally, the extracorporeal circuit100withdraws blood from the venous portion of the circulation system20(or cardiopulmonary system30), and returns the blood to the arterial portion of the circulation system. The withdrawn blood can be treated while it is withdrawn, or the withdrawn blood can be merely returned to the arterial portion of the circulation system20. The blood treatment, if applied, can be any of a variety of treatments including, but not limited to, oxygenation (and carbon dioxide withdrawal).

The venous line110extends from the venous portion of the circulation system20, and preferably from a venous portion of the cardiopulmonary system30. The venous line110typically includes a venous cannula112providing the fluid connection to the circulation system20.

The venous line110can also include or provide a site114for introduction of the dilution indicator. In a preferred configuration, the site114for introducing the dilution indicator is proximal to the interface of the venous line110and the circulation system20. In selected configurations, the introduction site114can be integrated into the venous cannula112.

If the venous dilution senor116is used, then the injection site114should be distal to the sensor so when flow is reversed, the injected indicator first passes the sensor prior entering venous system through cannula112.

It is also contemplated, that a component of the extracorporeal circuit100can be controlled to create or induce an indicator within the flow in the extracorporeal circuit. For example, a filtration or treatment rate can be sufficiently changed to create an effective indicator in the extracorporeal circuit100which then travels through the cardiopulmonary system30.

In addition, the venous line110can include a dilution sensor116. The dilution sensor116(as well as sensor146) can be any of a variety of sensors, and can cooperate with the particular indicator. The sensor116(as well as sensor146) can measure different blood properties: such as but not limited to temperature, electrical impedance, optical properties, density, ultrasound velocity, concentration of glucose and other blood substances (any physical or chemical blood properties). Preferably, the sensor116is close to the venous point of cannulation112, (the interface of the venous line110and the circulation system20).

The arterial line140connects the extracorporeal circuit100to an arterial portion of the circulation system20and preferably to an arterial portion of the cardiopulmonary system30. The arterial line140usually connects to the ascending aorta. However, the arterial line140can be placed downstream in the arterial portion of the vascular system for example into femoral artery, or carotid artery, where the vessel is sufficiently large to accommodate the necessary flow rate. The arterial line140typically includes an arterial cannula142providing the fluid connection to the circulation system20.

The arterial line140also includes the dilution sensor146. The sensor146can be any of a variety of sensors, as set forth in the description of the sensor116, and is typically selected to cooperate with the anticipated indicator. Preferably, the sensor146is proximal (close to) to the point of arterial cannulation, (the interface of the arterial line140and the circulation system20).

However, it is understood the sensor146can be located anywhere in the extracorporeal circuit100or even outside of the extracorporeal circuit. That is, the sensor146can be remotely located and measure in the extracorporeal circuit100, and preferably in the arterial line140, the changes produced in the blood from the indicator introduction or values related to the indicator introduction which can be transmitted or transferred by means of diffusion, electromagnetic or thermo fields or by other means to the location of the sensor.

Current oxygenators120are broadly classified into bubble type and oxygenators and membrane type oxygenators. The membrane type oxygenators fall under the laminate type, the coil type, and the hollow fiber type. Membrane type oxygenators offer advantages over the bubble type oxygenators as the membrane type oxygenators typically cause less blood damage, such as hemolysis, protein denaturation, and blood coagulation as compared with the bubble type oxygenators. Although the preferred configuration is set forth in terms of a membrane type oxygenator, it is understood any type of oxygenator can be employed or no oxygenator can be used in the extracorporeal circuit.

The pump130can be any of a variety of pumps types, including but not limited to a roller (or impeller) pump. The pump130induces a blood flow through the extracorporeal circuit100. At least one of the pump130and the controller160typically include control of the rpm of the pump and the flow rate of the blood through the pump, respectively. The pump130can be at any of a variety of locations in the extracorporeal circuit100, and is not limited to the position shown in the Figures.

The controller160is typically connectable to the oxygenator120, the pump130and the sensor(s)116,146. The controller160can be a stand alone device such as a personal computer, a dedicated device or embedded in one of the components, such as the pump130or the oxygenator120. Although the controller160is shown as connected to the sensors116and146, the pump130and the oxygenator120, it is understood the controller can be connected to only the sensors, the sensors and the pump, or any combination of the sensors, pump and oxygenator.

The normal or forward blood flow through the extracorporeal circuit100includes withdrawing blood through the venous line110from the venous side circulation system20(and particularly the cardiopulmonary circuit30), passing the withdrawn blood through the extracorporeal circuit (to optionally treat such as oxygenate, or merely circulate the withdrawn blood), and introducing the withdrawn (or treated) blood through the arterial line140into the arterial side of the circulation system (and particularly the arterial portion of the cardiopulmonary circuit). The pump130thereby normally induces a blood flow through the extracorporeal circuit100from the venous line110to the arterial line140.

Thus, the forward flow through the extracorporeal circuit100is parallel to the flow from the venous side of the circulation system20(through the cardiopulmonary system30, the right heart, the lungs, a left heart) and passing to the arterial portion of the systemic system40.

While cardiopulmonary support offers life-saving and life prolonging treatment, the intrusive nature of the cardiopulmonary support carries significant risks and potential complications. Each additional hour of unnecessary cardiopulmonary support increases the probability of negative complications as well as increasing the already substantial costs of the treatment. Therefore, it is desirable to limit the duration of cardiopulmonary support as required by the individual patient. One of the main criteria for decreasing or terminating cardiopulmonary support is an adequate increase in the heart flow—cardiac output. Typically, cardiopulmonary support can be decreased as the normal heart capacity is restored.

The heart capacity (flow) is typically measured by cardiac output CO. Cardiac output CO is the amount of blood pumped out by the left ventricles in a given period of time (typically a 1 minute interval).

Referring toFIG. 1, the total blood flow (TBF) passing to the systemic system40is the sum of the cardiac output CO (blood flowing from the cardiopulmonary system30) and the blood flow Q from the extracorporeal circuit100.
TBF=CO+Q  (Eq. 1)

To apply the present dilution technique to measure cardiac output CO during circulation in the extracorporeal circuit100, the blood flow is reversed in at least the arterial line140, and the venous line110if there is no buffer in the extracorporeal circuit100. Thus, blood is withdrawn from the arterial side of the circulation system20, passed through the extracorporeal circuit100and delivered to the venous side of the circulation system20. That is, the flow in the extracorporeal circuit100is reversed relative to the circulation system20. During the reversed flow in the extracorporeal circuit100, the dilution indicator injected into the venous line (of extracorporeal circuit100or intravascular system, such as circulation system20) will pass through any incorporated portion of the venous portion of the circulation system20, the cardiopulmonary circuit30(the right heart, the lungs, the left heart) and will be sampled (dilution curve recorded or measured) in the of the extracorporeal circuit, and preferably in arterial line140.

The flow can be reversed by a number of configurations. Referring toFIG. 2, the direction of the pump130can be temporarily reversed (for example, on the order of 2–5 minutes). In this case, injection of the indicator can be made anywhere into the extracorporeal circuit100, though preferably at the site114in the venous line110. The indicator will then pass through the cardiopulmonary system30and a portion of the indicator then passes into the arterial line140of the extracorporeal circuit100. The resulting dilution curve can be measured anywhere in the extracorporeal circuit100, and preferably in the arterial line140. The resulting dilution curve is recorded, or measured and from which the cardiac output CO is calculated.

However, components of the extracorporeal circuit100, such as (bubble traps, oxygenators) are typically designed to permit only single direction (normal or forward) flow. Therefore, for this configuration to be implemented, the components of the extracorporeal circuit100must be compatible for bidirectional flow.

Alternatively, as seen inFIG. 3, the lines between the pump130and the circulation system20(patient) can be reversed. This line reversal can be achieved by, for example, using a special line reversal device, line switching or with simple re-clamping of tubing in the extracorporeal circuit100(an example of the line clamping is seen inFIGS. 5 and 6). An advantage to this configuration of the flow reversal lies in a constant forward flow through the pump130and relevant extracorporeal components. Thus, only in the local region of the interface between the extracorporeal circuit100and the circulation system20(patient) is the flow reversed so that indicator if introduced into the extracorporeal circuit100passes into the venous portion of the cardiopulmonary system30, through the cardiopulmonary system and a portion is withdrawn through the arterial line140of the extracorporeal circuit. Again, the dilution curves can be measured anywhere within the extracorporeal circuit100, but preferably along the arterial line140.

In the extracorporeal circuit100of some cardiopulmonary support systems, such as ECMO, there is a bridge (bypass) tubing170between the withdraw and return lines, as seen inFIG. 4). The bridge tubing170is usually clamped to preclude flow therethrough. However, the bridge tubing170is periodically opened to insure that blood is not clotted. The bridge tubing170is established for emergency situations, for example to prevent flow from the ECMO if bubbles or clots appear or there is a need to substitute extracorporeal lines. The bridge tubing170can be used to measure the cardiac output CO. Specifically, the pump130can be stopped, clamped off by clamps150(shown inFIG. 4) or run through a parallel circuit (not shown). When the pump130is stopped (clamped or run through the parallel circuit), the blood will flow in the bridge tubing170by patient pressure gradient from the artery to vein. This flow can allow performing cardiac output measurements, as seen inFIG. 4. However, this configuration of flow reversal is subject to the limitation that blood flow through the bridge tubing170depends on heart performance (and hence pressure from the systemic system) and hydrodynamics resistance (length of the bridge, the diameter of the tubing as well as the size of the cannula). These factors can reduce the stability of the blood flow rate through the bridge tubing170. It is also understood that an additional pump (not shown) can be included in this configuration to assist in providing an acceptable flow.

Referring toFIG. 5, the extracorporeal circuit100can include the bridge tubing170to define a flow reversing structure in cooperation with clamps150. That is, the extracorporeal circuit100can be clamped, as seen inFIG. 5, to provide normal, forward flow through the circuit (the pump130and oxygenator120), with no flow through the bridge tubing. Although four clamps150are shown it is understood fewer, or more clamps can be used are desired to maintain the forward flow.

Referring toFIG. 6, wherein the clamps are removed (or selectively opened) on the bridge tubing170and closed on the shown portions of the venous line and arterial line, flow is diverted through the bridge tubing to effect a reversed flow through the venous cannula112and the arterial cannula142.

For each of the flow reversal configurations, it is understood the reversed flow is selected to reduce or minimize stress to the heart (and cardiopulmonary system30).

After flow in at least the parts of the venous and the arterial lines110,140of the extracorporeal circuit100is reversed, the indicator is introduced so as to pass from the venous portion of the circulation system20to pass through the cardiopulmonary system30and be withdrawn into the arterial line140of the extracorporeal circuit100to be measured (recorded) by the sensor146in (or associated with) the extracorporeal circuit. The indicator can be introduced via any place of the extracorporeal circuit100, and preferably the venous line100of the extracorporeal circuit, as well as intravenously into the venous portion of the circulation system20.

In one configuration of the invention, a single dilution indicator sensor146is employed in the arterial line. As set forth in the description of the extracorporeal circuit100, the preferred location of the single sensor146is close to the place of arterial cannulation, seen at142inFIG. 2, and the preferred location for indicator introduction is near the site of venous cannulation seen at112inFIG. 2. In this construction, the indicator will be less dispersed in the lines of extracorporeal circuit100.

It is also understood that location of dilution sensor146in the arterial line140is close to the arterial cannula142is beneficial as the measured or recorded dilution curves will be less disturbed after leaving cardiopulmonary system30, than if recorded in the venous line110after passing through the pump130or other components of extracorporeal system100.

The resulting dilution curve is measured or recorded by sensor, such as in the extracorporeal system100.

The cardiac output CO will be given by:

Where Vi is the volume of the introduced indicator and S is the area of the dilution curve (concentration of indicator) measured in the extracorporeal circuit100from the blood (and indicator) flowing from the arterial circulation system (cardiopulmonary circuit30). The introduction of the indicator can be a relatively quick (short duration) injection, a timed or measured injection, or a continuous injection.

It is understood that different formula can be used to determine the cardiac output, depending upon the specific indicator, the way the indicator is introduced, as is described in literature and textbooks.

It is also contemplated that the indicator can be introduced in the arterial line140just upstream of the sensor146(inFIGS. 2–4), such that a first dilution curve S1is measured (recorded) prior to the indicator passing the remainder of the extracorporeal circuit100and subsequently entering the cardiopulmonary system30, then passing into the arterial line to be measured again to provide a second dilution curve S. In this configuration, the cardiac output CO is given by:

If the sensor measures the pump flow Q, or it is known, then the cardiac output CO can be calculated by equation 3, were S1and S are areas under concentration curves or values that are proportional (related) to such curves. It is understood that different formula can be used to determine the cardiac output, depending upon the specific indicator and the way the indicator is introduced.

In the two sensor configuration, as shown inFIG. 2, (the second sensor116being disposed in the venous line110downstream of the indicator injection site), for two matched sensors, the cardiac output CO is given by:

Where S1is the area under the dilution concentration curve measured before the indicator enters the circulation system20by the sensor116on the venous line110, or value related to the dilution concentration.

It is further contemplated that analogous sensor and introduction sites can be employed for the configuration ofFIG. 3.

As a representative example of the configuration ofFIG. 2, the sensors are ultrasound dilution sensors, and wherein sensor116or146also measures blood flow in the extracorporeal circuit, and particularly the venous line. Injection of a 10 ml of saline indicator produces first dilution curve (1) from the sensor116, seen inFIG. 7, and after the indicator has traveling through the cardiopulmonary system30, the sensor146produces second dilution curve (2). From these measurements, the cardiac output is calculated via Equation 3a. InFIG. 7, the scale of the first curve is different than the scale of the second curve.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.