Introduction
During cardiopulmonary bypass (CPB), temperature and pump flow are closely interconnected variables. As the patient is cooled, metabolic demand and oxygen consumption decrease, allowing perfusionists to modify pump flow while maintaining adequate tissue oxygen delivery.
However, temperature alone should never be used to determine an appropriate pump flow. The ultimate objective is to maintain adequate oxygen delivery (DO₂), tissue perfusion, and metabolic stability throughout CPB.
A temperature–flow console can therefore be useful as a decision-support tool, helping the perfusion team understand how changes in temperature may influence flow requirements.
Why Does Temperature Matter?
Body temperature has a major influence on metabolic activity.
As temperature decreases:
- Oxygen consumption decreases.
- Carbon dioxide production decreases.
- Metabolic demand falls.
- The required oxygen delivery may decrease.
- Lower pump flows may sometimes be tolerated during controlled hypothermia.
Conversely, during rewarming, metabolic demand progressively increases. Pump flow and oxygen delivery therefore need to increase appropriately.
The important principle is:
Lower temperature reduces metabolic demand, but it does not eliminate the need for adequate oxygen delivery.
Temperature–Flow Relationship
A simplified conceptual relationship can be expressed as:
Required Flow ∝ Metabolic Demand / Oxygen Content
Oxygen delivery is commonly expressed as:
DO₂ = Cardiac Output × CaO₂
During CPB, cardiac output is replaced by pump flow:
DO₂ = Pump Flow × CaO₂
For indexed oxygen delivery:
DO₂i = Pump Flow Index × CaO₂
where:
CaO₂ = (1.34 × Hb × SaO₂) + (0.003 × PaO₂)
This demonstrates why temperature should not be considered in isolation.
For example, a patient at a relatively low temperature may tolerate a lower flow because metabolic demand is reduced. But if hemoglobin is low or arterial oxygen saturation is inadequate, oxygen delivery can still become critically low despite apparently acceptable pump flow.
Example: Hypothermic CPB
Consider a patient cooled to approximately 21°C during CPB.
At this temperature, metabolic demand is substantially reduced compared with normothermia. A perfusionist may therefore use a lower flow strategy depending on the surgical procedure, patient's condition, oxygen delivery, venous saturation, lactate, arterial pressure, and other perfusion parameters.
The key question should not simply be:
Is the flow appropriate for this temperature?
Instead, ask:
Is the current flow sufficient to meet the patient's oxygen-delivery requirement at this temperature?
This is a much more clinically meaningful approach.
Temperature and Flow During Rewarming
Rewarming is a particularly important phase.
As temperature increases:
1. Metabolic rate increases.
2. Oxygen consumption increases.
3. Oxygen demand increases.
4. Pump flow may need to increase.
5. Oxygen delivery should be reassessed continuously.
A flow that was appropriate during deep hypothermia may become inadequate during rewarming.
Therefore, the transition from cooling → maintenance → rewarming should be treated as a dynamic perfusion process, rather than using a single fixed flow target throughout CPB.
Do Not Use Temperature Alone
Several variables can change the required pump flow independently of temperature.
1. Hemoglobin
Hemoglobin is a major determinant of arterial oxygen content.
A patient with a low hemoglobin concentration may require a higher flow to maintain adequate DO₂.
For example:
Low Hb + low flow = potentially inadequate oxygen delivery
Therefore, a satisfactory pump flow does not necessarily mean satisfactory tissue oxygenation.
2. Arterial Oxygen Saturation
SaO₂ should remain appropriate during CPB.
A reduction in oxygen saturation decreases arterial oxygen content and consequently decreases DO₂.
3. Venous Oxygen Saturation
SvO₂ can provide useful information about the balance between oxygen delivery and oxygen consumption.
A falling SvO₂ may indicate increasing oxygen extraction and potentially inadequate oxygen delivery, although interpretation should always consider the entire clinical picture.
4. Lactate
Lactate trends can provide an additional indication of tissue perfusion.
A progressive lactate increase during CPB should prompt assessment of:
- Pump flow
- DO₂i
- Hemoglobin
- Oxygenation
- Venous saturation
- Mean arterial pressure
- Temperature
- Acid–base status
- Regional perfusion
5. Mean Arterial Pressure
MAP provides information about perfusion pressure but should not be considered a direct surrogate for oxygen delivery.
A normal MAP can coexist with inadequate systemic oxygen delivery.
A Better Approach: Temperature + Flow + DO₂
A modern perfusion strategy should integrate several variables rather than relying on temperature-based flow formulas alone.
A useful conceptual model is:
Temperature → Metabolic Demand → Required Oxygen Delivery → Flow Adjustment
while continuously checking:
Hb + SaO₂ + Pump Flow + SvO₂ + Lactate + MAP
This creates a more comprehensive picture of perfusion adequacy.
Practical Clinical Workflow
During Cooling
- Gradually reduce temperature according to the surgical plan.
- Monitor arterial and venous blood gases.
- Adjust pump flow according to metabolic demand and clinical circumstances.
- Monitor DO₂i where available.
- Avoid unnecessarily excessive flow while maintaining adequate perfusion.
During Maintenance Hypothermia
- Maintain the planned temperature.
- Continuously assess oxygen delivery.
- Monitor Hb and arterial oxygenation.
- Follow SvO₂ and lactate trends.
- Consider regional perfusion requirements during complex procedures.
During Rewarming
- Increase temperature gradually.
- Recognize the progressive increase in metabolic demand.
- Reassess pump flow.
- Ensure adequate DO₂.
- Monitor arterial and venous oxygenation.
- Avoid excessive temperature gradients and excessive rewarming.
The Role of a Temperature–Flow Console
A temperature–flow console can convert these concepts into a practical bedside decision-support system.
For example, the console can display:
Current temperature: 21°C
Current pump flow: 1.23 L/min
Hb: entered by the perfusionist
SaO₂: entered or measured
PaO₂: optional
SvO₂: optional
The system can then calculate:
CaO₂ → DO₂ → DO₂i
and present the result together with the current temperature and flow.
A graphical display can also show how the expected flow requirement changes as temperature rises or falls.
This makes the relationship between temperature, flow, and oxygen delivery easier to understand at the bedside.
Important Limitation
Temperature-based flow guidance should be regarded as decision support, not an automatic prescription.
Pump flow must always be individualized according to:
- Patient size and BSA
- Surgical procedure
- Temperature
- Hemoglobin concentration
- Oxygen saturation
- DO₂i
- SvO₂
- Lactate
- Arterial pressure
- Cerebral and regional perfusion requirements
- Acid–base status
- Patient-specific pathology
Particularly during aortic surgery, deep hypothermic circulatory arrest, selective cerebral perfusion, or other complex procedures, systemic flow calculations alone may not adequately describe regional oxygen delivery.
Key Takeaway
The relationship between temperature and pump flow is fundamentally a relationship between metabolic demand and oxygen delivery.
Cooling decreases metabolic demand and may permit lower pump flows, while rewarming increases metabolic demand and may require increasing flow. But the correct flow should never be determined from temperature alone.
The modern approach is:
Temperature → Flow → Oxygen Delivery → Tissue Perfusion
with continuous integration of Hb, SaO₂, SvO₂, lactate, MAP, and DO₂i.
Ultimately, the goal of temperature–flow management during CPB is not simply to achieve a particular pump flow—it is to ensure that every phase of bypass provides adequate oxygen delivery for the patient's metabolic requirements.