Pressure waveform analysis and plethysmography have emerged as valuable tools for assessing preload status in anesthetized animals, offering clinicians real-time, minimally invasive methods to guide fluid therapy decisions. These techniques analyze arterial pressure variations that occur during the respiratory cycle to estimate fluid responsiveness, enabling more precise goal-directed fluid therapy (GDFT).
Physiological Basis #
The fundamental principle behind these techniques is cardiopulmonary interaction during respiration. Changes in intrathoracic pressure during the respiratory cycle affect venous return, cardiac preload, and subsequently cardiac output and arterial pressure. In hypovolemic states, these respiratory-induced variations in arterial pressure become exaggerated, providing a dynamic measure of preload responsiveness.
During mechanical ventilation, positive pressure increases intrathoracic pressure, decreasing venous return and right ventricular preload. This leads to reduced left ventricular filling after a few heartbeats (due to pulmonary transit time). In fluid-responsive animals, these changes produce significant variations in stroke volume and arterial pressure. Conversely, in fluid-unresponsive animals, these changes may be minimal.
Ventilated Patients #
In mechanically ventilated animals, several parameters derived from arterial pressure waveform analysis have demonstrated utility:
Systolic Pressure Variation (SPV) #
SPV measures the difference between maximum and minimum systolic arterial pressure during a respiratory cycle. It consists of:
- Delta Up (dUp): increase in systolic pressure during early inspiration
- Delta Down (dDown): decrease in systolic pressure during mechanical inspiration
In hypovolemic states, dDown becomes more pronounced. An SPV >10 mmHg generally indicates fluid responsiveness.
Pulse Pressure Variation (PPV) #
PPV is calculated as: [(PPmax – PPmin) / ((PPmax + PPmin)/2)] × 100
Where PP is the difference between systolic and diastolic pressure. PPV >13-15% typically indicates fluid responsiveness in most species. The advantage of PPV over SPV is that it accounts for both systolic and diastolic pressure changes, making it less susceptible to changes in vascular tone.
Stroke Volume Variation (SVV) #
When using pulse contour analysis systems, SVV can be directly calculated from the pressure waveform. SVV >10-15% generally indicates fluid responsiveness, though threshold values vary by species.
In canine studies, a PPV threshold of 15% and SVV threshold of 14% have been shown to predict fluid responsiveness with sensitivities of 80-90% and specificities of 70-90%.
Example Case: Ventilated Dog Undergoing Abdominal Surgery #
A 5-year-old Labrador undergoing laparoscopic splenectomy under isoflurane anesthesia shows mean arterial pressure (MAP) of 65 mmHg with PPV of 18%. The anesthetist administers a 5 mL/kg fluid bolus over 10 minutes. Post-bolus, PPV decreases to 9% and MAP rises to 75 mmHg, indicating improved preload status. Without further surgical blood loss, PPV remains <10%, suggesting adequate volume status.
Spontaneously Breathing Patients #
Pressure waveform analysis is more challenging in spontaneously breathing animals due to irregular breathing patterns and negative intrathoracic pressure during inspiration (which increases venous return, opposite to mechanical ventilation). However, several approaches can be used:
Respiratory Variations in Plethysmographic Waveform #
Photoplethysmographic variability index (PVI) from pulse oximetry can be used in spontaneously breathing animals. While less reliable than in ventilated patients, trends in PVI can guide therapy. Values >14% may suggest fluid responsiveness in some species.
Example Case: Spontaneously Breathing Cat with Hypotension #
A 4-year-old cat undergoing dental treatment under propofol/sevoflurane anesthesia develops hypotension (MAP 55 mmHg). The plethysmographic variability index is 16%. A mini-fluid challenge (3 mL/kg crystalloid) is administered. The MAP increases to 70 mmHg and PVI decreases to 8%, suggesting fluid responsiveness. After a complete fluid bolus (10 mL/kg total), PVI stabilizes at 7% and MAP at 75 mmHg.
Differences Between Ventilated and Spontaneously Breathing Patients #
The key differences include:
- Reliability: Pressure variation indices are more reliable in ventilated animals receiving consistent tidal volumes. Spontaneous ventilation produces irregular breathing patterns that reduce specificity.
- Threshold Values: Threshold values differ significantly. In spontaneously breathing animals, higher thresholds may be needed, or alternative assessments should be considered.
- Interpretative Complexity: Ventilated animals show a more predictable pattern of increased right ventricular preload during expiration. In spontaneously breathing animals, the pattern is reversed (increased preload during inspiration).
- Technical Implementation: In ventilated patients, automated algorithms can reliably calculate PPV/SVV. In spontaneously breathing patients, clinician judgment and trend analysis become more important.
- Complementary Assessments: Spontaneously breathing animals often require additional assessments like mini-fluid challenges to confirm findings.
Using Delta Pressure to Guide Goal-Directed Fluid Therapy #
Goal-directed fluid therapy using pressure variations involves:
1. Establishing Baseline Measurements #
Record baseline arterial pressure waveform and calculate delta P (PPV or SVV), considering appropriate threshold values for the species and clinical context.
2. Evaluate Fluid Responsiveness #
If delta P exceeds threshold values:
- Administer a fluid challenge (typically 3-5 mL/kg crystalloid over 5-10 minutes, or alternatively 1 ml/kg over 60 sec)
- Reassess delta P after fluid administration
- If delta P decreases significantly and hemodynamics improve, the patient was fluid responsive
- If delta P remains elevated despite adequate fluid bolus, consider other factors (vasodilation, cardiac dysfunction)
If delta P is below threshold values initially, fluid bolus may not improve hemodynamics. However, small boluses may be repeated. Eventually the mean pressure will increase and the delta P will decrease. Initially the response may be transient as the volume expansion is fleeting as redistribution occurs quickly. However, once preload is restored the pressure effects usually persist. Remember total blood volume is typically the shock dose of fluids (85-90 ml/kg in dogs and ~65 ml/kg in cats.) 1 ml/kg is generally 1-2% of total blood volume and unlikely to cause fluid overload if titrated through repeated small boluses described above.
3. Iterative Assessment #
Repeat measurements throughout surgery, particularly after events that may alter volume status (hemorrhage, third-space losses).
4. Integration with Other Parameters #
Combine delta P with other parameters:
- Traditional vital signs (heart rate, blood pressure)
- Clinical assessment (mucous membrane color, capillary refill time)
- Laboratory values (lactate, base deficit)
- Advanced monitoring (cardiac output, central venous pressure) when available
5. Modified Approach for Spontaneously Breathing Animals #
For spontaneously breathing animals:
- Use trends rather than absolute values
- Consider mini-fluid challenges (1-3 mL/kg) and observe response
- Use plethysmographic variation as supportive evidence
Limitations #
Several factors affect the reliability of pressure variation analysis:
- Arrhythmias
- Right ventricular dysfunction
- Altered chest wall compliance
- Low tidal volumes (<8 mL/kg)
- Open-chest procedures
- Species variations
Conclusion #
Pressure waveform analysis and plethysmography offer valuable tools for assessing preload status and guiding fluid therapy in anesthetized animals. While more reliable in mechanically ventilated patients, modified approaches can be applied to spontaneously breathing animals. When integrated into a comprehensive hemodynamic assessment strategy, these techniques enable more precise fluid management, potentially improving outcomes and reducing complications associated with both hypovolemia and fluid overload.
