Overview
Definition:
Cardiac output (CO) monitoring, particularly via intermittent thermodilution (TD) and continuous cardiac output (CCO) techniques like PiCCO (Pulse Contour Cardiac Output), provides real-time or near real-time assessment of the volume of blood pumped by the heart per minute
This is crucial for optimizing hemodynamics in critically ill surgical patients
Thermodilution involves injecting a cold saline bolus or gas into a central vein and measuring temperature changes in the pulmonary artery or peripheral artery to calculate CO
PiCCO utilizes arterial pressure waveform analysis calibrated with intermittent thermodilution to provide continuous CO estimation.
Epidemiology:
Hemodynamic instability is common in surgical patients, especially those undergoing major, trauma, or sepsis-related surgeries
CO monitoring is indicated in a significant proportion of ICU patients, including post-cardiac surgery, severe sepsis/septic shock, and major non-cardiac surgery with high risk of complications.
Clinical Significance:
Accurate CO monitoring guides fluid management, vasopressor/inotropic support, and mechanical ventilation strategies
It helps differentiate between various forms of shock (hypovolemic, cardiogenic, distributive) and guides interventions to improve tissue perfusion, reduce organ dysfunction, and improve patient outcomes in the perioperative and critical care settings
Failure to optimize CO can lead to increased morbidity and mortality.
Diagnostic Approach
History Taking:
Focus on the reason for monitoring: recent surgery type, duration, and complexity
History of cardiac, renal, or pulmonary disease
Signs of shock: hypotension, tachycardia, altered mental status
Urine output
Fluid balance
Use of vasoactive drugs.
Physical Examination:
Assess for signs of hypoperfusion: cool extremities, delayed capillary refill, decreased urine output (<0.5 mL/kg/hr), altered mentation
Listen for murmurs
Palpate pulses
Assess skin turgor
Monitor heart rate and rhythm, respiratory rate, blood pressure (central and peripheral), and oxygen saturation.
Investigations:
Arterial blood gas analysis for lactate and acid-base status
Complete blood count and coagulation profile
Renal function tests (creatinine, BUN)
Liver function tests
Electrocardiogram (ECG)
Chest X-ray
Echocardiography (bedside)
Invasive hemodynamic monitoring: arterial line, central venous catheter, pulmonary artery catheter (if indicated), and CO monitoring device (PiCCO/thermodilution)
Interpretation of CO values requires considering the clinical context and trends.
Differential Diagnosis:
Hypotension in surgical patients can be due to hypovolemia (hemorrhage, third-spacing), cardiogenic shock (myocardial dysfunction, valvular dysfunction), distributive shock (sepsis, anaphylaxis), obstructive shock (pulmonary embolism, cardiac tamponade), or vasoplegia (post-bypass, sepsis)
CO monitoring helps differentiate these by revealing low CO with high systemic vascular resistance (cardiogenic, hypovolemic) or low CO with low SVR (sepsis)
Low CO can also be due to bradyarrhythmias or severe valvular regurgitation.
Interpretation Of Picco Thermodilution
Cardiac Output Values:
Normal CO range: 4-8 L/min
Low CO (<4 L/min) indicates inadequate cardiac performance or excessive afterload/preload
High CO (>8 L/min) may be seen in early sepsis (hyperdynamic state) or with excessive fluid resuscitation
Trends are more important than absolute values.
Stroke Volume Variation Svv:
SVV (measured by PiCCO) is an indicator of preload responsiveness in mechanically ventilated patients
SVV >10-15% suggests that the patient is preload dependent and likely to respond to fluid administration
Low SVV (<10%) suggests adequate preload or that the patient is not fluid responsive.
Systemic Vascular Resistance Svr:
Normal SVR: 800-1200 dynes*sec/cm^5
Low SVR (<800) indicates vasodilation, common in septic shock, and often requires vasopressors
High SVR (>1200) indicates vasoconstriction, potentially due to hypovolemia, cold stress, or early severe sepsis, and may necessitate vasodilators or inotropes.
Intrathoracic Blood Volume Itbv:
PiCCO also provides ITBV, which estimates the volume of blood in the thorax
Normal ITBV is typically 700-1000 mL
Elevated ITBV may suggest fluid overload, while low ITBV might indicate the need for fluid administration, especially if SVV is also elevated.
Pulmonary Vascular Perfusion Index Pvpi:
This index represents the ratio of CO to pulmonary artery occlusion pressure (PAOP)
A high PVPI suggests adequate pulmonary vascular perfusion, while a low PVPI may indicate pulmonary hypertension or increased resistance.
Clinical Application And Management Guidelines
Septic Shock:
Initial management includes fluid resuscitation (targeting adequate CO and SVV), followed by vasopressors (e.g., norepinephrine)
Inotropes (e.g., dobutamine) may be added if CO remains low despite adequate preload and vasopressor support
Monitor SVV and ITBV to guide fluid therapy
Goal is to maintain CO >4 L/min and SVR within the target range.
Cardiogenic Shock:
Low CO with high SVR and potentially elevated ITBV
Focus on improving contractility with inotropes (dobutamine, milrinone)
Vasodilators (e.g., nitroglycerin) may be used cautiously if blood pressure permits
Avoid excessive fluid administration.
Hypovolemic Shock:
Low CO, low SVR (initially), and low ITBV
Aggressive fluid resuscitation is the mainstay
Monitor SVV for fluid responsiveness
Transfusion of blood products if indicated
Vasopressors may be needed if hypovolemia is refractory.
Postoperative Care:
In patients undergoing major surgery, CO monitoring helps identify and manage early postoperative hemodynamic instability, guide fluid and vasoactive drug administration, and optimize organ perfusion
It is particularly valuable in high-risk surgical patients.
Complications
Early Complications:
Infection at insertion sites of catheters
Thrombosis or bleeding related to arterial lines
Arrhythmias during TD bolus injection
Inadvertent dislodgement of catheters
Misinterpretation of data leading to inappropriate therapy.
Late Complications:
Catheter-related bloodstream infections (CRBSI) if lines are indwelling for prolonged periods
Chronic venous thrombosis
Pneumothorax or hemothorax if central venous catheter insertion is complicated.
Prevention Strategies:
Strict aseptic technique during insertion
Regular catheter site care and daily assessment for necessity of indwelling lines
Early removal of invasive lines when no longer indicated
Use of ultrasound for central venous access
Careful patient selection and protocolized monitoring.
Key Points
Exam Focus:
Understand the principles of thermodilution and PiCCO algorithms
Be able to interpret CO, SVV, SVR, and ITBV values in different shock states
Recognize that trends are more critical than single values
Differentiate between preload responsiveness and contractility issues.
Clinical Pearls:
Always correlate CO monitoring data with the patient's clinical status, vital signs, and urine output
Recognize that SVV is only reliable in mechanically ventilated patients with regular sinus rhythm and without high PEEP or intra-abdominal hypertension
PiCCO is less invasive than PAC but requires arterial access and can be affected by arrhythmias or severe valvular disease.
Common Mistakes:
Over-reliance on single CO values without considering trends
Incorrect interpretation of SVV in non-compliant patients (spontaneously breathing, irregular rhythms)
Failure to recognize limitations of the technology
Treating numbers in isolation rather than in the context of the whole patient
Not discontinuing monitoring when no longer clinically indicated.