Monitoring critical patients (Proceedings)

The purpose of close physiologic monitoring of critically ill patients is to alert clinicians and nurses to acute changes in patient status.  In addition, the monitoring of various parameters can provide a means to assess patient responses to specific therapeutic interventions.  A well equipped emergency room or ICU should have a number of monitors available for patient assessment.  These may be in the form of separate machines, or may be combined into a multiparameter monitor.  It is also important to remember that even though machines are convenient and can generate numbers, the results must always be interpreted in light of physical examination findings, and serial physical exams are potentially the most sensitive way to identify acute changes in patient status.

Much of critical care medicine is concerned with tissue perfusion; this is because poor oxygen delivery to tissues is a common denominator in critical illness, and perfusion is an important determinant of oxygen delivery.  While it is difficult to measure the individual perfusion of organs in the body, we can measure a number of surrogate values, which are correlated with perfusion in the body to some degree.  Some of these, such as lactate, indicate that there is a tissue bed that has poor oxygen delivery (and is thus producing lactate from anaerobic metabolism).  Others, such as urine output, are more specific to a particular organ system, but can be correlated to adequate perfusion of the kidneys (assuming that the patient has normal renal function).

One of the most common objective surrogate indices of perfusion is the systemic blood pressure.  The mean arterial blood pressure (MAP) is an expression of the average pressure (and by a corollary flow) that supplies blood to organs.  In most cases, perfusion of these organs is kept constant via local mechanisms as long as the MAP is between 60 and 160 mm Hg.  While an adequate blood pressure does not necessarily guarantee adequate perfusion, a poor blood pressure almost certainly correlates with poor organ perfusion.

Blood pressure may be measured either directly, via catheterization of an artery, or indirectly, using an automated (oscillometric) or manual device (e.g. Doppler probe).  The Doppler probe is a piezoelectric crystal placed over a peripheral artery (e.g. radial or metatarsal), and is used in combination with a sphygmomanometer attached to a cuff.  The sphygmomanometer measures the pressure inside the cuff in mm Hg.  As blood flows through the artery, past the crystal, the movement is translated into a sound.   As the pressure is increased in the cuff, blood flow to the extremity is decreased, and the Doppler sound diminishes.  As the pressure in the cuff decreases, again allowing blood flow into the extremity, sound gradually returns, and the highest cuff pressure at which sound is first audible is the systolic arterial pressure (SAP).  Oscillometric blood pressure machines sense oscillations caused by the arterial pulse under an inflatable cuff.  As the cuff inflates to a pressure greater than the systolic pressure, no oscillations are detected.  By incrementally decreasing the pressure in the cuff, the oscillations return, and the machine can estimate the systolic, mean, and diastolic blood pressures.  Direct catheterization of an artery is performed in small animals using the dorsal metatarsal artery, the tail artery, or ear artery.  This is the most accurate way to measure blood pressure, but also the most invasive and technically difficult. 

Electrocardiographic (ECG) monitoring displays the electric activity within the heart.  It can demonstrate cardiac arrhythmias, and can also give an idea about the origin of arrhythmias (e.g. ventricular or supraventricular).  A lead II ECG is typically used to monitor basic cardiac rate and rhythm, but other leads may be useful if the electrode placement is changed.  Any lead that clearly demonstrates P, QRS, and T waves is sufficient for general monitoring.  Although the ECG shows the electrical activity within the heart, it does not give information about the adequacy of each beat, or even if the electrical activity translates into a cardiac contraction.  The information from other monitoring devices such as blood pressure monitors can be assessed along with the ECG reading to provide a better idea of cardiac output.  The ECG is also useful to monitor fluid resuscitation; for instance, a patient with hypovolemic shock will likely exhibit tachycardia.  As effective circulating volume is restored (e.g. by intravenous crystalloid administration), perfusion should improve, and the heart rate should slow down, and the ECG can thus be used as a rough guide to assess the adequacy of resuscitation.  Some changes in electrolytes (e.g. increased K+)

While arterial blood gas (ABG) analysis is the gold standard for assessing oxygenation, arterial blood samples may be difficult to obtain in some patients.  The non-invasive technique of pulse oximetry (the “pulse ox”) allows the estimation of the SpO2, which is analogous to the SaO2, and can be used as a substitute for direct oxygen measurements in some patients. The pulse oximeter is also very useful for following trends in patients.  Rather than performing serial blood gas measurements to assess PaO2 and SaO2, the pulse oximeter probe may be left in place and will give a continuous estimation of the SaO2 over that time period.  The SaO2 and the PaO2 are related to each other by the oxyhemoglobin dissociation curve.  It is useful to have ABG analysis of patients with respiratory difficulty, but the addition of the pulse oximeter to the monitoring regime can decrease the total amount of analyses needed (e.g. from every 4 hours to every 12 hours).

Because decreased tissue oxygen delivery will result in an increase in anaerobic metabolism, plasma lactate concentrations are expected to increase in patients with shock.  Lactate may also be measured to assess the success of therapeutic interventions: if a patient presents to the emergency room with a high lactate due to hypovolemic shock, as the patient is resuscitated, the lactate should decrease.  The drop in lactate with resuscitation has been evaluated as a prognostic indicator for some types of shock.  If your initial resuscitative efforts do not result in a decrease in plasma lactate, the patient either requires more aggressive resuscitation, or an alternate strategy.  In patients with hemolytic anemia, a persistently high lactate is associated with a poorer prognosis.  Lactate may be measured by hand-held devices, or larger blood gas machines.

Arterial or venous blood gas analysis is essentially the only way to measure the adequacy of ventilation in awake critically ill patients.  Ventilation refers only to the partial pressure of CO2 in the circulating blood, and arterial and venous CO2 measrues closely approximate each other.  Hypoventilation such as might be caused by potent opioid drugs or neuromuscular disease will result in a high (>60 mm Hg) PCO2, while hyperventilation as caused by excessive panting, or in response to a metabolic acidosis will result in low PCO2.  In patients that are intubated, or in those who do not mind having a nasal cannula in place, end-tidal CO2­ (ETCO2)may be measured as an estimate of arterial CO2 levels.

Vascular volume is another important parameter that can be estimated, although not directly measured.  While hypovolemia may manifest as a low systemic blood pressure, hypervolemia is more difficult to assess.  The measurement of central venous pressure is one way to assess the intravascular volume status of a patient.  By measuring the pressure in the right side of the heart, CVP indirectly indicates the volume on the venous side; as the intravascular volume increases, the measured CVP will also increase.  Echocardiography may also give estimates of vascular volume; by assessing cardiac chamber size and function, conclusions can be drawn about preload.

In patients with difficulty maintaining intravascular fluid volume, or in those at risk of third space fluid losses, close monitoring of the “ins and outs” is indicated.  The total amount of intravenous fluids infused into the pet is compared to the total fluid outputs of the pet.  Output includes urine, vomitus, and diarrhea, primarily.  Urine output can be measured by placement of a urinary catheter and direct measurement, or by collection of voided urine for the determination of the weight (1 L of fluid weighs 1 kg).  Other losses can be difficult to measure or quantify, but the fact that fluid has weight is immutable.  Regular patient weight assessment (up to 3-4 times daily) can give valuable information regarding the fluid status of a pet.  Fluid infused in will result in weight gain, fluid loss will result in weight loss.  Especially in animals where it is difficult to closely monitor fluid outs, the use of patient weight will help the clinician to monitor general trends. 

Blood work in the critical patient will also give valuable information as to patient status.  Sampling of a packed cell volume (PCV) and plasma total solids (TS) can give information regarding anemia, blood loss, hypoproteinemia, and other systemic disease (e.g. icterus seen in after the plasma has separated from the red blood cells).  In patients with active hemorrhage, or those who may be anemic for other reasons, PCV and TS should be measured at least twice daily.  If blood is collected, other important monitoring that can be performed include a measurement of plasma or serum electrolytes.  In critically ill patients, especially those receiving fluid therapy, electrolytes such as potassium and calcium can change rapidly and be associated with changing patient status.

Suggestions for monitoring protocols for animals with various conditions that are encountered in the ICU are listed below; please note that these are general suggestions, and because each case is different, the individual needs may vary.  Additionally, the most important monitoring tool is the physical exam, and in any animal where they are in critical condition and requiring intensive monitoring should have physical exams performed at least every 4 hours, or more frequently if indicated.