Diagnosis and Management of Shock
1. Identify the major types of shock
2. Understand the types of therapy used in shock: fluid therapy, inotropes, etc
3. Understand the concept of oxygen supply and delivery as it applies to shock
4. Become familiar with basic hemodynamic monitoring
Definitions/Classification of Different Types of Shock
Shock is defined as the condition wherein blood flow (and subsequently the delivery of oxygen and nutrients) is inadequate to meet tissue demand. Although patients are often hypotensive when in shock, shock can be present without hypotension: there must be associated hypoperfusion abnormalities. Hypoperfusion may lead to end-organ dysfunction, which can be manifest in many ways including mental status changes, oliguria, increasing LFT’s, lactic acidosis, etc.
Types of Shock
1. Cardiogenic – characterized by inadequate forward blood flow and contractility because of a primary defect in cardiac function; typically will have low cardiac output (CO), increased systemic vascular resistance (SVR), and increased left ventricular filling pressures (LVFP), which may lead to higher central venous pressures (CVP)
2. Hypovolemic – occurs when the intravascular volume becomes depleted; typically will have low CO, increased SVR, decreased LVFP, and low CVP
3. Distributive – characterized by loss of peripheral vascular tone so blood flow is maldistributed (i.e. some organs that need more blood will get less and those that need less will get more); these patients typically will have normal to increased CO, low SVR, low LVFP and low CVP; this type of shock is also known as “warm shock” and is seen with sepsis, anaphylaxis, adrenal insufficiency, and neurogenic shock.
4. Obstructive – characterized by impedance of cardiac filling as is seen with cardiac tamponade or pneumothorax or with interference of LV outflow as is seen with coarctation of the aorta or interrupted aortic arch,; typically will have low CO, increased SVR, and increased CVP
Management of Shock
Physiology of Cardiac Output
The treatment of shock depends on the type of shock the patient has, but the ultimate goal should be to restore adequate oxygen delivery to the tissues:
Delivery of O2 (DO2) = Cardiac Index (CI) x O2 Content (CaO2)
Normal values = 620 +/- 50 ml/min/m2
CaO2 = (1.34 x Hgb x SaO2) + (0.0031 x PaO2)
CO = HR x SV; Normal CI = 3.5 – 5.5 L/min/m2
Stroke volume is determined by preload, afterload, and contractility
So, in order to improve O2 delivery, one may have to optimize the heart’s preload, contractility, or afterload or affect the vascular system tone.
Preload: defined as the length that cardiac muscle is stretched prior to a contraction; for any given patient, there is an optimal preload where the muscle fibers are at their most efficient; when you surpass this length (i.e. volume overload), the heart is much less efficient when it pumps and “heart failure” can develop
Contractility: ability of cardiac muscle to generate force; because the compliance of a newborn infant’s heart is less than in an adult, contractility becomes less of an issue in an infant. The best way infants can increase their CO is to increase their heart rates (see above equations)
Afterload: defined as the load cardiac muscle encounters during a contraction
Treatment of Shock
In any given patient with signs of end-organ hypoperfusion, it is appropriate to give an initial fluid bolus of a crystalloid (NS or LR, NEVER A DEXTROSE CONTAINING SOLUTION), in order to optimize preload. However, if cardiac dysfunction is suspected, fluids need to be given judiciously as too much fluid will increase the preload but may worsen contractility and afterload (which are the primary problems with this type of shock). In other forms of shock, fluid boluses of 20cc/kg should be administered; when the patient has received 60cc/kg, inotropic support should be considered, although more fluid (up to 100cc/kg or greater) may need to be given. One method of deciding the optimal preload for a patient is to give enough fluid until the CVP is ~ 8-10 cm H2O (see Monitoring below) and to reevaluate the patient’s status for consideration of inotropes.
The use of colloids (albumin, hetastarch) in resuscitation is controversial and thus their uses are Attending dependent.
Blood products may also be considered when appropriate, although the use of packed RBC’s in order to maintain oxygen carrying capacity (see CaO2 equation above) is also controversial. In general, Hgb > 7g/dl is considered adequate.
Inotropes: Receptors and Mechanisms
Catecholamine receptors are divided into 4 main types
1. a1 -- vasoconstriction
2. a2 -- vasodilatation and negative chronotropy (works centrally)
3. b1 -- improves cardiac function (inotropy, chronotropy, dromotropy)
4. b2 -- affects vascular and bronchiolar tone by relaxing smooth muscle
1 - Epinephrine
-- Potent a1 and b1 agonism and moderate b2 agonism
-- At lower doses (0.04-0.1 mcg/kg/min), b effects predominate (increased CO, HR, decreased SVR)
-- At higher doses, vasoconstriction and venoconstriction
-- Potent renal/splanchnic vasoconstrictor even at low doses (countered by increased CO)
-- Indication: useful in providing inotropic support in patients with severe CV collapse
-- T½ = 3 minutes; dose = 0.1-1.0 mcg/kg/min;
-- Must be given through a central line
2 - Norepinephrine
-- Potent a1 agonism, moderate b1 – (+) vasoconstrictor and some inotropy
-- Decreases renal and mesenteric perfusion and increases afterload
-- Major indications: “warm” shock that does not respond to volume or Dopamine
-- T½ = 2 minutes; dose = 0.05-0.1 mcg/kg/min
-- Must be given through a central line
-- Adverse rxns: local infiltrate, which can cause necrosis – treated with phentolamine
3 - Dopamine
-- Naturally occurring precursor of Norepinephrine
-- Has a dose dependent effect on a, b, and Dopamine receptors:
-- < 2-5 mcg/kg/min stimulate DA1 receptors to increase renal, mesenteric, cerebral, and coronary perfusion
-- 5-10 mcg/kg/min lead to b agonism – increase HR, contractility, SV with little effect on SVR à increases CO
-- >10 mcg/kg/min leads to predominating a effects with vaso- and venoconstriction; also decreases mesenteric and renal perfusion, increases coronary resistance and myocardial work
-- increases pulmonary artery pressures in those with pulmonary hypertension
-- Indication: used for decreased CO, BP
-- T½ = 2 minutes; dose =1-20 mcg/kg/min
-- Must be given through a central line
4 - Dobutamine
-- synthetic catecholamine
-- b- selective (mainly b1) but l-isomer has little a effects
-- Increases contractility and HR with some decrease in SVR
-- Indications: used in patients with low CO states
-- T½ = 2 min; dose =1-20 mcg/kg/min
5 - Milrinone
-- non-catecholaminergic inotropic agent; phosphodiesterase inhibitor
-- Increases CO and vasodilates; pulmonary vasodilator
-- Indications: useful in patients with CHF, cardiogenic shock, hypodynamic septic shock, b receptor down regulation
-- Dose= 0.25-0.75 mcg/kg/min
Use of Inotropes
The treatment of shock with inotropes should involve careful thought as to the type of shock the patient has and how it is being manifested in the patient. For example, a person with hyperdynamic “warm”shock should not be treated solely with an agent that promotes more CO as this will not help.
Cardiogenic shock – since the primary problem is cardiac dysfunction, the goal of inotropic therapy should be to improve contractility and decrease afterload so that the heart may pump more efficiently. In this case, agents such as Dobutamine, Epinephrine, and Milrinone (Dopamine could also be considered) are the main drugs used.
Hypovolemic shock – should not require inotropic support after appropriate fluids are given
Distributive shock – after appropriate fluid resuscitation and the patient’s preload is optimized, the goal of inotropic support should be to improve vascular tone. In this case, agents such as Dopamine and Norepinephrine should be considered, as they are vasoconstrictors.
Obstructive shock – although fluid and intropes may be needed, correction of the problem (chest tube, surgical repair of a heart lesion, etc) are the treatments of choice
** In infants with suspected obstructed left heart lesions that are ductal dependent, a prostaglandin infusion of 0.05-0.1 mcg/kg/min should be initiated. These patients may present with a metabolic acidosis, absent or poor femoral pulses, poor urine output, etc. The side effects of PGE1 infusion include apnea and hypotension, particularly at the beginning of the infusion.
Patients with shock that necessitate multiple fluid boluses and/or inotropes should be considered for central venous and arterial access
Central Venous Catheter
Catheters with 1-3 lumens that are placed in large veins (femoral, subclavian, or internal jugular veins) for the purpose of administering certain vasoactive drugs (Dopamine, NE, Epi) and for obtaining central venous pressures (CVP). A CVP can be used to estimate optimal preload. However, it must be remembered that the CVP is measuring a pressure and assumptions are made about the patient’s volume status. In patients with any kind of cardiac dysfunction, the optimal preload may be manifest as a higher than average CVP.
Single lumen catheter that is placed in an artery (radial, axillary, femoral, dorsalis pedis) in order to continually monitor blood pressures and to obtain frequent labs, including blood gasses.
Bladder catheterization should be considered in any patient with signs of end-organ hypoperfusion in order to monitor urine output on an hourly basis
In addition to following a patient’s clinical status (i.e. urine output, perfusion, mental status, etc.) certain laboratory test and radiographic studies may be useful. The following list is not all inclusive of the studies that may be needed.
1. blood gas –the trends of pH and base deficits can be used to determine efficacy of resuscitation; central venous SvO2 as compared to arterial SaO2 can be used to follow cardiac output when titrating inotropes
2. CXR – can be used to evaluate the presence of pulmonary edema, pneumothorax, etc. Heart size can also be evaluated
3. Echo – can be useful in evaluating heart function in a patient in shock; should also be used in infants with shock and acidosis to rule out congenital heat disease
4. Electrolytes – useful in determining renal function; can also look at the bicarbonate as a measure of acid/base status
5. Hemoglobin – useful in cases of hemorrhagic shock as well as shock associated with anemia