Phases of Anesthesia and Isoflurane Concentration

 

Phases of Anesthesia and Isoflurane Concentration

Phase of Anesthesia	Dog	Cat	Notes
Induction	3% – 5%	3% – 5%	Administered with high oxygen flow to rapidly achieve unconsciousness after sedation. Commonly delivered via mask during induction, especially in cats.
Maintenance	1.5% – 2.5%	1.5% – 2.5%	Once the patient is stable and intubated, reduce to the lowest effective concentration to maintain surgical anesthesia.

Dog Parameter	Normal Range
Temperature	100.5–102.5 °F (38.1–39.2 °C)
Heart Rate	Puppies: 120–160 bpm Small: 100–140 bpm Medium: 80–120 bpm Large: 60–100 bpm
Respiratory Rate	10–30 breaths/min
Blood Pressure	Systolic: 110–160 mmHg Diastolic: 60–90 mmHg MAP: 85–120 mmHg
SpO₂	95–100%

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🐱 Cat

Parameter	Normal Range
Temperature	100.5–102.5 °F (38.1–39.2 °C)
Heart Rate	140–220 bpm
Respiratory Rate	20–30 breaths/min
Blood Pressure	Systolic: 120–170 mmHg Diastolic: 70–120 mmHg MAP: ~100 mmHg
SpO₂ 1. Synergistic Combinations Synergy occurs when the combined effect of two drugs is significantly greater than the sum of their individual effects (1 + 1 > 2). General Rule: Synergy is most likely when two drugs target different, sequential steps in a critical bacterial metabolic pathway. Common Mechanisms and Examples: Mechanism of Synergy Classic Example Rationale Cell Wall Disruption + Altered Membrane Permeability β-lactam (e.g., Penicillin/Ampicillin) + Aminoglycoside (e.g., Gentamicin) The β-lactam weakens and disrupts the cell wall, allowing the aminoglycoside to enter the bacterial cell more easily and reach its ribosomal target. This is a cornerstone of treatment for serious enterococcal and pseudomonal infections. Sequential Block of a Metabolic Pathway Sulfonamide + Trimethoprim Sulfonamides inhibit an early step in folate synthesis, and trimethoprim inhibits a subsequent step. This double blockade is highly effective and bactericidal. Inhibition of Different Vital Functions β-lactam + Glycopeptide (e.g., Vancomycin) Both target cell wall synthesis but at different points. This combination can be synergistic against some highly resistant Gram-positive organisms. Inhibition of Resistance Mechanisms Amoxicillin + Clavulanic Acid (a β-lactamase inhibitor) Clavulanic acid irreversibly inhibits the bacterial β-lactamase enzyme, protecting amoxicillin from destruction and restoring its activity. 2. Antagonistic Combinations Antagonism occurs when the activity of one antibiotic interferes with the activity of another, resulting in a combined effect that is less than the effect of the most effective drug alone (1 + 1 < 1). General Rule: Antagonism is most likely when a bacteriostatic drug is combined with a bactericidal drug that requires active bacterial growth to be effective. Common Mechanisms and Examples: Mechanism of Antagonism Classic Example Rationale Bacteriostatic vs. Bactericidal Tetracycline (Bacteriostatic) + Penicillin (Bactericidal) Bacteriostatic drugs (like tetracyclines, macrolides, chloramphenicol) inhibit protein synthesis and stop bacterial growth. Bactericidal drugs like penicillins are most effective against actively growing and dividing cells that are building cell walls. By halting growth, the bacteriostatic drug renders the bactericidal drug less effective. Direct Interference with Mechanism Chloramphenicol + other 50S inhibitors (e.g., Macrolides, Lincosamides like Clindamycin) These drugs bind to the same general site on the 50S ribosomal subunit. When administered together, they compete for binding, reducing the overall efficacy of both. Induction of Destructive Enzymes Certain Cephalosporins (e.g., Cefoxitin) + Penicillins Some drugs can induce the production of beta-lactamase enzymes. Using them with another beta-lactam can lead to the destruction of the companion drug. Important Clinical Note: The "static vs. cidal" antagonism rule is a general principle, not an absolute law. The outcome can depend on the specific organism, drug concentrations, and timing of administration. However, it is a critical consideration when designing a regimen. 3. Additive or Indifferent Combinations This is the most common outcome of antibiotic combinations. Additive: The combined effect equals the sum of the individual effects (1 + 1 = 2). Indifferent: The combined effect is no greater than that of the more effective drug used alone. There is no interaction, positive or negative. General Rule: Additive/indifferent effects are seen when drugs act on different targets or pathways without interacting or when their spectra simply overlap. Common Examples: Combining a drug that targets the cell wall (e.g., a penicillin) with a drug that targets DNA (e.g., a fluoroquinolone) for a polymicrobial infection often results in an indifferent effect—each drug acts independently on its own target organisms. Using two different beta-lactams together (e.g., ampicillin + ceftriaxone) is generally avoided as it is often indifferent and increases the risk of selecting for broad beta-lactam resistance without a clear benefit. Summary Table for Quick Reference Interaction Type Effect When It's Likely Clinical Implication Synergistic 1 + 1 > 2 Drugs target sequential steps or one enhances penetration of the other. Use for: Serious/resistant infections (e.g., endocarditis, sepsis in immunocompromised), to lower doses of toxic drugs. Antagonistic 1 + 1 < 1 Bacteriostatic + Bactericidal (especially on rapidly growing cultures). Generally AVOID. Can lead to treatment failure. Be cautious combining drug classes. Additive/Indifferent 1 + 1 = 2 Drugs act on completely independent targets or have overlapping spectra. Use for: Polymicrobial infections to broaden coverage. Most common outcome of random combinations. Key Clinical Takeaway Combining antibiotics is a double-edged sword. It should be a deliberate, evidence-based decision, not a guess. Prioritize Culture & Susceptibility: The best approach is to identify the pathogen and its susceptibility profile and choose the single, most effective, narrowest-spectrum antibiotic. Have a Clear Rationale: Only combine antibiotics if you have a specific reason (e.g., seeking synergy for a resistant infection, covering multiple likely pathogens in a critical patient). Be Aware of Toxicity: Combining drugs can also combine their adverse effect profiles (e.g., both an aminoglycoside and vancomycin are nephrotoxic). Follow Established Protocols: When synergy is desired, use known synergistic pairs (e.g., ampicillin + gentamicin for endocarditis) rather than experimenting with unproven combinations. Synergistic Interactions A synergistic interaction means the combined effect of the drugs is greater than the sum of their individual effects. This is often the goal when combining antimicrobials. Drug Class / Specific Drug Interaction and Rationale Clinical Application in Small Animals Aminoglycosides (e.g., Amikacin, Gentamicin) Synergy. Both drugs target the bacterial ribosome (50S and 30S subunits, respectively), leading to a more effective disruption of protein synthesis. Azithromycin can also enhance the penetration of aminoglycosides into bacterial cells. Used for severe, difficult-to-treat infections like pyoderma, deep tissue abscesses, or pneumonia caused by susceptible but resilient organisms (e.g., some Pseudomonas strains, though azithromycin has poor inherent anti-pseudomonal activity). Major Caution: Increased risk of nephrotoxicity and ototoxicity from the aminoglycoside. Fluoroquinolones (e.g., Enrofloxacin, Pradofloxacin) Potential Synergy. The combination attacks two different bacterial targets: DNA replication (fluoroquinolones) and protein synthesis (azithromycin). This can be particularly effective against some intracellular bacteria. Consider for stubborn Bartonellosis or other deep-seated intracellular infections. However, this combination is not always synergistic and can be antagonistic for some bacteria, so it should be used judiciously and ideally based on culture/susceptibility testing. Rifampin Strong Synergy. Rifampin inhibits RNA synthesis, while azithromycin inhibits protein synthesis. This dual attack is highly effective against intracellular organisms and can help prevent the development of resistance. A classic combination for treating atypical mycobacterial infections and sometimes severe Bartonella infections. Monitoring is crucial: Rifampin is a potent liver enzyme inducer and can cause hepatotoxicity. It also turns bodily fluids (urine, tears, saliva) orange-red.	95–100%